Fix test

This fixes the issue where updating a Git input does a non-shallow
fetch, and then a subsequent eval does a shallow refetch because
the revCount is already known. Now the subsequent eval will use the
repo used in the first fetch.

doc/manual/package.nix

@@ -37,6 +37,7 @@ mkMesonDerivation (finalAttrs: {
       (fileset.unions [
         ../../.version
         # For example JSON
+        ../../src/libutil-tests/data/memory-source-accessor
         ../../src/libutil-tests/data/hash
         ../../src/libstore-tests/data/content-address
         ../../src/libstore-tests/data/store-path

doc/manual/rl-next/libcurl-pausing.md

@@ -0,0 +1,12 @@
+---
+synopsis: Fix "download buffer is full; consider increasing the 'download-buffer-size' setting" warning
+prs: [14614]
+issues: [11728]
+---
+
+The underlying issue that led to [#11728](https://github.com/NixOS/nix/issues/11728) has been resolved by utilizing
+[libcurl write pausing functionality](https://curl.se/libcurl/c/curl_easy_pause.html) to control backpressure when unpacking to slow destinations like the git-backed tarball cache. The default value of `download-buffer-size` is now 1 MiB and it's no longer recommended to increase it, since the root cause has been fixed.
+
+This is expected to improve download performance on fast connections, since previously a single slow download consumer would stall the thread and prevent any other transfers from progressing.
+
+Many thanks go out to the [Lix project](https://lix.systems/) for the [implementation](https://git.lix.systems/lix-project/lix/commit/4ae6fb5a8f0d456b8d2ba2aaca3712b4e49057fc) that served as inspiration for this change and for triaging libcurl [issues with pausing](https://github.com/curl/curl/issues/19334).

doc/manual/source/SUMMARY.md.in

@@ -121,6 +121,7 @@
 - [Architecture and Design](architecture/architecture.md)
 - [Formats and Protocols](protocols/index.md)
   - [JSON Formats](protocols/json/index.md)
+    - [File System Object](protocols/json/file-system-object.md)
     - [Hash](protocols/json/hash.md)
     - [Content Address](protocols/json/content-address.md)
     - [Store Path](protocols/json/store-path.md)

doc/manual/source/development/debugging.md

@@ -15,7 +15,7 @@ In the development shell, set the `mesonBuildType` environment variable to `debu
 Then, proceed to build Nix as described in [Building Nix](./building.md).
 This will build Nix with debug symbols, which are essential for effective debugging.
 
-It is also possible to build without debugging for faster build:
+It is also possible to build without optimization for faster build:
 
 ```console
 [nix-shell]$ NIX_HARDENING_ENABLE=$(printLines $NIX_HARDENING_ENABLE | grep -v fortify)

doc/manual/source/protocols/json/file-system-object.md

@@ -0,0 +1,21 @@
+{{#include file-system-object-v1-fixed.md}}
+
+## Examples
+
+### Simple
+
+```json
+{{#include schema/file-system-object-v1/simple.json}}
+```
+
+### Complex
+
+```json
+{{#include schema/file-system-object-v1/complex.json}}
+```
+
+<!-- need to convert YAML to JSON first
+## Raw Schema
+
+[JSON Schema for File System Object v1](schema/file-system-object-v1.json)
+-->

doc/manual/source/protocols/json/fixup-json-schema-generated-doc.sed

@@ -11,6 +11,7 @@ s/\\`/`/g
 #
 # As we have more such relative links, more replacements of this nature
 # should appear below.
+s^#/\$defs/\(regular\|symlink\|directory\)^In this schema^g
 s^\(./hash-v1.yaml\)\?#/$defs/algorithm^[JSON format for `Hash`](./hash.html#algorithm)^g
 s^\(./hash-v1.yaml\)^[JSON format for `Hash`](./hash.html)^g
 s^\(./content-address-v1.yaml\)\?#/$defs/method^[JSON format for `ContentAddress`](./content-address.html#method)^g

doc/manual/source/protocols/json/meson.build

@@ -9,6 +9,7 @@ json_schema_for_humans = find_program('generate-schema-doc', required : false)
 json_schema_config = files('json-schema-for-humans-config.yaml')
 
 schemas = [
+  'file-system-object-v1',
   'hash-v1',
   'content-address-v1',
   'store-path-v1',

doc/manual/source/protocols/json/schema/file-system-object-v1

@@ -0,0 +1 @@
+../../../../../../src/libutil-tests/data/memory-source-accessor

doc/manual/source/protocols/json/schema/file-system-object-v1.yaml

@@ -0,0 +1,71 @@
+"$schema": http://json-schema.org/draft-04/schema#
+"$id": https://nix.dev/manual/nix/latest/protocols/json/schema/file-system-object-v1.json
+title: File System Object
+description: |
+  This schema describes the JSON representation of Nix's [File System Object](@docroot@/store/file-system-object.md).
+
+  The schema is recursive because file system objects contain other file system objects.
+type: object
+required: ["type"]
+properties:
+  type:
+    type: string
+    enum: ["regular", "symlink", "directory"]
+
+# Enforce conditional structure based on `type`
+anyOf:
+  - $ref: "#/$defs/regular"
+    required: ["type", "contents"]
+
+  - $ref: "#/$defs/directory"
+    required: ["type", "entries"]
+
+  - $ref: "#/$defs/symlink"
+    required: ["type", "target"]
+
+"$defs":
+  regular:
+    title: Regular File
+    description: |
+      See [Regular File](@docroot@/store/file-system-object.md#regular) in the manual for details.
+    required: ["contents"]
+    properties:
+      type:
+        const: "regular"
+      contents:
+        type: string
+        description: File contents
+      executable:
+        type: boolean
+        description: Whether the file is executable.
+        default: false
+    additionalProperties: false
+
+  directory:
+    title: Directory
+    description: |
+      See [Directory](@docroot@/store/file-system-object.md#directory) in the manual for details.
+    required: ["entries"]
+    properties:
+      type:
+        const: "directory"
+      entries:
+        type: object
+        description: |
+          Map of names to nested file system objects (for type=directory)
+        additionalProperties:
+          $ref: "#"
+    additionalProperties: false
+
+  symlink:
+    title: Symbolic Link
+    description: |
+      See [Symbolic Link](@docroot@/store/file-system-object.md#symlink) in the manual for details.
+    required: ["target"]
+    properties:
+      type:
+        const: "symlink"
+      target:
+        type: string
+        description: Target path of the symlink.
+    additionalProperties: false

doc/manual/source/store/file-system-object.md

@@ -3,19 +3,23 @@
 Nix uses a simplified model of the file system, which consists of file system objects.
 Every file system object is one of the following:
 
- - File
+ - [**Regular File**]{#regular}
 
    - A possibly empty sequence of bytes for contents
    - A single boolean representing the [executable](https://en.m.wikipedia.org/wiki/File-system_permissions#Permissions) permission
 
- - Directory
+ - [**Directory**]{#directory}
 
    Mapping of names to child file system objects
 
- - [Symbolic link](https://en.m.wikipedia.org/wiki/Symbolic_link)
+ - [**Symbolic link**]{#symlink}
 
-   An arbitrary string.
-   Nix does not assign any semantics to symbolic links.
+   An arbitrary string, known as the *target* of the symlink.
+
+   In general, Nix does not assign any semantics to symbolic links.
+   Certain operations however, may make additional assumptions and attempt to use the target to find another file system object.
+
+   > See [the Wikpedia article on symbolic links](https://en.m.wikipedia.org/wiki/Symbolic_link) for background information if you are unfamiliar with this Unix concept.
 
 File system objects and their children form a tree.
 A bare file or symlink can be a root file system object.

src/json-schema-checks/file-system-object

@@ -0,0 +1 @@
+../../src/libutil-tests/data/memory-source-accessor

src/json-schema-checks/meson.build

@@ -20,6 +20,14 @@ schema_dir = meson.current_source_dir() / 'schema'
 
 # Get all example files
 schemas = [
+  {
+    'stem' : 'file-system-object',
+    'schema' : schema_dir / 'file-system-object-v1.yaml',
+    'files' : [
+      'simple.json',
+      'complex.json',
+    ],
+  },
   {
     'stem' : 'hash',
     'schema' : schema_dir / 'hash-v1.yaml',

src/json-schema-checks/package.nix

@@ -20,6 +20,7 @@ mkMesonDerivation (finalAttrs: {
   fileset = lib.fileset.unions [
     ../../.version
     ../../doc/manual/source/protocols/json/schema
+    ../../src/libutil-tests/data/memory-source-accessor
     ../../src/libutil-tests/data/hash
     ../../src/libstore-tests/data/content-address
     ../../src/libstore-tests/data/store-path

src/libcmd/common-eval-args.cc

@@ -168,9 +168,9 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
                                 ? state.rootPath(absPath(getCommandBaseDir()))
                                 : state.rootPath(".")));
                 },
-                [&](const AutoArgString & arg) { v->mkString(arg.s); },
-                [&](const AutoArgFile & arg) { v->mkString(readFile(arg.path.string())); },
-                [&](const AutoArgStdin & arg) { v->mkString(readFile(STDIN_FILENO)); }},
+                [&](const AutoArgString & arg) { v->mkString(arg.s, state.mem); },
+                [&](const AutoArgFile & arg) { v->mkString(readFile(arg.path.string()), state.mem); },
+                [&](const AutoArgStdin & arg) { v->mkString(readFile(STDIN_FILENO), state.mem); }},
             arg);
         res.insert(state.symbols.create(name), v);
     }

src/libexpr-c/nix_api_expr.cc

@@ -69,8 +69,8 @@ nix_err nix_expr_eval_from_string(
         context->last_err_code = NIX_OK;
     try {
         nix::Expr * parsedExpr = state->state.parseExprFromString(expr, state->state.rootPath(nix::CanonPath(path)));
-        state->state.eval(parsedExpr, value->value);
-        state->state.forceValue(value->value, nix::noPos);
+        state->state.eval(parsedExpr, *value->value);
+        state->state.forceValue(*value->value, nix::noPos);
     }
     NIXC_CATCH_ERRS
 }
@@ -80,8 +80,8 @@ nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, n
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        state->state.callFunction(fn->value, arg->value, value->value, nix::noPos);
-        state->state.forceValue(value->value, nix::noPos);
+        state->state.callFunction(*fn->value, *arg->value, *value->value, nix::noPos);
+        state->state.forceValue(*value->value, nix::noPos);
     }
     NIXC_CATCH_ERRS
 }
@@ -91,9 +91,15 @@ nix_err nix_value_call_multi(
 {
     if (context)
         context->last_err_code = NIX_OK;
+
+    std::vector<nix::Value *> internal_args;
+    internal_args.reserve(nargs);
+    for (size_t i = 0; i < nargs; i++)
+        internal_args.push_back(args[i]->value);
+
     try {
-        state->state.callFunction(fn->value, {(nix::Value **) args, nargs}, value->value, nix::noPos);
-        state->state.forceValue(value->value, nix::noPos);
+        state->state.callFunction(*fn->value, {internal_args.data(), nargs}, *value->value, nix::noPos);
+        state->state.forceValue(*value->value, nix::noPos);
     }
     NIXC_CATCH_ERRS
 }
@@ -103,7 +109,7 @@ nix_err nix_value_force(nix_c_context * context, EvalState * state, nix_value *
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        state->state.forceValue(value->value, nix::noPos);
+        state->state.forceValue(*value->value, nix::noPos);
     }
     NIXC_CATCH_ERRS
 }
@@ -113,7 +119,7 @@ nix_err nix_value_force_deep(nix_c_context * context, EvalState * state, nix_val
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        state->state.forceValueDeep(value->value);
+        state->state.forceValueDeep(*value->value);
     }
     NIXC_CATCH_ERRS
 }

src/libexpr-c/nix_api_expr_internal.h

@@ -39,7 +39,13 @@ struct ListBuilder
 
 struct nix_value
 {
-    nix::Value value;
+    nix::Value * value;
+    /**
+     * As we move to a managed heap, we need EvalMemory in more places. Ideally, we would take in EvalState or
+     * EvalMemory as an argument when we need it, but we don't want to make changes to the stable C api, so we stuff it
+     * into the nix_value that will get passed in to the relevant functions.
+     */
+    nix::EvalMemory * mem;
 };
 
 struct nix_string_return

src/libexpr-c/nix_api_value.cc

@@ -20,7 +20,7 @@ static const nix::Value & check_value_not_null(const nix_value * value)
     if (!value) {
         throw std::runtime_error("nix_value is null");
     }
-    return *((const nix::Value *) value);
+    return *value->value;
 }
 
 static nix::Value & check_value_not_null(nix_value * value)
@@ -28,7 +28,7 @@ static nix::Value & check_value_not_null(nix_value * value)
     if (!value) {
         throw std::runtime_error("nix_value is null");
     }
-    return value->value;
+    return *value->value;
 }
 
 static const nix::Value & check_value_in(const nix_value * value)
@@ -58,9 +58,14 @@ static nix::Value & check_value_out(nix_value * value)
     return v;
 }
 
-static inline nix_value * as_nix_value_ptr(nix::Value * v)
+static inline nix_value * new_nix_value(nix::Value * v, nix::EvalMemory & mem)
 {
-    return reinterpret_cast<nix_value *>(v);
+    nix_value * ret = new (mem.allocBytes(sizeof(nix_value))) nix_value{
+        .value = v,
+        .mem = &mem,
+    };
+    nix_gc_incref(nullptr, ret);
+    return ret;
 }
 
 /**
@@ -69,7 +74,13 @@ static inline nix_value * as_nix_value_ptr(nix::Value * v)
  * Deals with errors and converts arguments from C++ into C types.
  */
 static void nix_c_primop_wrapper(
-    PrimOpFun f, void * userdata, nix::EvalState & state, const nix::PosIdx pos, nix::Value ** args, nix::Value & v)
+    PrimOpFun f,
+    void * userdata,
+    int arity,
+    nix::EvalState & state,
+    const nix::PosIdx pos,
+    nix::Value ** args,
+    nix::Value & v)
 {
     nix_c_context ctx;
 
@@ -85,8 +96,15 @@ static void nix_c_primop_wrapper(
     // ok because we don't see a need for this yet (e.g. inspecting thunks,
     // or maybe something to make blackholes work better; we don't know).
     nix::Value vTmp;
+    nix_value * vTmpPtr = new_nix_value(&vTmp, state.mem);
 
-    f(userdata, &ctx, (EvalState *) &state, (nix_value **) args, (nix_value *) &vTmp);
+    std::vector<nix_value *> external_args;
+    external_args.reserve(arity);
+    for (int i = 0; i < arity; i++) {
+        nix_value * external_arg = new_nix_value(args[i], state.mem);
+        external_args.push_back(external_arg);
+    }
+    f(userdata, &ctx, (EvalState *) &state, external_args.data(), vTmpPtr);
 
     if (ctx.last_err_code != NIX_OK) {
         /* TODO: Throw different errors depending on the error code */
@@ -135,7 +153,7 @@ PrimOp * nix_alloc_primop(
                     .args = {},
                     .arity = (size_t) arity,
                     .doc = doc,
-                    .fun = std::bind(nix_c_primop_wrapper, fun, user_data, _1, _2, _3, _4)};
+                    .fun = std::bind(nix_c_primop_wrapper, fun, user_data, arity, _1, _2, _3, _4)};
         if (args)
             for (size_t i = 0; args[i]; i++)
                 p->args.emplace_back(*args);
@@ -160,8 +178,7 @@ nix_value * nix_alloc_value(nix_c_context * context, EvalState * state)
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        nix_value * res = as_nix_value_ptr(state->state.allocValue());
-        nix_gc_incref(nullptr, res);
+        nix_value * res = new_nix_value(state->state.allocValue(), state->state.mem);
         return res;
     }
     NIXC_CATCH_ERRS_NULL
@@ -331,10 +348,10 @@ nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value,
             return nullptr;
         }
         auto * p = v.listView()[ix];
-        nix_gc_incref(nullptr, p);
-        if (p != nullptr)
-            state->state.forceValue(*p, nix::noPos);
-        return as_nix_value_ptr(p);
+        if (p == nullptr)
+            return nullptr;
+        state->state.forceValue(*p, nix::noPos);
+        return new_nix_value(p, state->state.mem);
     }
     NIXC_CATCH_ERRS_NULL
 }
@@ -352,9 +369,8 @@ nix_get_list_byidx_lazy(nix_c_context * context, const nix_value * value, EvalSt
             return nullptr;
         }
         auto * p = v.listView()[ix];
-        nix_gc_incref(nullptr, p);
         // Note: intentionally NOT calling forceValue() to keep the element lazy
-        return as_nix_value_ptr(p);
+        return new_nix_value(p, state->state.mem);
     }
     NIXC_CATCH_ERRS_NULL
 }
@@ -369,9 +385,8 @@ nix_value * nix_get_attr_byname(nix_c_context * context, const nix_value * value
         nix::Symbol s = state->state.symbols.create(name);
         auto attr = v.attrs()->get(s);
         if (attr) {
-            nix_gc_incref(nullptr, attr->value);
             state->state.forceValue(*attr->value, nix::noPos);
-            return as_nix_value_ptr(attr->value);
+            return new_nix_value(attr->value, state->state.mem);
         }
         nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
         return nullptr;
@@ -390,9 +405,8 @@ nix_get_attr_byname_lazy(nix_c_context * context, const nix_value * value, EvalS
         nix::Symbol s = state->state.symbols.create(name);
         auto attr = v.attrs()->get(s);
         if (attr) {
-            nix_gc_incref(nullptr, attr->value);
             // Note: intentionally NOT calling forceValue() to keep the attribute lazy
-            return as_nix_value_ptr(attr->value);
+            return new_nix_value(attr->value, state->state.mem);
         }
         nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
         return nullptr;
@@ -440,9 +454,8 @@ nix_get_attr_byidx(nix_c_context * context, nix_value * value, EvalState * state
         }
         const nix::Attr & a = (*v.attrs())[i];
         *name = state->state.symbols[a.name].c_str();
-        nix_gc_incref(nullptr, a.value);
         state->state.forceValue(*a.value, nix::noPos);
-        return as_nix_value_ptr(a.value);
+        return new_nix_value(a.value, state->state.mem);
     }
     NIXC_CATCH_ERRS_NULL
 }
@@ -461,9 +474,8 @@ nix_value * nix_get_attr_byidx_lazy(
         }
         const nix::Attr & a = (*v.attrs())[i];
         *name = state->state.symbols[a.name].c_str();
-        nix_gc_incref(nullptr, a.value);
         // Note: intentionally NOT calling forceValue() to keep the attribute lazy
-        return as_nix_value_ptr(a.value);
+        return new_nix_value(a.value, state->state.mem);
     }
     NIXC_CATCH_ERRS_NULL
 }
@@ -503,7 +515,7 @@ nix_err nix_init_string(nix_c_context * context, nix_value * value, const char *
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_out(value);
-        v.mkString(std::string_view(str));
+        v.mkString(std::string_view(str), *value->mem);
     }
     NIXC_CATCH_ERRS
 }
@@ -514,7 +526,7 @@ nix_err nix_init_path_string(nix_c_context * context, EvalState * s, nix_value *
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_out(value);
-        v.mkPath(s->state.rootPath(nix::CanonPath(str)));
+        v.mkPath(s->state.rootPath(nix::CanonPath(str)), s->state.mem);
     }
     NIXC_CATCH_ERRS
 }

src/libexpr-tests/json.cc

@@ -73,7 +73,7 @@ TEST_F(JSONValueTest, StringQuotes)
 TEST_F(JSONValueTest, DISABLED_Path)
 {
     Value v;
-    v.mkPath(state.rootPath(CanonPath("/test")));
+    v.mkPath(state.rootPath(CanonPath("/test")), state.mem);
     ASSERT_EQ(getJSONValue(v), "\"/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x\"");
 }
 } /* namespace nix */

src/libexpr-tests/value/print.cc

@@ -268,7 +268,7 @@ struct StringPrintingTests : LibExprTest
     void test(std::string_view literal, std::string_view expected, unsigned int maxLength, A... args)
     {
         Value v;
-        v.mkString(literal);
+        v.mkString(literal, state.mem);
 
         std::stringstream out;
         printValue(state, out, v, PrintOptions{.maxStringLength = maxLength});
@@ -353,7 +353,7 @@ TEST_F(ValuePrintingTests, ansiColorsStringElided)
 TEST_F(ValuePrintingTests, ansiColorsPath)
 {
     Value v;
-    v.mkPath(state.rootPath(CanonPath("puppy")));
+    v.mkPath(state.rootPath(CanonPath("puppy")), state.mem);
 
     test(v, ANSI_GREEN "/puppy" ANSI_NORMAL, PrintOptions{.ansiColors = true});
 }

src/libexpr/eval.cc

@@ -81,20 +81,20 @@ static const char * makeImmutableString(std::string_view s)
     return t;
 }
 
-StringData & StringData::alloc(size_t size)
+StringData & StringData::alloc(EvalMemory & mem, size_t size)
 {
-    void * t = GC_MALLOC_ATOMIC(sizeof(StringData) + size + 1);
+    void * t = mem.allocBytes(sizeof(StringData) + size + 1);
     if (!t)
         throw std::bad_alloc();
     auto res = new (t) StringData(size);
     return *res;
 }
 
-const StringData & StringData::make(std::string_view s)
+const StringData & StringData::make(EvalMemory & mem, std::string_view s)
 {
     if (s.empty())
         return ""_sds;
-    auto & res = alloc(s.size());
+    auto & res = alloc(mem, s.size());
     std::memcpy(&res.data_, s.data(), s.size());
     res.data_[s.size()] = '\0';
     return res;
@@ -849,9 +849,9 @@ DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
         evalState.runDebugRepl(nullptr, trace.env, trace.expr);
 }
 
-void Value::mkString(std::string_view s)
+void Value::mkString(std::string_view s, EvalMemory & mem)
 {
-    mkStringNoCopy(StringData::make(s));
+    mkStringNoCopy(StringData::make(mem, s));
 }
 
 Value::StringWithContext::Context *
@@ -862,13 +862,13 @@ Value::StringWithContext::Context::fromBuilder(const NixStringContext & context,
 
     auto ctx = new (mem.allocBytes(sizeof(Context) + context.size() * sizeof(value_type))) Context(context.size());
     std::ranges::transform(
-        context, ctx->elems, [](const NixStringContextElem & elt) { return &StringData::make(elt.to_string()); });
+        context, ctx->elems, [&](const NixStringContextElem & elt) { return &StringData::make(mem, elt.to_string()); });
     return ctx;
 }
 
 void Value::mkString(std::string_view s, const NixStringContext & context, EvalMemory & mem)
 {
-    mkStringNoCopy(StringData::make(s), Value::StringWithContext::Context::fromBuilder(context, mem));
+    mkStringNoCopy(StringData::make(mem, s), Value::StringWithContext::Context::fromBuilder(context, mem));
 }
 
 void Value::mkStringMove(const StringData & s, const NixStringContext & context, EvalMemory & mem)
@@ -876,9 +876,9 @@ void Value::mkStringMove(const StringData & s, const NixStringContext & context,
     mkStringNoCopy(s, Value::StringWithContext::Context::fromBuilder(context, mem));
 }
 
-void Value::mkPath(const SourcePath & path)
+void Value::mkPath(const SourcePath & path, EvalMemory & mem)
 {
-    mkPath(&*path.accessor, StringData::make(path.path.abs()));
+    mkPath(&*path.accessor, StringData::make(mem, path.path.abs()));
 }
 
 inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
@@ -943,7 +943,7 @@ void EvalState::mkPos(Value & v, PosIdx p)
     auto origin = positions.originOf(p);
     if (auto path = std::get_if<SourcePath>(&origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(s.file).mkString(path->path.abs());
+        attrs.alloc(s.file).mkString(path->path.abs(), mem);
         makePositionThunks(*this, p, attrs.alloc(s.line), attrs.alloc(s.column));
         v.mkAttrs(attrs);
     } else
@@ -1751,9 +1751,9 @@ void ExprCall::eval(EvalState & state, Env & env, Value & v)
     // 4: about 60
     // 5: under 10
     // This excluded attrset lambdas (`{...}:`). Contributions of mixed lambdas appears insignificant at ~150 total.
-    SmallValueVector<4> vArgs(args.size());
-    for (size_t i = 0; i < args.size(); ++i)
-        vArgs[i] = args[i]->maybeThunk(state, env);
+    SmallValueVector<4> vArgs(args->size());
+    for (size_t i = 0; i < args->size(); ++i)
+        vArgs[i] = (*args)[i]->maybeThunk(state, env);
 
     state.callFunction(vFun, vArgs, v, pos);
 }
@@ -2139,9 +2139,9 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         for (const auto & part : strings) {
             resultStr += *part;
         }
-        v.mkPath(state.rootPath(CanonPath(resultStr)));
+        v.mkPath(state.rootPath(CanonPath(resultStr)), state.mem);
     } else {
-        auto & resultStr = StringData::alloc(sSize);
+        auto & resultStr = StringData::alloc(state.mem, sSize);
         auto * tmp = resultStr.data();
         for (const auto & part : strings) {
             std::memcpy(tmp, part->data(), part->size());
@@ -2188,6 +2188,8 @@ void EvalState::forceValueDeep(Value & v)
     std::set<const Value *> seen;
 
     [&, &state(*this)](this const auto & recurse, Value & v) {
+        auto _level = state.addCallDepth(v.determinePos(noPos));
+
         if (!seen.insert(&v).second)
             return;
 
@@ -2214,8 +2216,15 @@ void EvalState::forceValueDeep(Value & v)
         }
 
         else if (v.isList()) {
+            size_t index = 0;
             for (auto v2 : v.listView())
-                recurse(*v2);
+                try {
+                    recurse(*v2);
+                    index++;
+                } catch (Error & e) {
+                    state.addErrorTrace(e, "while evaluating list element at index %1%", index);
+                    throw;
+                }
         }
     }(v);
 }

src/libexpr/include/nix/expr/nixexpr.hh

@@ -592,11 +592,14 @@ public:
 struct ExprCall : Expr
 {
     Expr * fun;
-    std::vector<Expr *> args;
+    /**
+     * args will never be null. See comment on ExprAttrs::AttrDefs below.
+     */
+    std::optional<std::pmr::vector<Expr *>> args;
     PosIdx pos;
     std::optional<PosIdx> cursedOrEndPos; // used during parsing to warn about https://github.com/NixOS/nix/issues/11118
 
-    ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args)
+    ExprCall(const PosIdx & pos, Expr * fun, std::pmr::vector<Expr *> && args)
         : fun(fun)
         , args(args)
         , pos(pos)
@@ -604,7 +607,7 @@ struct ExprCall : Expr
     {
     }
 
-    ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args, PosIdx && cursedOrEndPos)
+    ExprCall(const PosIdx & pos, Expr * fun, std::pmr::vector<Expr *> && args, PosIdx && cursedOrEndPos)
         : fun(fun)
         , args(args)
         , pos(pos)
@@ -836,7 +839,7 @@ public:
     // we define some calls to add explicitly so that the argument can be passed in as initializer lists
     template<class C>
     [[gnu::always_inline]]
-    C * add(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args)
+    C * add(const PosIdx & pos, Expr * fun, std::pmr::vector<Expr *> && args)
         requires(std::same_as<C, ExprCall>)
     {
         return alloc.new_object<C>(pos, fun, std::move(args));
@@ -844,7 +847,7 @@ public:
 
     template<class C>
     [[gnu::always_inline]]
-    C * add(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args, PosIdx && cursedOrEndPos)
+    C * add(const PosIdx & pos, Expr * fun, std::pmr::vector<Expr *> && args, PosIdx && cursedOrEndPos)
         requires(std::same_as<C, ExprCall>)
     {
         return alloc.new_object<C>(pos, fun, std::move(args), std::move(cursedOrEndPos));

src/libexpr/include/nix/expr/value.hh

@@ -232,13 +232,13 @@ public:
      * Allocate StringData on the (possibly) GC-managed heap and copy
      * the contents of s to it.
      */
-    static const StringData & make(std::string_view s);
+    static const StringData & make(EvalMemory & mem, std::string_view s);
 
     /**
      * Allocate StringData on the (possibly) GC-managed heap.
      * @param size Length of the string (without the NUL terminator).
      */
-    static StringData & alloc(size_t size);
+    static StringData & alloc(EvalMemory & mem, size_t size);
 
     size_t size() const
     {
@@ -1147,13 +1147,13 @@ public:
         setStorage(StringWithContext{.str = &s, .context = context});
     }
 
-    void mkString(std::string_view s);
+    void mkString(std::string_view s, EvalMemory & mem);
 
     void mkString(std::string_view s, const NixStringContext & context, EvalMemory & mem);
 
     void mkStringMove(const StringData & s, const NixStringContext & context, EvalMemory & mem);
 
-    void mkPath(const SourcePath & path);
+    void mkPath(const SourcePath & path, EvalMemory & mem);
 
     inline void mkPath(SourceAccessor * accessor, const StringData & path) noexcept
     {

src/libexpr/json-to-value.cc

@@ -151,7 +151,7 @@ public:
     bool string(string_t & val) override
     {
         forceNoNullByte(val);
-        rs->value(state).mkString(val);
+        rs->value(state).mkString(val, state.mem);
         rs->add();
         return true;
     }

src/libexpr/nixexpr.cc

@@ -191,7 +191,7 @@ void ExprCall::show(const SymbolTable & symbols, std::ostream & str) const
 {
     str << '(';
     fun->show(symbols, str);
-    for (auto e : args) {
+    for (auto e : *args) {
         str << ' ';
         e->show(symbols, str);
     }
@@ -486,11 +486,17 @@ void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
 
 void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    // Move storage into the Exprs arena
+    {
+        auto arena = es.mem.exprs.alloc;
+        std::pmr::vector<Expr *> newArgs{std::move(*args), arena};
+        args.emplace(std::move(newArgs), arena);
+    }
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, env));
 
     fun->bindVars(es, env);
-    for (auto e : args)
+    for (auto e : *args)
         e->bindVars(es, env);
 }
 

src/libexpr/parser.y

@@ -115,7 +115,7 @@ static void setDocPosition(const LexerState & lexerState, ExprLambda * lambda, P
 
 static Expr * makeCall(Exprs & exprs, PosIdx pos, Expr * fn, Expr * arg) {
     if (auto e2 = dynamic_cast<ExprCall *>(fn)) {
-        e2->args.push_back(arg);
+        e2->args->push_back(arg);
         return fn;
     }
     return exprs.add<ExprCall>(pos, fn, {arg});
@@ -129,7 +129,7 @@ static Expr * makeCall(Exprs & exprs, PosIdx pos, Expr * fn, Expr * arg) {
 %type <Expr *> start expr expr_function expr_if expr_op
 %type <Expr *> expr_select expr_simple expr_app
 %type <Expr *> expr_pipe_from expr_pipe_into
-%type <std::vector<Expr *>> list
+%type <std::pmr::vector<Expr *>> list
 %type <ExprAttrs *> binds binds1
 %type <FormalsBuilder> formals formal_set
 %type <Formal> formal

src/libexpr/primops.cc

@@ -53,7 +53,7 @@ RegisterPrimOp::PrimOps & RegisterPrimOp::primOps()
 static inline Value * mkString(EvalState & state, const std::csub_match & match)
 {
     Value * v = state.allocValue();
-    v->mkString({match.first, match.second});
+    v->mkString({match.first, match.second}, state.mem);
     return v;
 }
 
@@ -230,12 +230,12 @@ void derivationToValue(
                 NixStringContextElem::DrvDeep{.drvPath = storePath},
             },
             state.mem);
-    attrs.alloc(state.s.name).mkString(drv.env["name"]);
+    attrs.alloc(state.s.name).mkString(drv.env["name"], state.mem);
 
     auto list = state.buildList(drv.outputs.size());
     for (const auto & [i, o] : enumerate(drv.outputs)) {
         mkOutputString(state, attrs, storePath, o);
-        (list[i] = state.allocValue())->mkString(o.first);
+        (list[i] = state.allocValue())->mkString(o.first, state.mem);
     }
     attrs.alloc(state.s.outputs).mkList(list);
 
@@ -519,7 +519,7 @@ static void prim_typeOf(EvalState & state, const PosIdx pos, Value ** args, Valu
         v.mkStringNoCopy("lambda"_sds);
         break;
     case nExternal:
-        v.mkString(args[0]->external()->typeOf());
+        v.mkString(args[0]->external()->typeOf(), state.mem);
         break;
     case nFloat:
         v.mkStringNoCopy("float"_sds);
@@ -1176,7 +1176,7 @@ static void prim_getEnv(EvalState & state, const PosIdx pos, Value ** args, Valu
 {
     std::string name(
         state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.getEnv"));
-    v.mkString(state.settings.restrictEval || state.settings.pureEval ? "" : getEnv(name).value_or(""));
+    v.mkString(state.settings.restrictEval || state.settings.pureEval ? "" : getEnv(name).value_or(""), state.mem);
 }
 
 static RegisterPrimOp primop_getEnv({
@@ -1842,8 +1842,10 @@ static RegisterPrimOp primop_derivationStrict(
    ‘out’. */
 static void prim_placeholder(EvalState & state, const PosIdx pos, Value ** args, Value & v)
 {
-    v.mkString(hashPlaceholder(
-        state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.placeholder")));
+    v.mkString(
+        hashPlaceholder(state.forceStringNoCtx(
+            *args[0], pos, "while evaluating the first argument passed to builtins.placeholder")),
+        state.mem);
 }
 
 static RegisterPrimOp primop_placeholder({
@@ -2027,7 +2029,7 @@ static void prim_dirOf(EvalState & state, const PosIdx pos, Value ** args, Value
     state.forceValue(*args[0], pos);
     if (args[0]->type() == nPath) {
         auto path = args[0]->path();
-        v.mkPath(path.path.isRoot() ? path : path.parent());
+        v.mkPath(path.path.isRoot() ? path : path.parent(), state.mem);
     } else {
         NixStringContext context;
         auto path = state.coerceToString(
@@ -2144,7 +2146,7 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value ** args, Va
     auto path =
         state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.findFile");
 
-    v.mkPath(state.findFile(lookupPath, path, pos));
+    v.mkPath(state.findFile(lookupPath, path, pos), state.mem);
 }
 
 static RegisterPrimOp primop_findFile(
@@ -2293,7 +2295,7 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value ** args, Va
 
     auto path = realisePath(state, pos, *args[1]);
 
-    v.mkString(hashString(*ha, path.readFile()).to_string(HashFormat::Base16, false));
+    v.mkString(hashString(*ha, path.readFile()).to_string(HashFormat::Base16, false), state.mem);
 }
 
 static RegisterPrimOp primop_hashFile({
@@ -2382,7 +2384,7 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value ** args, Val
             // detailed node info quickly in this case we produce a thunk to
             // query the file type lazily.
             auto epath = state.allocValue();
-            epath->mkPath(path / name);
+            epath->mkPath(path / name, state.mem);
             if (!readFileType)
                 readFileType = &state.getBuiltin("readFileType");
             attr.mkApp(readFileType, epath);
@@ -2763,7 +2765,7 @@ bool EvalState::callPathFilter(Value * filterFun, const SourcePath & path, PosId
     /* Call the filter function.  The first argument is the path, the
        second is a string indicating the type of the file. */
     Value arg1;
-    arg1.mkString(path.path.abs());
+    arg1.mkString(path.path.abs(), mem);
 
     // assert that type is not "unknown"
     Value * args[]{&arg1, const_cast<Value *>(&fileTypeToString(*this, st.type))};
@@ -4541,7 +4543,7 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value ** args,
     auto s =
         state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString");
 
-    v.mkString(hashString(*ha, s).to_string(HashFormat::Base16, false));
+    v.mkString(hashString(*ha, s).to_string(HashFormat::Base16, false), state.mem);
 }
 
 static RegisterPrimOp primop_hashString({
@@ -4574,7 +4576,7 @@ static void prim_convertHash(EvalState & state, const PosIdx pos, Value ** args,
     HashFormat hf = parseHashFormat(
         state.forceStringNoCtx(*iteratorToHashFormat->value, pos, "while evaluating the attribute 'toHashFormat'"));
 
-    v.mkString(Hash::parseAny(hash, ha).to_string(hf, hf == HashFormat::SRI));
+    v.mkString(Hash::parseAny(hash, ha).to_string(hf, hf == HashFormat::SRI), state.mem);
 }
 
 static RegisterPrimOp primop_convertHash({
@@ -4992,8 +4994,8 @@ static void prim_parseDrvName(EvalState & state, const PosIdx pos, Value ** args
         state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.parseDrvName");
     DrvName parsed(name);
     auto attrs = state.buildBindings(2);
-    attrs.alloc(state.s.name).mkString(parsed.name);
-    attrs.alloc("version").mkString(parsed.version);
+    attrs.alloc(state.s.name).mkString(parsed.name, state.mem);
+    attrs.alloc("version").mkString(parsed.version, state.mem);
     v.mkAttrs(attrs);
 }
 
@@ -5048,7 +5050,7 @@ static void prim_splitVersion(EvalState & state, const PosIdx pos, Value ** args
     }
     auto list = state.buildList(components.size());
     for (const auto & [n, component] : enumerate(components))
-        (list[n] = state.allocValue())->mkString(std::move(component));
+        (list[n] = state.allocValue())->mkString(std::move(component), state.mem);
     v.mkList(list);
 }
 
@@ -5192,7 +5194,7 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
         });
 
     if (!settings.pureEval)
-        v.mkString(settings.getCurrentSystem());
+        v.mkString(settings.getCurrentSystem(), mem);
     addConstant(
         "__currentSystem",
         v,
@@ -5224,7 +5226,7 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
             .impureOnly = true,
         });
 
-    v.mkString(nixVersion);
+    v.mkString(nixVersion, mem);
     addConstant(
         "__nixVersion",
         v,
@@ -5249,7 +5251,7 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
         )",
         });
 
-    v.mkString(store->storeDir);
+    v.mkString(store->storeDir, mem);
     addConstant(
         "__storeDir",
         v,
@@ -5314,8 +5316,8 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
     auto list = buildList(lookupPath.elements.size());
     for (const auto & [n, i] : enumerate(lookupPath.elements)) {
         auto attrs = buildBindings(2);
-        attrs.alloc("path").mkString(i.path.s);
-        attrs.alloc("prefix").mkString(i.prefix.s);
+        attrs.alloc("path").mkString(i.path.s, mem);
+        attrs.alloc("prefix").mkString(i.prefix.s, mem);
         (list[n] = allocValue())->mkAttrs(attrs);
     }
     v.mkList(list);

src/libexpr/primops/context.cc

@@ -11,7 +11,7 @@ static void prim_unsafeDiscardStringContext(EvalState & state, const PosIdx pos,
     NixStringContext context;
     auto s = state.coerceToString(
         pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardStringContext");
-    v.mkString(*s);
+    v.mkString(*s, state.mem);
 }
 
 static RegisterPrimOp primop_unsafeDiscardStringContext({
@@ -218,7 +218,7 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value ** args,
         if (!info.second.outputs.empty()) {
             auto list = state.buildList(info.second.outputs.size());
             for (const auto & [i, output] : enumerate(info.second.outputs))
-                (list[i] = state.allocValue())->mkString(output);
+                (list[i] = state.allocValue())->mkString(output, state.mem);
             infoAttrs.alloc(state.s.outputs).mkList(list);
         }
         attrs.alloc(state.store->printStorePath(info.first)).mkAttrs(infoAttrs);

src/libexpr/primops/fetchMercurial.cc

@@ -86,12 +86,12 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value ** ar
     auto attrs2 = state.buildBindings(8);
     state.mkStorePathString(storePath, attrs2.alloc(state.s.outPath));
     if (input2.getRef())
-        attrs2.alloc("branch").mkString(*input2.getRef());
+        attrs2.alloc("branch").mkString(*input2.getRef(), state.mem);
     // Backward compatibility: set 'rev' to
     // 0000000000000000000000000000000000000000 for a dirty tree.
     auto rev2 = input2.getRev().value_or(Hash(HashAlgorithm::SHA1));
-    attrs2.alloc("rev").mkString(rev2.gitRev());
-    attrs2.alloc("shortRev").mkString(rev2.gitRev().substr(0, 12));
+    attrs2.alloc("rev").mkString(rev2.gitRev(), state.mem);
+    attrs2.alloc("shortRev").mkString(rev2.gitRev().substr(0, 12), state.mem);
     if (auto revCount = input2.getRevCount())
         attrs2.alloc("revCount").mkInt(*revCount);
     v.mkAttrs(attrs2);

src/libexpr/primops/fetchTree.cc

@@ -35,7 +35,7 @@ void emitTreeAttrs(
     // FIXME: support arbitrary input attributes.
 
     if (auto narHash = input.getNarHash())
-        attrs.alloc("narHash").mkString(narHash->to_string(HashFormat::SRI, true));
+        attrs.alloc("narHash").mkString(narHash->to_string(HashFormat::SRI, true), state.mem);
 
     if (input.getType() == "git")
         attrs.alloc("submodules").mkBool(fetchers::maybeGetBoolAttr(input.attrs, "submodules").value_or(false));
@@ -43,13 +43,13 @@ void emitTreeAttrs(
     if (!forceDirty) {
 
         if (auto rev = input.getRev()) {
-            attrs.alloc("rev").mkString(rev->gitRev());
-            attrs.alloc("shortRev").mkString(rev->gitShortRev());
+            attrs.alloc("rev").mkString(rev->gitRev(), state.mem);
+            attrs.alloc("shortRev").mkString(rev->gitShortRev(), state.mem);
         } else if (emptyRevFallback) {
             // Backwards compat for `builtins.fetchGit`: dirty repos return an empty sha1 as rev
             auto emptyHash = Hash(HashAlgorithm::SHA1);
-            attrs.alloc("rev").mkString(emptyHash.gitRev());
-            attrs.alloc("shortRev").mkString(emptyHash.gitShortRev());
+            attrs.alloc("rev").mkString(emptyHash.gitRev(), state.mem);
+            attrs.alloc("shortRev").mkString(emptyHash.gitShortRev(), state.mem);
         }
 
         if (auto revCount = input.getRevCount())
@@ -59,13 +59,14 @@ void emitTreeAttrs(
     }
 
     if (auto dirtyRev = fetchers::maybeGetStrAttr(input.attrs, "dirtyRev")) {
-        attrs.alloc("dirtyRev").mkString(*dirtyRev);
-        attrs.alloc("dirtyShortRev").mkString(*fetchers::maybeGetStrAttr(input.attrs, "dirtyShortRev"));
+        attrs.alloc("dirtyRev").mkString(*dirtyRev, state.mem);
+        attrs.alloc("dirtyShortRev").mkString(*fetchers::maybeGetStrAttr(input.attrs, "dirtyShortRev"), state.mem);
     }
 
     if (auto lastModified = input.getLastModified()) {
         attrs.alloc("lastModified").mkInt(*lastModified);
-        attrs.alloc("lastModifiedDate").mkString(fmt("%s", std::put_time(std::gmtime(&*lastModified), "%Y%m%d%H%M%S")));
+        attrs.alloc("lastModifiedDate")
+            .mkString(fmt("%s", std::put_time(std::gmtime(&*lastModified), "%Y%m%d%H%M%S")), state.mem);
     }
 
     v.mkAttrs(attrs);
@@ -150,11 +151,6 @@ static void fetchTree(
             attrs.emplace("exportIgnore", Explicit<bool>{true});
         }
 
-        // fetchTree should fetch git repos with shallow = true by default
-        if (type == "git" && !params.isFetchGit && !attrs.contains("shallow")) {
-            attrs.emplace("shallow", Explicit<bool>{true});
-        }
-
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
                 state.error<EvalError>("argument 'name' isn’t supported in call to '%s'", fetcher)

src/libexpr/primops/fromTOML.cc

@@ -126,7 +126,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value ** args, Va
         case toml::value_t::string: {
             auto s = toml::get<std::string_view>(t);
             forceNoNullByte(s);
-            v.mkString(s);
+            v.mkString(s, state.mem);
         } break;
         case toml::value_t::local_datetime:
         case toml::value_t::offset_datetime:
@@ -142,7 +142,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value ** args, Va
                 s << t;
                 auto str = s.view();
                 forceNoNullByte(str);
-                attrs.alloc("value").mkString(str);
+                attrs.alloc("value").mkString(str, state.mem);
                 v.mkAttrs(attrs);
             } else {
                 throw std::runtime_error("Dates and times are not supported");

src/libexpr/value-to-json.cc

@@ -16,6 +16,8 @@ json printValueAsJSON(
 {
     checkInterrupt();
 
+    auto _level = state.addCallDepth(pos);
+
     if (strict)
         state.forceValue(v, pos);
 

src/libfetchers-tests/git-utils.cc

@@ -92,7 +92,7 @@ TEST_F(GitUtilsTest, sink_basic)
     // sink->createHardlink("foo-1.1/links/foo-2", CanonPath("foo-1.1/hello"));
 
     auto result = repo->dereferenceSingletonDirectory(sink->flush());
-    auto accessor = repo->getAccessor(result, false, getRepoName());
+    auto accessor = repo->getAccessor(result, {}, getRepoName());
     auto entries = accessor->readDirectory(CanonPath::root);
     ASSERT_EQ(entries.size(), 5u);
     ASSERT_EQ(accessor->readFile(CanonPath("hello")), "hello world");

src/libfetchers/fetchers.cc

@@ -269,24 +269,10 @@ void Input::checkLocks(Input specified, Input & result)
         }
     }
 
-    if (auto prevLastModified = specified.getLastModified()) {
-        if (result.getLastModified() != prevLastModified)
-            throw Error(
-                "'lastModified' attribute mismatch in input '%s', expected %d, got %d",
-                result.to_string(),
-                *prevLastModified,
-                result.getLastModified().value_or(-1));
-    }
-
     if (auto prevRev = specified.getRev()) {
         if (result.getRev() != prevRev)
             throw Error("'rev' attribute mismatch in input '%s', expected %s", result.to_string(), prevRev->gitRev());
     }
-
-    if (auto prevRevCount = specified.getRevCount()) {
-        if (result.getRevCount() != prevRevCount)
-            throw Error("'revCount' attribute mismatch in input '%s', expected %d", result.to_string(), *prevRevCount);
-    }
 }
 
 std::pair<ref<SourceAccessor>, Input> Input::getAccessor(const Settings & settings, Store & store) const
@@ -495,9 +481,9 @@ void InputScheme::clone(
 
     Activity act(*logger, lvlTalkative, actUnknown, fmt("copying '%s' to %s...", input2.to_string(), destDir));
 
-    auto source = sinkToSource([&](Sink & sink) { accessor->dumpPath(CanonPath::root, sink); });
-
-    restorePath(destDir, *source);
+    RestoreSink sink(/*startFsync=*/false);
+    sink.dstPath = destDir;
+    copyRecursive(*accessor, CanonPath::root, sink, CanonPath::root);
 }
 
 std::optional<ExperimentalFeature> InputScheme::experimentalFeature() const

src/libfetchers/git-utils.cc

@@ -541,14 +541,15 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
     }
 
     /**
-     * A 'GitSourceAccessor' with no regard for export-ignore or any other transformations.
+     * A 'GitSourceAccessor' with no regard for export-ignore.
      */
-    ref<GitSourceAccessor> getRawAccessor(const Hash & rev, bool smudgeLfs = false);
+    ref<GitSourceAccessor> getRawAccessor(const Hash & rev, const GitAccessorOptions & options);
 
     ref<SourceAccessor>
-    getAccessor(const Hash & rev, bool exportIgnore, std::string displayPrefix, bool smudgeLfs = false) override;
+    getAccessor(const Hash & rev, const GitAccessorOptions & options, std::string displayPrefix) override;
 
-    ref<SourceAccessor> getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllowedError e) override;
+    ref<SourceAccessor>
+    getAccessor(const WorkdirInfo & wd, const GitAccessorOptions & options, MakeNotAllowedError e) override;
 
     ref<GitFileSystemObjectSink> getFileSystemObjectSink() override;
 
@@ -668,7 +669,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
     Hash treeHashToNarHash(const fetchers::Settings & settings, const Hash & treeHash) override
     {
-        auto accessor = getAccessor(treeHash, false, "");
+        auto accessor = getAccessor(treeHash, {}, "");
 
         fetchers::Cache::Key cacheKey{"treeHashToNarHash", {{"treeHash", treeHash.gitRev()}}};
 
@@ -716,15 +717,17 @@ struct GitSourceAccessor : SourceAccessor
         ref<GitRepoImpl> repo;
         Object root;
         std::optional<lfs::Fetch> lfsFetch = std::nullopt;
+        GitAccessorOptions options;
     };
 
     Sync<State> state_;
 
-    GitSourceAccessor(ref<GitRepoImpl> repo_, const Hash & rev, bool smudgeLfs)
+    GitSourceAccessor(ref<GitRepoImpl> repo_, const Hash & rev, const GitAccessorOptions & options)
         : state_{State{
               .repo = repo_,
               .root = peelToTreeOrBlob(lookupObject(*repo_, hashToOID(rev)).get()),
-              .lfsFetch = smudgeLfs ? std::make_optional(lfs::Fetch(*repo_, hashToOID(rev))) : std::nullopt,
+              .lfsFetch = options.smudgeLfs ? std::make_optional(lfs::Fetch(*repo_, hashToOID(rev))) : std::nullopt,
+              .options = options,
           }}
     {
     }
@@ -1254,26 +1257,26 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
     }
 };
 
-ref<GitSourceAccessor> GitRepoImpl::getRawAccessor(const Hash & rev, bool smudgeLfs)
+ref<GitSourceAccessor> GitRepoImpl::getRawAccessor(const Hash & rev, const GitAccessorOptions & options)
 {
     auto self = ref<GitRepoImpl>(shared_from_this());
-    return make_ref<GitSourceAccessor>(self, rev, smudgeLfs);
+    return make_ref<GitSourceAccessor>(self, rev, options);
 }
 
 ref<SourceAccessor>
-GitRepoImpl::getAccessor(const Hash & rev, bool exportIgnore, std::string displayPrefix, bool smudgeLfs)
+GitRepoImpl::getAccessor(const Hash & rev, const GitAccessorOptions & options, std::string displayPrefix)
 {
     auto self = ref<GitRepoImpl>(shared_from_this());
-    ref<GitSourceAccessor> rawGitAccessor = getRawAccessor(rev, smudgeLfs);
+    ref<GitSourceAccessor> rawGitAccessor = getRawAccessor(rev, options);
     rawGitAccessor->setPathDisplay(std::move(displayPrefix));
-    if (exportIgnore)
+    if (options.exportIgnore)
         return make_ref<GitExportIgnoreSourceAccessor>(self, rawGitAccessor, rev);
     else
         return rawGitAccessor;
 }
 
-ref<SourceAccessor>
-GitRepoImpl::getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllowedError makeNotAllowedError)
+ref<SourceAccessor> GitRepoImpl::getAccessor(
+    const WorkdirInfo & wd, const GitAccessorOptions & options, MakeNotAllowedError makeNotAllowedError)
 {
     auto self = ref<GitRepoImpl>(shared_from_this());
     ref<SourceAccessor> fileAccessor = AllowListSourceAccessor::create(
@@ -1283,10 +1286,9 @@ GitRepoImpl::getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllow
                                            boost::unordered_flat_set<CanonPath>{CanonPath::root},
                                            std::move(makeNotAllowedError))
                                            .cast<SourceAccessor>();
-    if (exportIgnore)
-        return make_ref<GitExportIgnoreSourceAccessor>(self, fileAccessor, std::nullopt);
-    else
-        return fileAccessor;
+    if (options.exportIgnore)
+        fileAccessor = make_ref<GitExportIgnoreSourceAccessor>(self, fileAccessor, std::nullopt);
+    return fileAccessor;
 }
 
 ref<GitFileSystemObjectSink> GitRepoImpl::getFileSystemObjectSink()
@@ -1299,7 +1301,7 @@ std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules
     /* Read the .gitmodules files from this revision. */
     CanonPath modulesFile(".gitmodules");
 
-    auto accessor = getAccessor(rev, exportIgnore, "");
+    auto accessor = getAccessor(rev, {.exportIgnore = exportIgnore}, "");
     if (!accessor->pathExists(modulesFile))
         return {};
 
@@ -1316,7 +1318,7 @@ std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules
 
     std::vector<std::tuple<Submodule, Hash>> result;
 
-    auto rawAccessor = getRawAccessor(rev);
+    auto rawAccessor = getRawAccessor(rev, {});
 
     for (auto & submodule : parseSubmodules(pathTemp)) {
         /* Filter out .gitmodules entries that don't exist or are not
@@ -1332,10 +1334,8 @@ namespace fetchers {
 
 ref<GitRepo> Settings::getTarballCache() const
 {
-    auto tarballCache(_tarballCache.lock());
-    if (!*tarballCache)
-        *tarballCache = GitRepo::openRepo(std::filesystem::path(getCacheDir()) / "tarball-cache", true, true);
-    return ref<GitRepo>(*tarballCache);
+    static auto repoDir = std::filesystem::path(getCacheDir()) / "tarball-cache";
+    return GitRepo::openRepo(repoDir, true, true);
 }
 
 } // namespace fetchers

src/libfetchers/git.cc

@@ -778,6 +778,16 @@ struct GitInputScheme : InputScheme
         }
     }
 
+    /**
+     * Decide whether we can do a shallow clone, which is faster. This is possible if the user explicitly specified
+     * `shallow = true`, or if we already have a `revCount`.
+     */
+    bool canDoShallow(const Input & input) const
+    {
+        bool shallow = getShallowAttr(input);
+        return shallow || input.getRevCount().has_value();
+    }
+
     std::pair<ref<SourceAccessor>, Input>
     getAccessorFromCommit(const Settings & settings, Store & store, RepoInfo & repoInfo, Input && input) const
     {
@@ -786,7 +796,7 @@ struct GitInputScheme : InputScheme
         auto origRev = input.getRev();
 
         auto originalRef = input.getRef();
-        bool shallow = getShallowAttr(input);
+        bool shallow = canDoShallow(input);
         auto ref = originalRef ? *originalRef : getDefaultRef(repoInfo, shallow);
         input.attrs.insert_or_assign("ref", ref);
 
@@ -797,11 +807,27 @@ struct GitInputScheme : InputScheme
             if (!input.getRev())
                 input.attrs.insert_or_assign("rev", GitRepo::openRepo(repoDir)->resolveRef(ref).gitRev());
         } else {
+            auto rev = input.getRev();
             auto repoUrl = std::get<ParsedURL>(repoInfo.location);
             std::filesystem::path cacheDir = getCachePath(repoUrl.to_string(), shallow);
             repoDir = cacheDir;
             repoInfo.gitDir = ".";
 
+            /* If shallow = false, but we have a non-shallow repo that already contains the desired rev, then use that
+             * repo instead. */
+            std::filesystem::path cacheDirNonShallow = getCachePath(repoUrl.to_string(), false);
+            if (rev && shallow && pathExists(cacheDirNonShallow)) {
+                auto nonShallowRepo = GitRepo::openRepo(cacheDirNonShallow, true, true);
+                if (nonShallowRepo->hasObject(*rev)) {
+                    debug(
+                        "using non-shallow cached repo for '%s' since it contains rev '%s'",
+                        repoUrl.to_string(),
+                        rev->gitRev());
+                    repoDir = cacheDirNonShallow;
+                    goto have_rev;
+                }
+            }
+
             std::filesystem::create_directories(cacheDir.parent_path());
             PathLocks cacheDirLock({cacheDir.string()});
 
@@ -817,7 +843,7 @@ struct GitInputScheme : InputScheme
 
             /* If a rev was specified, we need to fetch if it's not in the
                repo. */
-            if (auto rev = input.getRev()) {
+            if (rev) {
                 doFetch = !repo->hasObject(*rev);
             } else {
                 if (getAllRefsAttr(input)) {
@@ -831,7 +857,6 @@ struct GitInputScheme : InputScheme
             }
 
             if (doFetch) {
-                bool shallow = getShallowAttr(input);
                 try {
                     auto fetchRef = getAllRefsAttr(input)             ? "refs/*:refs/*"
                                     : input.getRev()                  ? input.getRev()->gitRev()
@@ -859,7 +884,7 @@ struct GitInputScheme : InputScheme
                     warn("could not update cached head '%s' for '%s'", ref, repoInfo.locationToArg());
             }
 
-            if (auto rev = input.getRev()) {
+            if (rev) {
                 if (!repo->hasObject(*rev))
                     throw Error(
                         "Cannot find Git revision '%s' in ref '%s' of repository '%s'! "
@@ -876,23 +901,30 @@ struct GitInputScheme : InputScheme
             // the remainder
         }
 
+    have_rev:
         auto repo = GitRepo::openRepo(repoDir);
 
-        auto isShallow = repo->isShallow();
-
-        if (isShallow && !getShallowAttr(input))
-            throw Error(
-                "'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified",
-                repoInfo.locationToArg());
-
         // FIXME: check whether rev is an ancestor of ref?
 
         auto rev = *input.getRev();
 
-        input.attrs.insert_or_assign("lastModified", getLastModified(settings, repoInfo, repoDir, rev));
+        /* Skip lastModified computation if it's already supplied by the caller.
+           We don't care if they specify an incorrect value; it doesn't
+           matter for security, unlike narHash. */
+        if (!input.attrs.contains("lastModified"))
+            input.attrs.insert_or_assign("lastModified", getLastModified(settings, repoInfo, repoDir, rev));
+
+        /* Like lastModified, skip revCount if supplied by the caller. */
+        if (!shallow && !input.attrs.contains("revCount")) {
+            auto isShallow = repo->isShallow();
+
+            if (isShallow && !shallow)
+                throw Error(
+                    "'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified",
+                    repoInfo.locationToArg());
 
-        if (!getShallowAttr(input))
             input.attrs.insert_or_assign("revCount", getRevCount(settings, repoInfo, repoDir, rev));
+        }
 
         printTalkative("using revision %s of repo '%s'", rev.gitRev(), repoInfo.locationToArg());
 
@@ -900,7 +932,8 @@ struct GitInputScheme : InputScheme
 
         bool exportIgnore = getExportIgnoreAttr(input);
         bool smudgeLfs = getLfsAttr(input);
-        auto accessor = repo->getAccessor(rev, exportIgnore, "«" + input.to_string() + "»", smudgeLfs);
+        auto accessor = repo->getAccessor(
+            rev, {.exportIgnore = exportIgnore, .smudgeLfs = smudgeLfs}, "«" + input.to_string() + "»");
 
         /* If the repo has submodules, fetch them and return a mounted
            input accessor consisting of the accessor for the top-level
@@ -967,7 +1000,7 @@ struct GitInputScheme : InputScheme
         auto exportIgnore = getExportIgnoreAttr(input);
 
         ref<SourceAccessor> accessor =
-            repo->getAccessor(repoInfo.workdirInfo, exportIgnore, makeNotAllowedError(repoPath));
+            repo->getAccessor(repoInfo.workdirInfo, {.exportIgnore = exportIgnore}, makeNotAllowedError(repoPath));
 
         /* If the repo has submodules, return a mounted input accessor
            consisting of the accessor for the top-level repo and the

src/libfetchers/github.cc

@@ -351,7 +351,7 @@ struct GitArchiveInputScheme : InputScheme
         input.attrs.insert_or_assign("lastModified", uint64_t(tarballInfo.lastModified));
 
         auto accessor =
-            settings.getTarballCache()->getAccessor(tarballInfo.treeHash, false, "«" + input.to_string() + "»");
+            settings.getTarballCache()->getAccessor(tarballInfo.treeHash, {}, "«" + input.to_string() + "»");
 
         return {accessor, input};
     }

src/libfetchers/include/nix/fetchers/fetch-settings.hh

@@ -135,8 +135,6 @@ struct Settings : public Config
 
 private:
     mutable Sync<std::shared_ptr<Cache>> _cache;
-
-    mutable Sync<std::shared_ptr<GitRepo>> _tarballCache;
 };
 
 } // namespace nix::fetchers

src/libfetchers/include/nix/fetchers/git-utils.hh

@@ -22,6 +22,12 @@ struct GitFileSystemObjectSink : ExtendedFileSystemObjectSink
     virtual Hash flush() = 0;
 };
 
+struct GitAccessorOptions
+{
+    bool exportIgnore = false;
+    bool smudgeLfs = false;
+};
+
 struct GitRepo
 {
     virtual ~GitRepo() {}
@@ -89,10 +95,10 @@ struct GitRepo
     virtual bool hasObject(const Hash & oid) = 0;
 
     virtual ref<SourceAccessor>
-    getAccessor(const Hash & rev, bool exportIgnore, std::string displayPrefix, bool smudgeLfs = false) = 0;
+    getAccessor(const Hash & rev, const GitAccessorOptions & options, std::string displayPrefix) = 0;
 
-    virtual ref<SourceAccessor>
-    getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllowedError makeNotAllowedError) = 0;
+    virtual ref<SourceAccessor> getAccessor(
+        const WorkdirInfo & wd, const GitAccessorOptions & options, MakeNotAllowedError makeNotAllowedError) = 0;
 
     virtual ref<GitFileSystemObjectSink> getFileSystemObjectSink() = 0;
 

src/libfetchers/tarball.cc

@@ -136,7 +136,7 @@ static DownloadTarballResult downloadTarball_(
             .treeHash = treeHash,
             .lastModified = (time_t) getIntAttr(infoAttrs, "lastModified"),
             .immutableUrl = maybeGetStrAttr(infoAttrs, "immutableUrl"),
-            .accessor = settings.getTarballCache()->getAccessor(treeHash, false, displayPrefix),
+            .accessor = settings.getTarballCache()->getAccessor(treeHash, {}, displayPrefix),
         };
     };
 

src/libflake-c/nix_api_flake.cc

@@ -200,7 +200,7 @@ nix_value * nix_locked_flake_get_output_attrs(
     nix_clear_err(context);
     try {
         auto v = nix_alloc_value(context, evalState);
-        nix::flake::callFlake(evalState->state, *lockedFlake->lockedFlake, v->value);
+        nix::flake::callFlake(evalState->state, *lockedFlake->lockedFlake, *v->value);
         return v;
     }
     NIXC_CATCH_ERRS_NULL

src/libflake/flake-primops.cc

@@ -93,7 +93,7 @@ static void prim_parseFlakeRef(EvalState & state, const PosIdx pos, Value ** arg
         auto & vv = binds.alloc(s);
         std::visit(
             overloaded{
-                [&vv](const std::string & value) { vv.mkString(value); },
+                [&vv, &state](const std::string & value) { vv.mkString(value, state.mem); },
                 [&vv](const uint64_t & value) { vv.mkInt(value); },
                 [&vv](const Explicit<bool> & value) { vv.mkBool(value.t); }},
             value);
@@ -156,7 +156,7 @@ static void prim_flakeRefToString(EvalState & state, const PosIdx pos, Value **
         }
     }
     auto flakeRef = FlakeRef::fromAttrs(state.fetchSettings, attrs);
-    v.mkString(flakeRef.to_string());
+    v.mkString(flakeRef.to_string(), state.mem);
 }
 
 nix::PrimOp flakeRefToString({

src/libflake/flake.cc

@@ -956,7 +956,7 @@ void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & vRes)
         auto key = keyMap.find(node);
         assert(key != keyMap.end());
 
-        override.alloc(state.symbols.create("dir")).mkString(CanonPath(subdir).rel());
+        override.alloc(state.symbols.create("dir")).mkString(CanonPath(subdir).rel(), state.mem);
 
         overrides.alloc(state.symbols.create(key->second)).mkAttrs(override);
     }
@@ -966,7 +966,7 @@ void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & vRes)
     Value * vCallFlake = requireInternalFile(state, CanonPath("call-flake.nix"));
 
     auto vLocks = state.allocValue();
-    vLocks->mkString(lockFileStr);
+    vLocks->mkString(lockFileStr, state.mem);
 
     auto vFetchFinalTree = get(state.internalPrimOps, "fetchFinalTree");
     assert(vFetchFinalTree);

src/libstore-tests/references.cc

@@ -99,7 +99,7 @@ TEST(references, scanForReferencesDeep)
     // Create an in-memory file system with various reference patterns
     auto accessor = make_ref<MemorySourceAccessor>();
     accessor->root = File::Directory{
-        .contents{
+        .entries{
             {
                 // file1.txt: contains hash1
                 "file1.txt",
@@ -125,7 +125,7 @@ TEST(references, scanForReferencesDeep)
                 // subdir: a subdirectory
                 "subdir",
                 File::Directory{
-                    .contents{
+                    .entries{
                         {
                             // subdir/file4.txt: contains hash1 again
                             "file4.txt",

src/libstore/binary-cache-store.cc

@@ -8,7 +8,7 @@
 #include "nix/util/sync.hh"
 #include "nix/store/remote-fs-accessor.hh"
 #include "nix/store/nar-info-disk-cache.hh"
-#include "nix/store/nar-accessor.hh"
+#include "nix/util/nar-accessor.hh"
 #include "nix/util/thread-pool.hh"
 #include "nix/util/callback.hh"
 #include "nix/util/signals.hh"
@@ -208,7 +208,7 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
     if (config.writeNARListing) {
         nlohmann::json j = {
             {"version", 1},
-            {"root", listNar(ref<SourceAccessor>(narAccessor), CanonPath::root, true)},
+            {"root", listNarDeep(*narAccessor, CanonPath::root)},
         };
 
         upsertFile(std::string(info.path.hashPart()) + ".ls", j.dump(), "application/json");

src/libstore/filetransfer.cc

@@ -48,7 +48,7 @@ struct curlFileTransfer : public FileTransfer
     std::random_device rd;
     std::mt19937 mt19937;
 
-    struct TransferItem : public std::enable_shared_from_this<TransferItem>
+    struct TransferItem : public std::enable_shared_from_this<TransferItem>, public FileTransfer::Item
     {
         curlFileTransfer & fileTransfer;
         FileTransferRequest request;
@@ -60,6 +60,7 @@ struct curlFileTransfer : public FileTransfer
         // buffer to accompany the `req` above
         char errbuf[CURL_ERROR_SIZE];
         bool active = false; // whether the handle has been added to the multi object
+        bool paused = false; // whether the request has been paused previously
         std::string statusMsg;
 
         unsigned int attempt = 0;
@@ -116,7 +117,13 @@ struct curlFileTransfer : public FileTransfer
                        successful response. */
                     if (successfulStatuses.count(httpStatus)) {
                         writtenToSink += data.size();
-                        this->request.dataCallback(data);
+                        PauseTransfer needsPause = this->request.dataCallback(data);
+                        if (needsPause == PauseTransfer::Yes) {
+                            /* Smuggle the boolean flag into writeCallback. Note that
+                               the finalSink might get called multiple times if there's
+                               decompression going on. */
+                            paused = true;
+                        }
                     }
                 } else
                     this->result.data.append(data);
@@ -195,6 +202,14 @@ struct curlFileTransfer : public FileTransfer
             }
 
             (*decompressionSink)({(char *) contents, realSize});
+            if (paused) {
+                /* The callback has signaled that the transfer needs to be
+                   paused. Already consumed data won't be returned twice unlike
+                   when returning CURL_WRITEFUNC_PAUSE.
+                   https://curl-library.cool.haxx.narkive.com/larE1cRA/curl-easy-pause-documentation-question
+                   */
+                curl_easy_pause(req, CURLPAUSE_RECV);
+            }
 
             return realSize;
         } catch (...) {
@@ -364,6 +379,15 @@ struct curlFileTransfer : public FileTransfer
             return ((TransferItem *) clientp)->seekCallback(offset, origin);
         }
 
+        void unpause()
+        {
+            /* Unpausing an already unpaused transfer is a no-op. */
+            if (paused) {
+                curl_easy_pause(req, CURLPAUSE_CONT);
+                paused = false;
+            }
+        }
+
         void init()
         {
             if (!req)
@@ -624,7 +648,7 @@ struct curlFileTransfer : public FileTransfer
                     errorSink.reset();
                     embargo = std::chrono::steady_clock::now() + std::chrono::milliseconds(ms);
                     try {
-                        fileTransfer.enqueueItem(shared_from_this());
+                        fileTransfer.enqueueItem(ref{shared_from_this()});
                     } catch (const nix::Error & e) {
                         // If enqueue fails (e.g., during shutdown), fail the transfer properly
                         // instead of letting the exception propagate, which would leave done=false
@@ -641,24 +665,24 @@ struct curlFileTransfer : public FileTransfer
     {
         struct EmbargoComparator
         {
-            bool operator()(const std::shared_ptr<TransferItem> & i1, const std::shared_ptr<TransferItem> & i2)
+            bool operator()(const ref<TransferItem> & i1, const ref<TransferItem> & i2)
             {
                 return i1->embargo > i2->embargo;
             }
         };
 
-        std::
-            priority_queue<std::shared_ptr<TransferItem>, std::vector<std::shared_ptr<TransferItem>>, EmbargoComparator>
-                incoming;
+        std::priority_queue<ref<TransferItem>, std::vector<ref<TransferItem>>, EmbargoComparator> incoming;
+        std::vector<ref<TransferItem>> unpause;
     private:
         bool quitting = false;
     public:
         void quit()
         {
             quitting = true;
-            /* We wil not be processing any more incoming requests */
+            /* We will not be processing any more incoming requests */
             while (!incoming.empty())
                 incoming.pop();
+            unpause.clear();
         }
 
         bool isQuitting()
@@ -827,6 +851,17 @@ struct curlFileTransfer : public FileTransfer
                 item->active = true;
                 items[item->req] = item;
             }
+
+            /* NOTE: Unpausing may invoke callbacks to flush all buffers. */
+            auto unpause = [&]() {
+                auto state(state_.lock());
+                auto res = state->unpause;
+                state->unpause.clear();
+                return res;
+            }();
+
+            for (auto & item : unpause)
+                item->unpause();
         }
 
         debug("download thread shutting down");
@@ -851,7 +886,7 @@ struct curlFileTransfer : public FileTransfer
         }
     }
 
-    void enqueueItem(std::shared_ptr<TransferItem> item)
+    ItemHandle enqueueItem(ref<TransferItem> item)
     {
         if (item->request.data && item->request.uri.scheme() != "http" && item->request.uri.scheme() != "https"
             && item->request.uri.scheme() != "s3")
@@ -866,19 +901,34 @@ struct curlFileTransfer : public FileTransfer
 #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         writeFull(wakeupPipe.writeSide.get(), " ");
 #endif
+
+        return ItemHandle(static_cast<Item &>(*item));
     }
 
-    void enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) override
+    ItemHandle enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) override
     {
         /* Handle s3:// URIs by converting to HTTPS and optionally adding auth */
         if (request.uri.scheme() == "s3") {
             auto modifiedRequest = request;
             modifiedRequest.setupForS3();
-            enqueueItem(std::make_shared<TransferItem>(*this, std::move(modifiedRequest), std::move(callback)));
-            return;
+            return enqueueItem(make_ref<TransferItem>(*this, std::move(modifiedRequest), std::move(callback)));
         }
 
-        enqueueItem(std::make_shared<TransferItem>(*this, request, std::move(callback)));
+        return enqueueItem(make_ref<TransferItem>(*this, request, std::move(callback)));
+    }
+
+    void unpauseTransfer(ref<TransferItem> item)
+    {
+        auto state(state_.lock());
+        state->unpause.push_back(std::move(item));
+#ifndef _WIN32 // TODO need graceful async exit support on Windows?
+        writeFull(wakeupPipe.writeSide.get(), " ");
+#endif
+    }
+
+    void unpauseTransfer(ItemHandle handle) override
+    {
+        unpauseTransfer(ref{static_cast<TransferItem &>(handle.item.get()).shared_from_this()});
     }
 };
 
@@ -975,6 +1025,7 @@ void FileTransfer::download(
     struct State
     {
         bool quit = false;
+        bool paused = false;
         std::exception_ptr exc;
         std::string data;
         std::condition_variable avail, request;
@@ -990,31 +1041,38 @@ void FileTransfer::download(
         state->request.notify_one();
     });
 
-    request.dataCallback = [_state](std::string_view data) {
+    request.dataCallback = [_state, uri = request.uri.to_string()](std::string_view data) -> PauseTransfer {
         auto state(_state->lock());
 
         if (state->quit)
-            return;
-
-        /* If the buffer is full, then go to sleep until the calling
-           thread wakes us up (i.e. when it has removed data from the
-           buffer). We don't wait forever to prevent stalling the
-           download thread. (Hopefully sleeping will throttle the
-           sender.) */
-        if (state->data.size() > fileTransferSettings.downloadBufferSize) {
-            debug("download buffer is full; going to sleep");
-            static bool haveWarned = false;
-            warnOnce(haveWarned, "download buffer is full; consider increasing the 'download-buffer-size' setting");
-            state.wait_for(state->request, std::chrono::seconds(10));
-        }
+            return PauseTransfer::No;
 
         /* Append data to the buffer and wake up the calling
            thread. */
         state->data.append(data);
         state->avail.notify_one();
+
+        if (state->data.size() <= fileTransferSettings.downloadBufferSize)
+            return PauseTransfer::No;
+
+        /* dataCallback gets called multiple times by an intermediate sink. Only
+           issue the debug message the first time around. */
+        if (!state->paused)
+            debug(
+                "pausing transfer for '%s': download buffer is full (%d > %d)",
+                uri,
+                state->data.size(),
+                fileTransferSettings.downloadBufferSize);
+
+        state->paused = true;
+
+        /* Technically the buffer might become larger than
+           downloadBufferSize, but with sinks there's no way to avoid
+           consuming data. */
+        return PauseTransfer::Yes;
     };
 
-    enqueueFileTransfer(
+    auto handle = enqueueFileTransfer(
         request, {[_state, resultCallback{std::move(resultCallback)}](std::future<FileTransferResult> fut) {
             auto state(_state->lock());
             state->quit = true;
@@ -1047,6 +1105,10 @@ void FileTransfer::download(
                     return;
                 }
 
+                if (state->paused) {
+                    unpauseTransfer(handle);
+                    state->paused = false;
+                }
                 state.wait(state->avail);
 
                 if (state->data.empty())

src/libstore/include/nix/store/filetransfer.hh

@@ -70,12 +70,12 @@ struct FileTransferSettings : Config
 
     Setting<size_t> downloadBufferSize{
         this,
-        64 * 1024 * 1024,
+        1 * 1024 * 1024,
         "download-buffer-size",
         R"(
           The size of Nix's internal download buffer in bytes during `curl` transfers. If data is
           not processed quickly enough to exceed the size of this buffer, downloads may stall.
-          The default is 67108864 (64 MiB).
+          The default is 1048576 (1 MiB).
         )"};
 };
 
@@ -105,6 +105,11 @@ struct UsernameAuth
     std::optional<std::string> password;
 };
 
+enum class PauseTransfer : bool {
+    No = false,
+    Yes = true,
+};
+
 struct FileTransferRequest
 {
     VerbatimURL uri;
@@ -136,7 +141,14 @@ struct FileTransferRequest
 
     std::optional<UploadData> data;
     std::string mimeType;
-    std::function<void(std::string_view data)> dataCallback;
+
+    /**
+     * Callbacked invoked with a chunk of received data.
+     * Can pause the transfer by returning PauseTransfer::Yes. No data must be consumed
+     * if transfer is paused.
+     */
+    std::function<PauseTransfer(std::string_view data)> dataCallback;
+
     /**
      * Optional username and password for HTTP basic authentication.
      * When provided, these credentials will be used with curl's CURLOPT_USERNAME/PASSWORD option.
@@ -226,6 +238,25 @@ class Store;
 
 struct FileTransfer
 {
+protected:
+    class Item
+    {};
+
+public:
+    /**
+     * An opaque handle to the file transfer. Can be used to reference an in-flight transfer operations.
+     */
+    struct ItemHandle
+    {
+        std::reference_wrapper<Item> item;
+        friend struct FileTransfer;
+
+        ItemHandle(Item & item)
+            : item(item)
+        {
+        }
+    };
+
     virtual ~FileTransfer() {}
 
     /**
@@ -233,7 +264,13 @@ struct FileTransfer
      * the download. The future may throw a FileTransferError
      * exception.
      */
-    virtual void enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) = 0;
+    virtual ItemHandle
+    enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) = 0;
+
+    /**
+     * Unpause a transfer that has been previously paused by a dataCallback.
+     */
+    virtual void unpauseTransfer(ItemHandle handle) = 0;
 
     std::future<FileTransferResult> enqueueFileTransfer(const FileTransferRequest & request);
 

src/libstore/include/nix/store/meson.build

@@ -55,7 +55,6 @@ headers = [ config_pub_h ] + files(
   'machines.hh',
   'make-content-addressed.hh',
   'names.hh',
-  'nar-accessor.hh',
   'nar-info-disk-cache.hh',
   'nar-info.hh',
   'outputs-spec.hh',

src/libstore/include/nix/store/nar-accessor.hh

@@ -1,43 +0,0 @@
-#pragma once
-///@file
-
-#include "nix/util/source-accessor.hh"
-
-#include <functional>
-
-#include <nlohmann/json_fwd.hpp>
-
-namespace nix {
-
-struct Source;
-
-/**
- * Return an object that provides access to the contents of a NAR
- * file.
- */
-ref<SourceAccessor> makeNarAccessor(std::string && nar);
-
-ref<SourceAccessor> makeNarAccessor(Source & source);
-
-/**
- * Create a NAR accessor from a NAR listing (in the format produced by
- * listNar()). The callback getNarBytes(offset, length) is used by the
- * readFile() method of the accessor to get the contents of files
- * inside the NAR.
- */
-using GetNarBytes = std::function<std::string(uint64_t, uint64_t)>;
-
-/**
- * The canonical GetNarBytes function for a seekable Source.
- */
-GetNarBytes seekableGetNarBytes(const Path & path);
-
-ref<SourceAccessor> makeLazyNarAccessor(const nlohmann::json & listing, GetNarBytes getNarBytes);
-
-/**
- * Write a JSON representation of the contents of a NAR (except file
- * contents).
- */
-nlohmann::json listNar(ref<SourceAccessor> accessor, const CanonPath & path, bool recurse);
-
-} // namespace nix

src/libstore/meson.build

@@ -114,6 +114,17 @@ boost = dependency(
 deps_other += boost
 
 curl = dependency('libcurl', 'curl', version : '>= 7.75.0')
+if curl.version().version_compare('>=8.16.0') and curl.version().version_compare(
+  '<8.17.0',
+)
+  # Out of precaution, avoid building with libcurl version that suffer from https://github.com/curl/curl/issues/19334.
+  error(
+    'curl @0@ has issues with write pausing, please use libcurl < 8.16 or >= 8.17, see https://github.com/curl/curl/issues/19334'.format(
+      curl.version(),
+    ),
+  )
+endif
+
 deps_private += curl
 
 # seccomp only makes sense on Linux
@@ -300,7 +311,6 @@ sources = files(
   'make-content-addressed.cc',
   'misc.cc',
   'names.cc',
-  'nar-accessor.cc',
   'nar-info-disk-cache.cc',
   'nar-info.cc',
   'optimise-store.cc',

src/libstore/remote-fs-accessor.cc

@@ -1,6 +1,6 @@
 #include <nlohmann/json.hpp>
 #include "nix/store/remote-fs-accessor.hh"
-#include "nix/store/nar-accessor.hh"
+#include "nix/util/nar-accessor.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -39,7 +39,7 @@ ref<SourceAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std:
 
     if (cacheDir != "") {
         try {
-            nlohmann::json j = listNar(narAccessor, CanonPath::root, true);
+            nlohmann::json j = listNarDeep(*narAccessor, CanonPath::root);
             writeFile(makeCacheFile(hashPart, "ls"), j.dump());
         } catch (...) {
             ignoreExceptionExceptInterrupt();

src/libutil-tests/data/memory-source-accessor/complex.json

@@ -0,0 +1,24 @@
+{
+  "entries": {
+    "bar": {
+      "entries": {
+        "baz": {
+          "contents": "good day,\n\u0000\n\tworld!",
+          "executable": true,
+          "type": "regular"
+        },
+        "quux": {
+          "target": "/over/there",
+          "type": "symlink"
+        }
+      },
+      "type": "directory"
+    },
+    "foo": {
+      "contents": "hello\n\u0000\n\tworld!",
+      "executable": false,
+      "type": "regular"
+    }
+  },
+  "type": "directory"
+}

src/libutil-tests/data/memory-source-accessor/simple.json

@@ -0,0 +1,5 @@
+{
+  "contents": "asdf",
+  "executable": false,
+  "type": "regular"
+}

src/libutil-tests/data/nar-listing/deep.json

@@ -0,0 +1,23 @@
+{
+  "entries": {
+    "bar": {
+      "entries": {
+        "baz": {
+          "executable": true,
+          "size": 19,
+          "type": "regular"
+        },
+        "quux": {
+          "target": "/over/there",
+          "type": "symlink"
+        }
+      },
+      "type": "directory"
+    },
+    "foo": {
+      "size": 15,
+      "type": "regular"
+    }
+  },
+  "type": "directory"
+}

src/libutil-tests/data/nar-listing/shallow.json

@@ -0,0 +1,7 @@
+{
+  "entries": {
+    "bar": {},
+    "foo": {}
+  },
+  "type": "directory"
+}

src/libutil-tests/git.cc

@@ -224,42 +224,15 @@ TEST_F(GitTest, tree_sha256_write)
     });
 }
 
+namespace memory_source_accessor {
+
+extern ref<MemorySourceAccessor> exampleComplex();
+
+}
+
 TEST_F(GitTest, both_roundrip)
 {
-    using File = MemorySourceAccessor::File;
-
-    auto files = make_ref<MemorySourceAccessor>();
-    files->root = File::Directory{
-        .contents{
-            {
-                "foo",
-                File::Regular{
-                    .contents = "hello\n\0\n\tworld!",
-                },
-            },
-            {
-                "bar",
-                File::Directory{
-                    .contents =
-                        {
-                            {
-                                "baz",
-                                File::Regular{
-                                    .executable = true,
-                                    .contents = "good day,\n\0\n\tworld!",
-                                },
-                            },
-                            {
-                                "quux",
-                                File::Symlink{
-                                    .target = "/over/there",
-                                },
-                            },
-                        },
-                },
-            },
-        },
-    };
+    auto files = memory_source_accessor::exampleComplex();
 
     for (const auto hashAlgo : {HashAlgorithm::SHA1, HashAlgorithm::SHA256}) {
         std::map<Hash, std::string> cas;

src/libutil-tests/memory-source-accessor.cc

@@ -0,0 +1,116 @@
+#include <string_view>
+
+#include "nix/util/memory-source-accessor.hh"
+#include "nix/util/tests/json-characterization.hh"
+
+namespace nix {
+
+namespace memory_source_accessor {
+
+using namespace std::literals;
+using File = MemorySourceAccessor::File;
+
+ref<MemorySourceAccessor> exampleSimple()
+{
+    auto sc = make_ref<MemorySourceAccessor>();
+    sc->root = File{File::Regular{
+        .executable = false,
+        .contents = "asdf",
+    }};
+    return sc;
+}
+
+ref<MemorySourceAccessor> exampleComplex()
+{
+    auto files = make_ref<MemorySourceAccessor>();
+    files->root = File::Directory{
+        .entries{
+            {
+                "foo",
+                File::Regular{
+                    .contents = "hello\n\0\n\tworld!"s,
+                },
+            },
+            {
+                "bar",
+                File::Directory{
+                    .entries =
+                        {
+                            {
+                                "baz",
+                                File::Regular{
+                                    .executable = true,
+                                    .contents = "good day,\n\0\n\tworld!"s,
+                                },
+                            },
+                            {
+                                "quux",
+                                File::Symlink{
+                                    .target = "/over/there",
+                                },
+                            },
+                        },
+                },
+            },
+        },
+    };
+    return files;
+}
+
+} // namespace memory_source_accessor
+
+/* ----------------------------------------------------------------------------
+ * JSON
+ * --------------------------------------------------------------------------*/
+
+class MemorySourceAccessorTest : public virtual CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "memory-source-accessor";
+
+public:
+
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / testStem;
+    }
+};
+
+using nlohmann::json;
+
+struct MemorySourceAccessorJsonTest : MemorySourceAccessorTest,
+                                      JsonCharacterizationTest<MemorySourceAccessor>,
+                                      ::testing::WithParamInterface<std::pair<std::string_view, MemorySourceAccessor>>
+{};
+
+TEST_P(MemorySourceAccessorJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    /* Cannot use `readJsonTest` because need to compare `root` field of
+       the source accessors for equality. */
+    readTest(Path{name} + ".json", [&](const auto & encodedRaw) {
+        auto encoded = json::parse(encodedRaw);
+        auto decoded = static_cast<MemorySourceAccessor>(encoded);
+        ASSERT_EQ(decoded.root, expected.root);
+    });
+}
+
+TEST_P(MemorySourceAccessorJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    MemorySourceAccessorJSON,
+    MemorySourceAccessorJsonTest,
+    ::testing::Values(
+        std::pair{
+            "simple",
+            *memory_source_accessor::exampleSimple(),
+        },
+        std::pair{
+            "complex",
+            *memory_source_accessor::exampleComplex(),
+        }));
+
+} // namespace nix

src/libutil-tests/meson.build

@@ -63,7 +63,9 @@ sources = files(
   'json-utils.cc',
   'logging.cc',
   'lru-cache.cc',
+  'memory-source-accessor.cc',
   'monitorfdhup.cc',
+  'nar-listing.cc',
   'nix_api_util.cc',
   'nix_api_util_internal.cc',
   'pool.cc',

src/libutil-tests/nar-listing.cc

@@ -0,0 +1,83 @@
+#include <string_view>
+
+#include "nix/util/nar-accessor.hh"
+#include "nix/util/tests/json-characterization.hh"
+
+namespace nix {
+
+// Forward declaration from memory-source-accessor.cc
+namespace memory_source_accessor {
+ref<MemorySourceAccessor> exampleComplex();
+}
+
+/* ----------------------------------------------------------------------------
+ * JSON
+ * --------------------------------------------------------------------------*/
+
+class NarListingTest : public virtual CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "nar-listing";
+
+public:
+
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / testStem;
+    }
+};
+
+using nlohmann::json;
+
+struct NarListingJsonTest : NarListingTest,
+                            JsonCharacterizationTest<NarListing>,
+                            ::testing::WithParamInterface<std::pair<std::string_view, NarListing>>
+{};
+
+TEST_P(NarListingJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    readJsonTest(name, expected);
+}
+
+TEST_P(NarListingJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    NarListingJSON,
+    NarListingJsonTest,
+    ::testing::Values(
+        std::pair{
+            "deep",
+            listNarDeep(*memory_source_accessor::exampleComplex(), CanonPath::root),
+        }));
+
+struct ShallowNarListingJsonTest : NarListingTest,
+                                   JsonCharacterizationTest<ShallowNarListing>,
+                                   ::testing::WithParamInterface<std::pair<std::string_view, ShallowNarListing>>
+{};
+
+TEST_P(ShallowNarListingJsonTest, from_json)
+{
+    auto & [name, expected] = GetParam();
+    readJsonTest(name, expected);
+}
+
+TEST_P(ShallowNarListingJsonTest, to_json)
+{
+    auto & [name, value] = GetParam();
+    writeJsonTest(name, value);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    ShallowNarListingJSON,
+    ShallowNarListingJsonTest,
+    ::testing::Values(
+        std::pair{
+            "shallow",
+            listNarShallow(*memory_source_accessor::exampleComplex(), CanonPath::root),
+        }));
+
+} // namespace nix

src/libutil/current-process.cc

@@ -134,6 +134,11 @@ std::optional<Path> getSelfExe()
             return std::nullopt;
         }
 
+        // FreeBSD's sysctl(KERN_PROC_PATHNAME) includes the null terminator in
+        // pathLen. Strip it to prevent Nix evaluation errors when the path is
+        // serialized to JSON and evaluated as a Nix string.
+        path.pop_back();
+
         return Path(path.begin(), path.end());
 #else
         return std::nullopt;

src/libutil/fs-sink.cc

@@ -37,7 +37,6 @@ void copyRecursive(SourceAccessor & accessor, const CanonPath & from, FileSystem
         sink.createDirectory(to, [&](FileSystemObjectSink & dirSink, const CanonPath & relDirPath) {
             for (auto & [name, _] : accessor.readDirectory(from)) {
                 copyRecursive(accessor, from / name, dirSink, relDirPath / name);
-                break;
             }
         });
         break;

src/libutil/include/nix/util/json-impls.hh

@@ -6,15 +6,18 @@
 #include "nix/util/experimental-features.hh"
 
 // Following https://github.com/nlohmann/json#how-can-i-use-get-for-non-default-constructiblenon-copyable-types
-#define JSON_IMPL(TYPE)                                   \
-    namespace nlohmann {                                  \
-    using namespace nix;                                  \
-    template<>                                            \
+#define JSON_IMPL_INNER(TYPE)                             \
     struct adl_serializer<TYPE>                           \
     {                                                     \
         static TYPE from_json(const json & json);         \
         static void to_json(json & json, const TYPE & t); \
-    };                                                    \
+    };
+
+#define JSON_IMPL(TYPE)   \
+    namespace nlohmann {  \
+    using namespace nix;  \
+    template<>            \
+    JSON_IMPL_INNER(TYPE) \
     }
 
 #define JSON_IMPL_WITH_XP_FEATURES(TYPE)                                                                            \

src/libutil/include/nix/util/memory-source-accessor.hh

@@ -4,59 +4,111 @@
 #include "nix/util/source-path.hh"
 #include "nix/util/fs-sink.hh"
 #include "nix/util/variant-wrapper.hh"
+#include "nix/util/json-impls.hh"
 
 namespace nix {
 
+/**
+ * File System Object definitions
+ *
+ * @see https://nix.dev/manual/nix/latest/store/file-system-object.html
+ */
+namespace fso {
+
+template<typename RegularContents>
+struct Regular
+{
+    bool executable = false;
+    RegularContents contents;
+
+    auto operator<=>(const Regular &) const = default;
+};
+
+/**
+ * Child parameter because sometimes we want "shallow" directories without
+ * full file children.
+ */
+template<typename Child>
+struct DirectoryT
+{
+    using Name = std::string;
+
+    std::map<Name, Child, std::less<>> entries;
+
+    inline bool operator==(const DirectoryT &) const noexcept;
+    inline std::strong_ordering operator<=>(const DirectoryT &) const noexcept;
+};
+
+struct Symlink
+{
+    std::string target;
+
+    auto operator<=>(const Symlink &) const = default;
+};
+
+/**
+ * For when we know there is child, but don't know anything about it.
+ *
+ * This is not part of the core File System Object data model --- this
+ * represents not knowing, not an additional type of file.
+ */
+struct Opaque
+{
+    auto operator<=>(const Opaque &) const = default;
+};
+
+/**
+ * `File<std::string>` nicely defining what a "file system object"
+ * is in Nix.
+ *
+ * With a different type arugment, it is also can be a "skeletal"
+ * version is that abstract syntax for a "NAR listing".
+ */
+template<typename RegularContents, bool recur>
+struct VariantT
+{
+    bool operator==(const VariantT &) const noexcept;
+    std::strong_ordering operator<=>(const VariantT &) const noexcept;
+
+    using Regular = nix::fso::Regular<RegularContents>;
+
+    /**
+     * In the default case, we do want full file children for our directory.
+     */
+    using Directory = nix::fso::DirectoryT<std::conditional_t<recur, VariantT, Opaque>>;
+
+    using Symlink = nix::fso::Symlink;
+
+    using Raw = std::variant<Regular, Directory, Symlink>;
+    Raw raw;
+
+    MAKE_WRAPPER_CONSTRUCTOR(VariantT);
+
+    SourceAccessor::Stat lstat() const;
+};
+
+template<typename Child>
+inline bool DirectoryT<Child>::operator==(const DirectoryT &) const noexcept = default;
+
+template<typename Child>
+inline std::strong_ordering DirectoryT<Child>::operator<=>(const DirectoryT &) const noexcept = default;
+
+template<typename RegularContents, bool recur>
+inline bool
+VariantT<RegularContents, recur>::operator==(const VariantT<RegularContents, recur> &) const noexcept = default;
+
+template<typename RegularContents, bool recur>
+inline std::strong_ordering
+VariantT<RegularContents, recur>::operator<=>(const VariantT<RegularContents, recur> &) const noexcept = default;
+
+} // namespace fso
+
 /**
  * An source accessor for an in-memory file system.
  */
 struct MemorySourceAccessor : virtual SourceAccessor
 {
-    /**
-     * In addition to being part of the implementation of
-     * `MemorySourceAccessor`, this has a side benefit of nicely
-     * defining what a "file system object" is in Nix.
-     */
-    struct File
-    {
-        bool operator==(const File &) const noexcept;
-        std::strong_ordering operator<=>(const File &) const noexcept;
-
-        struct Regular
-        {
-            bool executable = false;
-            std::string contents;
-
-            bool operator==(const Regular &) const = default;
-            auto operator<=>(const Regular &) const = default;
-        };
-
-        struct Directory
-        {
-            using Name = std::string;
-
-            std::map<Name, File, std::less<>> contents;
-
-            bool operator==(const Directory &) const noexcept;
-            // TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
-            bool operator<(const Directory &) const noexcept;
-        };
-
-        struct Symlink
-        {
-            std::string target;
-
-            bool operator==(const Symlink &) const = default;
-            auto operator<=>(const Symlink &) const = default;
-        };
-
-        using Raw = std::variant<Regular, Directory, Symlink>;
-        Raw raw;
-
-        MAKE_WRAPPER_CONSTRUCTOR(File);
-
-        Stat lstat() const;
-    };
+    using File = fso::VariantT<std::string, true>;
 
     std::optional<File> root;
 
@@ -89,19 +141,6 @@ struct MemorySourceAccessor : virtual SourceAccessor
     SourcePath addFile(CanonPath path, std::string && contents);
 };
 
-inline bool MemorySourceAccessor::File::Directory::operator==(
-    const MemorySourceAccessor::File::Directory &) const noexcept = default;
-
-inline bool
-MemorySourceAccessor::File::Directory::operator<(const MemorySourceAccessor::File::Directory & other) const noexcept
-{
-    return contents < other.contents;
-}
-
-inline bool MemorySourceAccessor::File::operator==(const MemorySourceAccessor::File &) const noexcept = default;
-inline std::strong_ordering
-MemorySourceAccessor::File::operator<=>(const MemorySourceAccessor::File &) const noexcept = default;
-
 /**
  * Write to a `MemorySourceAccessor` at the given path
  */
@@ -121,4 +160,53 @@ struct MemorySink : FileSystemObjectSink
     void createSymlink(const CanonPath & path, const std::string & target) override;
 };
 
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Regular> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Directory> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File::Symlink> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor::File> : std::true_type
+{};
+
+template<>
+struct json_avoids_null<MemorySourceAccessor> : std::true_type
+{};
+
 } // namespace nix
+
+namespace nlohmann {
+
+using namespace nix;
+
+#define ARG fso::Regular<RegularContents>
+template<typename RegularContents>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+#define ARG fso::DirectoryT<Child>
+template<typename Child>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+template<>
+JSON_IMPL_INNER(fso::Symlink)
+
+template<>
+JSON_IMPL_INNER(fso::Opaque)
+
+#define ARG fso::VariantT<RegularContents, recur>
+template<typename RegularContents, bool recur>
+JSON_IMPL_INNER(ARG)
+#undef ARG
+
+} // namespace nlohmann
+
+JSON_IMPL(MemorySourceAccessor)

src/libutil/include/nix/util/meson.build

@@ -50,6 +50,7 @@ headers = files(
   'memory-source-accessor.hh',
   'mounted-source-accessor.hh',
   'muxable-pipe.hh',
+  'nar-accessor.hh',
   'os-string.hh',
   'pool.hh',
   'pos-idx.hh',

src/libutil/include/nix/util/nar-accessor.hh

@@ -0,0 +1,80 @@
+#pragma once
+///@file
+
+#include "nix/util/memory-source-accessor.hh"
+
+#include <functional>
+
+#include <nlohmann/json_fwd.hpp>
+
+namespace nix {
+
+struct Source;
+
+/**
+ * Return an object that provides access to the contents of a NAR
+ * file.
+ */
+ref<SourceAccessor> makeNarAccessor(std::string && nar);
+
+ref<SourceAccessor> makeNarAccessor(Source & source);
+
+/**
+ * Create a NAR accessor from a NAR listing (in the format produced by
+ * listNar()). The callback getNarBytes(offset, length) is used by the
+ * readFile() method of the accessor to get the contents of files
+ * inside the NAR.
+ */
+using GetNarBytes = std::function<std::string(uint64_t, uint64_t)>;
+
+/**
+ * The canonical GetNarBytes function for a seekable Source.
+ */
+GetNarBytes seekableGetNarBytes(const Path & path);
+
+ref<SourceAccessor> makeLazyNarAccessor(const nlohmann::json & listing, GetNarBytes getNarBytes);
+
+struct NarListingRegularFile
+{
+    /**
+     * @see `SourceAccessor::Stat::fileSize`
+     */
+    std::optional<uint64_t> fileSize;
+
+    /**
+     * @see `SourceAccessor::Stat::narOffset`
+     *
+     * We only set to non-`std::nullopt` if it is also non-zero.
+     */
+    std::optional<uint64_t> narOffset;
+
+    auto operator<=>(const NarListingRegularFile &) const = default;
+};
+
+/**
+ * Abstract syntax for a "NAR listing".
+ */
+using NarListing = fso::VariantT<NarListingRegularFile, true>;
+
+/**
+ * Shallow NAR listing where directory children are not recursively expanded.
+ * Uses a variant that can hold Regular/Symlink fully, but Directory children
+ * are just unit types indicating presence without content.
+ */
+using ShallowNarListing = fso::VariantT<NarListingRegularFile, false>;
+
+/**
+ * Return a deep structured representation of the contents of a NAR (except file
+ * contents), recursively listing all children.
+ */
+NarListing listNarDeep(SourceAccessor & accessor, const CanonPath & path);
+
+/**
+ * Return a shallow structured representation of the contents of a NAR (except file
+ * contents), only listing immediate children without recursing.
+ */
+ShallowNarListing listNarShallow(SourceAccessor & accessor, const CanonPath & path);
+
+// All json_avoids_null and JSON_IMPL covered by generic templates in memory-source-accessor.hh
+
+} // namespace nix

src/libutil/memory-source-accessor.cc

@@ -1,4 +1,5 @@
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/json-utils.hh"
 
 namespace nix {
 
@@ -29,13 +30,13 @@ MemorySourceAccessor::File * MemorySourceAccessor::open(const CanonPath & path,
             return nullptr;
         auto & curDir = *curDirP;
 
-        auto i = curDir.contents.find(name);
-        if (i == curDir.contents.end()) {
+        auto i = curDir.entries.find(name);
+        if (i == curDir.entries.end()) {
             if (!create)
                 return nullptr;
             else {
                 newF = true;
-                i = curDir.contents.insert(
+                i = curDir.entries.insert(
                     i,
                     {
                         std::string{name},
@@ -68,25 +69,26 @@ bool MemorySourceAccessor::pathExists(const CanonPath & path)
     return open(path, std::nullopt);
 }
 
-MemorySourceAccessor::Stat MemorySourceAccessor::File::lstat() const
+template<>
+SourceAccessor::Stat MemorySourceAccessor::File::lstat() const
 {
     return std::visit(
         overloaded{
             [](const Regular & r) {
-                return Stat{
-                    .type = tRegular,
+                return SourceAccessor::Stat{
+                    .type = SourceAccessor::tRegular,
                     .fileSize = r.contents.size(),
                     .isExecutable = r.executable,
                 };
             },
             [](const Directory &) {
-                return Stat{
-                    .type = tDirectory,
+                return SourceAccessor::Stat{
+                    .type = SourceAccessor::tDirectory,
                 };
             },
             [](const Symlink &) {
-                return Stat{
-                    .type = tSymlink,
+                return SourceAccessor::Stat{
+                    .type = SourceAccessor::tSymlink,
                 };
             },
         },
@@ -106,7 +108,7 @@ MemorySourceAccessor::DirEntries MemorySourceAccessor::readDirectory(const Canon
         throw Error("file '%s' does not exist", path);
     if (auto * d = std::get_if<File::Directory>(&f->raw)) {
         DirEntries res;
-        for (auto & [name, file] : d->contents)
+        for (auto & [name, file] : d->entries)
             res.insert_or_assign(name, file.lstat().type);
         return res;
     } else

src/libutil/memory-source-accessor/json.cc

@@ -0,0 +1,162 @@
+#include "nix/util/memory-source-accessor.hh"
+#include "nix/util/nar-accessor.hh"
+#include "nix/util/json-utils.hh"
+
+#include <nlohmann/json.hpp>
+
+namespace nlohmann {
+
+using namespace nix;
+
+// fso::Regular<RegularContents>
+template<>
+MemorySourceAccessor::File::Regular adl_serializer<MemorySourceAccessor::File::Regular>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return MemorySourceAccessor::File::Regular{
+        .executable = getBoolean(valueAt(obj, "executable")),
+        .contents = getString(valueAt(obj, "contents")),
+    };
+}
+
+template<>
+void adl_serializer<MemorySourceAccessor::File::Regular>::to_json(
+    json & json, const MemorySourceAccessor::File::Regular & r)
+{
+    json = {
+        {"executable", r.executable},
+        {"contents", r.contents},
+    };
+}
+
+template<>
+NarListing::Regular adl_serializer<NarListing::Regular>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    auto * execPtr = optionalValueAt(obj, "executable");
+    auto * sizePtr = optionalValueAt(obj, "size");
+    auto * offsetPtr = optionalValueAt(obj, "narOffset");
+    return NarListing::Regular{
+        .executable = execPtr ? getBoolean(*execPtr) : false,
+        .contents{
+            .fileSize = ptrToOwned<uint64_t>(sizePtr),
+            .narOffset = ptrToOwned<uint64_t>(offsetPtr).and_then(
+                [](auto v) { return v != 0 ? std::optional{v} : std::nullopt; }),
+        },
+    };
+}
+
+template<>
+void adl_serializer<NarListing::Regular>::to_json(json & j, const NarListing::Regular & r)
+{
+    if (r.contents.fileSize)
+        j["size"] = *r.contents.fileSize;
+    if (r.executable)
+        j["executable"] = true;
+    if (r.contents.narOffset)
+        j["narOffset"] = *r.contents.narOffset;
+}
+
+template<typename Child>
+void adl_serializer<fso::DirectoryT<Child>>::to_json(json & j, const fso::DirectoryT<Child> & d)
+{
+    j["entries"] = d.entries;
+}
+
+template<typename Child>
+fso::DirectoryT<Child> adl_serializer<fso::DirectoryT<Child>>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return fso::DirectoryT<Child>{
+        .entries = valueAt(obj, "entries"),
+    };
+}
+
+// fso::Symlink
+fso::Symlink adl_serializer<fso::Symlink>::from_json(const json & json)
+{
+    auto & obj = getObject(json);
+    return fso::Symlink{
+        .target = getString(valueAt(obj, "target")),
+    };
+}
+
+void adl_serializer<fso::Symlink>::to_json(json & json, const fso::Symlink & s)
+{
+    json = {
+        {"target", s.target},
+    };
+}
+
+// fso::Opaque
+fso::Opaque adl_serializer<fso::Opaque>::from_json(const json &)
+{
+    return fso::Opaque{};
+}
+
+void adl_serializer<fso::Opaque>::to_json(json & j, const fso::Opaque &)
+{
+    j = nlohmann::json::object();
+}
+
+// fso::VariantT<RegularContents, recur> - generic implementation
+template<typename RegularContents, bool recur>
+void adl_serializer<fso::VariantT<RegularContents, recur>>::to_json(
+    json & j, const fso::VariantT<RegularContents, recur> & val)
+{
+    using Variant = fso::VariantT<RegularContents, recur>;
+    j = nlohmann::json::object();
+    std::visit(
+        overloaded{
+            [&](const typename Variant::Regular & r) {
+                j = r;
+                j["type"] = "regular";
+            },
+            [&](const typename Variant::Directory & d) {
+                j = d;
+                j["type"] = "directory";
+            },
+            [&](const typename Variant::Symlink & s) {
+                j = s;
+                j["type"] = "symlink";
+            },
+        },
+        val.raw);
+}
+
+template<typename RegularContents, bool recur>
+fso::VariantT<RegularContents, recur>
+adl_serializer<fso::VariantT<RegularContents, recur>>::from_json(const json & json)
+{
+    using Variant = fso::VariantT<RegularContents, recur>;
+    auto & obj = getObject(json);
+    auto type = getString(valueAt(obj, "type"));
+    if (type == "regular")
+        return static_cast<typename Variant::Regular>(json);
+    if (type == "directory")
+        return static_cast<typename Variant::Directory>(json);
+    if (type == "symlink")
+        return static_cast<typename Variant::Symlink>(json);
+    else
+        throw Error("unknown type of file '%s'", type);
+}
+
+// Explicit instantiations for VariantT types we use
+template struct adl_serializer<MemorySourceAccessor::File>;
+template struct adl_serializer<NarListing>;
+template struct adl_serializer<ShallowNarListing>;
+
+// MemorySourceAccessor
+MemorySourceAccessor adl_serializer<MemorySourceAccessor>::from_json(const json & json)
+{
+    MemorySourceAccessor res;
+    res.root = json;
+    return res;
+}
+
+void adl_serializer<MemorySourceAccessor>::to_json(json & json, const MemorySourceAccessor & val)
+{
+    json = val.root;
+}
+
+} // namespace nlohmann

src/libutil/meson.build

@@ -146,7 +146,9 @@ sources = [ config_priv_h ] + files(
   'json-utils.cc',
   'logging.cc',
   'memory-source-accessor.cc',
+  'memory-source-accessor/json.cc',
   'mounted-source-accessor.cc',
+  'nar-accessor.cc',
   'pos-table.cc',
   'position.cc',
   'posix-source-accessor.cc',

src/libutil/nar-accessor.cc

@@ -1,4 +1,4 @@
-#include "nix/store/nar-accessor.hh"
+#include "nix/util/nar-accessor.hh"
 #include "nix/util/archive.hh"
 
 #include <map>
@@ -272,41 +272,39 @@ GetNarBytes seekableGetNarBytes(const Path & path)
     };
 }
 
-using nlohmann::json;
+template<bool deep>
+using ListNarResult = std::conditional_t<deep, NarListing, ShallowNarListing>;
 
-json listNar(ref<SourceAccessor> accessor, const CanonPath & path, bool recurse)
+template<bool deep>
+static ListNarResult<deep> listNarImpl(SourceAccessor & accessor, const CanonPath & path)
 {
-    auto st = accessor->lstat(path);
-
-    json obj = json::object();
+    auto st = accessor.lstat(path);
 
     switch (st.type) {
     case SourceAccessor::Type::tRegular:
-        obj["type"] = "regular";
-        if (st.fileSize)
-            obj["size"] = *st.fileSize;
-        if (st.isExecutable)
-            obj["executable"] = true;
-        if (st.narOffset && *st.narOffset)
-            obj["narOffset"] = *st.narOffset;
-        break;
-    case SourceAccessor::Type::tDirectory:
-        obj["type"] = "directory";
-        {
-            obj["entries"] = json::object();
-            json & res2 = obj["entries"];
-            for (const auto & [name, type] : accessor->readDirectory(path)) {
-                if (recurse) {
-                    res2[name] = listNar(accessor, path / name, true);
-                } else
-                    res2[name] = json::object();
+        return typename ListNarResult<deep>::Regular{
+            .executable = st.isExecutable,
+            .contents =
+                NarListingRegularFile{
+                    .fileSize = st.fileSize,
+                    .narOffset = st.narOffset && *st.narOffset ? st.narOffset : std::nullopt,
+                },
+        };
+    case SourceAccessor::Type::tDirectory: {
+        typename ListNarResult<deep>::Directory dir;
+        for (const auto & [name, type] : accessor.readDirectory(path)) {
+            if constexpr (deep) {
+                dir.entries.emplace(name, listNarImpl<true>(accessor, path / name));
+            } else {
+                dir.entries.emplace(name, fso::Opaque{});
             }
         }
-        break;
+        return dir;
+    }
     case SourceAccessor::Type::tSymlink:
-        obj["type"] = "symlink";
-        obj["target"] = accessor->readLink(path);
-        break;
+        return typename ListNarResult<deep>::Symlink{
+            .target = accessor.readLink(path),
+        };
     case SourceAccessor::Type::tBlock:
     case SourceAccessor::Type::tChar:
     case SourceAccessor::Type::tSocket:
@@ -314,7 +312,16 @@ json listNar(ref<SourceAccessor> accessor, const CanonPath & path, bool recurse)
     case SourceAccessor::Type::tUnknown:
         assert(false); // cannot happen for NARs
     }
-    return obj;
+}
+
+NarListing listNarDeep(SourceAccessor & accessor, const CanonPath & path)
+{
+    return listNarImpl<true>(accessor, path);
+}
+
+ShallowNarListing listNarShallow(SourceAccessor & accessor, const CanonPath & path)
+{
+    return listNarImpl<false>(accessor, path);
 }
 
 } // namespace nix

src/libutil/unix/file-system.cc

@@ -16,7 +16,7 @@ namespace nix {
 
 Descriptor openDirectory(const std::filesystem::path & path)
 {
-    return open(path.c_str(), O_RDONLY | O_DIRECTORY);
+    return open(path.c_str(), O_RDONLY | O_DIRECTORY | O_CLOEXEC);
 }
 
 void setWriteTime(

src/nix/build-remote/build-remote.cc

@@ -322,7 +322,7 @@ static int main_build_remote(int argc, char ** argv)
             //    output ids, which break CA derivations
             if (!drv.inputDrvs.map.empty())
                 drv.inputSrcs = store->parseStorePathSet(inputs);
-            optResult = sshStore->buildDerivation(*drvPath, (const BasicDerivation &) drv);
+            optResult = sshStore->buildDerivation(*drvPath, static_cast<const BasicDerivation &>(drv));
             auto & result = *optResult;
             if (auto * failureP = result.tryGetFailure()) {
                 if (settings.keepFailed) {

src/nix/cat.cc

@@ -1,6 +1,6 @@
 #include "nix/cmd/command.hh"
 #include "nix/store/store-api.hh"
-#include "nix/store/nar-accessor.hh"
+#include "nix/util/nar-accessor.hh"
 #include "nix/util/serialise.hh"
 #include "nix/util/source-accessor.hh"
 
@@ -80,7 +80,7 @@ struct CmdCatNar : StoreCommand, MixCat
             throw SysError("opening NAR file '%s'", narPath);
         auto source = FdSource{fd.get()};
         auto narAccessor = makeNarAccessor(source);
-        auto listing = listNar(narAccessor, CanonPath::root, true);
+        nlohmann::json listing = listNarDeep(*narAccessor, CanonPath::root);
         cat(makeLazyNarAccessor(listing, seekableGetNarBytes(narPath)), CanonPath{path});
     }
 };

src/nix/ls.cc

@@ -1,6 +1,6 @@
 #include "nix/cmd/command.hh"
 #include "nix/store/store-api.hh"
-#include "nix/store/nar-accessor.hh"
+#include "nix/util/nar-accessor.hh"
 #include "nix/main/common-args.hh"
 #include <nlohmann/json.hpp>
 
@@ -85,7 +85,12 @@ struct MixLs : virtual Args, MixJSON
         if (json) {
             if (showDirectory)
                 throw UsageError("'--directory' is useless with '--json'");
-            logger->cout("%s", listNar(accessor, path, recursive));
+            nlohmann::json j;
+            if (recursive)
+                j = listNarDeep(*accessor, path);
+            else
+                j = listNarShallow(*accessor, path);
+            logger->cout("%s", j.dump());
         } else
             listText(accessor, std::move(path));
     }
@@ -150,7 +155,7 @@ struct CmdLsNar : Command, MixLs
             throw SysError("opening NAR file '%s'", narPath);
         auto source = FdSource{fd.get()};
         auto narAccessor = makeNarAccessor(source);
-        auto listing = listNar(narAccessor, CanonPath::root, true);
+        nlohmann::json listing = listNarDeep(*narAccessor, CanonPath::root);
         list(makeLazyNarAccessor(listing, seekableGetNarBytes(narPath)), CanonPath{path});
     }
 };

src/nix/main.cc

@@ -271,7 +271,7 @@ static void showHelp(std::vector<std::string> subcommand, NixArgs & toplevel)
     );
 
     auto vDump = state.allocValue();
-    vDump->mkString(toplevel.dumpCli());
+    vDump->mkString(toplevel.dumpCli(), state.mem);
 
     auto vRes = state.allocValue();
     Value * args[]{&state.getBuiltin("false"), vDump};

src/nix/meson.build

@@ -229,6 +229,7 @@ foreach linkname : nix_symlinks
     env : {'MSYS' : 'winsymlinks:lnk'},
     # TODO(Ericson2314): Don't do this once we have the `meson.override_find_program` working)
     build_by_default : true,
+    depends : this_exe,
   )
   # TODO(Ericson3214): Doesn't yet work
   #meson.override_find_program(linkname, t)
@@ -250,6 +251,7 @@ custom_target(
   env : {'MSYS' : 'winsymlinks:lnk'},
   # TODO(Ericson2314): Don't do this once we have the `meson.override_find_program` working)
   build_by_default : true,
+  depends : this_exe,
 )
 # TODO(Ericson3214): Doesn't yet work
 #meson.override_find_program(linkname, t)

src/nix/nix-env/nix-env.cc

@@ -132,7 +132,7 @@ static void getAllExprs(EvalState & state, const SourcePath & path, StringSet &
             }
             /* Load the expression on demand. */
             auto vArg = state.allocValue();
-            vArg->mkPath(path2);
+            vArg->mkPath(path2, state.mem);
             if (seen.size() == maxAttrs)
                 throw Error("too many Nix expressions in directory '%1%'", path);
             attrs.alloc(attrName).mkApp(&state.getBuiltin("import"), vArg);
@@ -483,7 +483,7 @@ static bool keep(PackageInfo & drv)
 static void setMetaFlag(EvalState & state, PackageInfo & drv, const std::string & name, const std::string & value)
 {
     auto v = state.allocValue();
-    v->mkString(value);
+    v->mkString(value, state.mem);
     drv.setMeta(name, v);
 }
 

src/nix/nix-env/user-env.cc

@@ -58,20 +58,20 @@ bool createUserEnv(
         auto attrs = state.buildBindings(7 + outputs.size());
 
         attrs.alloc(state.s.type).mkStringNoCopy("derivation"_sds);
-        attrs.alloc(state.s.name).mkString(i.queryName());
+        attrs.alloc(state.s.name).mkString(i.queryName(), state.mem);
         auto system = i.querySystem();
         if (!system.empty())
-            attrs.alloc(state.s.system).mkString(system);
-        attrs.alloc(state.s.outPath).mkString(state.store->printStorePath(i.queryOutPath()));
+            attrs.alloc(state.s.system).mkString(system, state.mem);
+        attrs.alloc(state.s.outPath).mkString(state.store->printStorePath(i.queryOutPath()), state.mem);
         if (drvPath)
-            attrs.alloc(state.s.drvPath).mkString(state.store->printStorePath(*drvPath));
+            attrs.alloc(state.s.drvPath).mkString(state.store->printStorePath(*drvPath), state.mem);
 
         // Copy each output meant for installation.
         auto outputsList = state.buildList(outputs.size());
         for (const auto & [m, j] : enumerate(outputs)) {
-            (outputsList[m] = state.allocValue())->mkString(j.first);
+            (outputsList[m] = state.allocValue())->mkString(j.first, state.mem);
             auto outputAttrs = state.buildBindings(2);
-            outputAttrs.alloc(state.s.outPath).mkString(state.store->printStorePath(*j.second));
+            outputAttrs.alloc(state.s.outPath).mkString(state.store->printStorePath(*j.second), state.mem);
             attrs.alloc(j.first).mkAttrs(outputAttrs);
 
             /* This is only necessary when installing store paths, e.g.,

src/nix/prefetch.cc

@@ -59,7 +59,7 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
 std::tuple<StorePath, Hash> prefetchFile(
     ref<Store> store,
     const VerbatimURL & url,
-    std::optional<std::string> name,
+    std::optional<std::string> maybeName,
     HashAlgorithm hashAlgo,
     std::optional<Hash> expectedHash,
     bool unpack,
@@ -68,16 +68,21 @@ std::tuple<StorePath, Hash> prefetchFile(
     ContentAddressMethod method =
         unpack || executable ? ContentAddressMethod::Raw::NixArchive : ContentAddressMethod::Raw::Flat;
 
-    /* Figure out a name in the Nix store. */
-    if (!name) {
-        name = url.lastPathSegment();
-        if (!name || name->empty())
-            throw Error("cannot figure out file name for '%s'", url.to_string());
-    }
+    std::string name = maybeName
+                           .or_else([&]() {
+                               /* Figure out a name in the Nix store. */
+                               auto derivedFromUrl = url.lastPathSegment();
+                               if (!derivedFromUrl || derivedFromUrl->empty())
+                                   throw Error("cannot figure out file name for '%s'", url.to_string());
+                               return derivedFromUrl;
+                           })
+                           .value();
+
     try {
-        checkName(*name);
+        checkName(name);
     } catch (BadStorePathName & e) {
-        e.addTrace({}, "file name '%s' was extracted from URL '%s'", *name, url.to_string());
+        if (!maybeName)
+            e.addTrace({}, "file name '%s' was extracted from URL '%s'", name, url.to_string());
         throw;
     }
 
@@ -89,7 +94,7 @@ std::tuple<StorePath, Hash> prefetchFile(
     if (expectedHash) {
         hashAlgo = expectedHash->algo;
         storePath =
-            store->makeFixedOutputPathFromCA(*name, ContentAddressWithReferences::fromParts(method, *expectedHash, {}));
+            store->makeFixedOutputPathFromCA(name, ContentAddressWithReferences::fromParts(method, *expectedHash, {}));
         if (store->isValidPath(*storePath))
             hash = expectedHash;
         else
@@ -138,7 +143,7 @@ std::tuple<StorePath, Hash> prefetchFile(
 
         Activity act(*logger, lvlChatty, actUnknown, fmt("adding '%s' to the store", url.to_string()));
 
-        auto info = store->addToStoreSlow(*name, makeFSSourceAccessor(tmpFile), method, hashAlgo, {}, expectedHash);
+        auto info = store->addToStoreSlow(name, makeFSSourceAccessor(tmpFile), method, hashAlgo, {}, expectedHash);
         storePath = info.path;
         assert(info.ca);
         hash = info.ca->hash;

tests/functional/flakes/common.sh

@@ -32,6 +32,8 @@ writeSimpleFlake() {
     baseName = builtins.baseNameOf ./.;
 
     root = ./.;
+
+    number = 123;
   };
 }
 EOF

tests/functional/flakes/meson.build

@@ -34,6 +34,7 @@ suites += {
     'source-paths.sh',
     'old-lockfiles.sh',
     'trace-ifd.sh',
+    'shallow.sh',
   ],
   'workdir' : meson.current_source_dir(),
 }

tests/functional/flakes/shallow.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+
+export _NIX_FORCE_HTTP=1
+
+source ./common.sh
+
+requireGit
+TODO_NixOS
+
+createFlake1
+
+repoDir="$TEST_ROOT/repo"
+mkdir -p "$repoDir"
+echo "# foo" >> "$flake1Dir/flake.nix"
+git -C "$flake1Dir" commit -a -m bla
+
+cat > "$repoDir"/flake.nix <<EOF
+{
+  inputs.dep = {
+    type = "git";
+    url = "file://$flake1Dir";
+  };
+  outputs = inputs: rec {
+    revs = assert inputs.dep.number == 123; inputs.dep.revCount;
+  };
+}
+EOF
+
+# This will do a non-shallow fetch.
+[[ $(nix eval "path:$repoDir#revs") = 2 ]]
+
+# This should re-use the existing non-shallow clone.
+clearStore
+mv "$flake1Dir" "$flake1Dir.moved"
+[[ $(nix eval "path:$repoDir#revs") = 2 ]]

tests/functional/lang/eval-fail-deepseq-list-attr.err.exp

@@ -0,0 +1,25 @@
+error:
+       … while calling the 'deepSeq' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:3:1:
+            2|
+            3| builtins.deepSeq [
+             | ^
+            4|   1
+
+       … while evaluating list element at index 1
+
+       … while evaluating the attribute 'b'
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:5:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |     ^
+            8|   }
+
+       … while calling the 'throw' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:9:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |         ^
+            8|   }
+
+       error: error in attr in list element

tests/functional/lang/eval-fail-deepseq-list-attr.nix

@@ -0,0 +1,10 @@
+# Test that deepSeq reports list index and attribute name in error traces.
+
+builtins.deepSeq [
+  1
+  {
+    a = 2;
+    b = throw "error in attr in list element";
+  }
+  3
+] "unexpected success"

tests/functional/lang/eval-fail-deepseq-stack-overflow.err.exp

@@ -0,0 +1,30 @@
+error:
+       … while calling the 'deepSeq' builtin
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:8:1:
+            7| in
+            8| builtins.deepSeq reverseLinkedList (
+             | ^
+            9|   throw "unexpected success; expected a controlled stack overflow instead"
+
+       … while evaluating the attribute 'tail'
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:6:67:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                   ^
+            7| in
+
+       (9997 duplicate frames omitted)
+
+       … while evaluating the attribute 'head'
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:6:62:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                              ^
+            7| in
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:5:28:
+            4| let
+            5|   long = builtins.genList (x: x) 100000;
+             |                            ^
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;

tests/functional/lang/eval-fail-deepseq-stack-overflow.nix

@@ -0,0 +1,10 @@
+# Test that deepSeq on a deeply nested structure produces a controlled
+# stack overflow error rather than a segfault.
+
+let
+  long = builtins.genList (x: x) 100000;
+  reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+in
+builtins.deepSeq reverseLinkedList (
+  throw "unexpected success; expected a controlled stack overflow instead"
+)

tests/functional/lang/eval-fail-derivation-structuredAttrs-stack-overflow.err.exp

@@ -0,0 +1,54 @@
+error:
+       … while evaluating the attribute 'outPath'
+         at «nix-internal»/derivation-internal.nix:50:7:
+           49|     value = commonAttrs // {
+           50|       outPath = builtins.getAttr outputName strict;
+             |       ^
+           51|       drvPath = strict.drvPath;
+
+       … while calling the 'getAttr' builtin
+         at «nix-internal»/derivation-internal.nix:50:17:
+           49|     value = commonAttrs // {
+           50|       outPath = builtins.getAttr outputName strict;
+             |                 ^
+           51|       drvPath = strict.drvPath;
+
+       … while calling the 'derivationStrict' builtin
+         at «nix-internal»/derivation-internal.nix:37:12:
+           36|
+           37|   strict = derivationStrict drvAttrs;
+             |            ^
+           38|
+
+       … while evaluating derivation 'test'
+         whose name attribute is located at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:5:3
+
+       … while evaluating attribute 'nested' of derivation 'test'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:9:3:
+            8|   __structuredAttrs = true;
+            9|   nested =
+             |   ^
+           10|     let
+
+       … while evaluating attribute 'tail'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:71:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                       ^
+           13|     in
+
+       (9994 duplicate frames omitted)
+
+       … while evaluating attribute 'head'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:66:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                  ^
+           13|     in
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:66:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                  ^
+           13|     in

tests/functional/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix

@@ -0,0 +1,15 @@
+# Test that derivations with __structuredAttrs and deeply nested structures
+# produce a controlled stack overflow error rather than a segfault.
+
+derivation {
+  name = "test";
+  system = "x86_64-linux";
+  builder = "/bin/sh";
+  __structuredAttrs = true;
+  nested =
+    let
+      long = builtins.genList (x: x) 100000;
+      reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+    in
+    reverseLinkedList;
+}

tests/functional/lang/eval-fail-toJSON-stack-overflow.err.exp

@@ -0,0 +1,30 @@
+error:
+       … while calling the 'toJSON' builtin
+         at /pwd/lang/eval-fail-toJSON-stack-overflow.nix:8:1:
+            7| in
+            8| builtins.toJSON reverseLinkedList
+             | ^
+            9|
+
+       … while evaluating attribute 'tail'
+         at /pwd/lang/eval-fail-toJSON-stack-overflow.nix:6:67:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                   ^
+            7| in
+
+       (9997 duplicate frames omitted)
+
+       … while evaluating attribute 'head'
+         at /pwd/lang/eval-fail-toJSON-stack-overflow.nix:6:62:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                              ^
+            7| in
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/lang/eval-fail-toJSON-stack-overflow.nix:6:62:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                              ^
+            7| in

tests/functional/lang/eval-fail-toJSON-stack-overflow.nix

@@ -0,0 +1,8 @@
+# Test that toJSON on a deeply nested structure produces a controlled
+# stack overflow error rather than a segfault.
+
+let
+  long = builtins.genList (x: x) 100000;
+  reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+in
+builtins.toJSON reverseLinkedList

tests/nixos/content-encoding.nix

@@ -131,6 +131,7 @@ in
     start_all()
 
     machine.wait_for_unit("nginx.service")
+    machine.wait_for_open_port(80)
 
     # Original test: zstd archive with gzip content-encoding
     # Make sure that the file is properly compressed as the test would be meaningless otherwise

tests/nixos/fetch-git/test-cases/fetchTree-shallow/default.nix

@@ -1,5 +1,5 @@
 {
-  description = "fetchTree fetches git repos shallowly by default";
+  description = "fetchTree fetches git repos shallowly if possible";
   script = ''
     # purge nix git cache to make sure we start with a clean slate
     client.succeed("rm -rf ~/.cache/nix")
@@ -28,6 +28,7 @@
         type = "git";
         url = "{repo.remote}";
         rev = "{commit2_rev}";
+        revCount = 1234;
       }}
     """
 

tests/nixos/tarball-flakes.nix

@@ -99,7 +99,6 @@ in
 
       # Check that fetching fails if we provide incorrect attributes.
       machine.fail("nix flake metadata --json http://localhost/tags/latest.tar.gz?rev=493300eb13ae6fb387fbd47bf54a85915acc31c0")
-      machine.fail("nix flake metadata --json http://localhost/tags/latest.tar.gz?revCount=789")
       machine.fail("nix flake metadata --json http://localhost/tags/latest.tar.gz?narHash=sha256-tbudgBSg+bHWHiHnlteNzN8TUvI80ygS9IULh4rklEw=")
     '';