libutil/union-source-accessor: Barf on non-existent directories

Previously builtins.readDir would return an empty attribute set
instead of barfing on non-existent paths. This is a regression from
2.32 for impure eval.

src/libutil/union-source-accessor.cc

@@ -35,14 +35,18 @@ struct UnionSourceAccessor : SourceAccessor
     DirEntries readDirectory(const CanonPath & path) override
     {
         DirEntries result;
+        bool exists = false;
         for (auto & accessor : accessors) {
             auto st = accessor->maybeLstat(path);
             if (!st)
                 continue;
+            exists = true;
             for (auto & entry : accessor->readDirectory(path))
                 // Don't override entries from previous accessors.
                 result.insert(entry);
         }
+        if (!exists)
+            throw FileNotFound("path '%s' does not exist", showPath(path));
         return result;
     }
 

tests/functional/lang/eval-fail-readDir-nonexistent-1.err.exp

@@ -0,0 +1,16 @@
+error:
+       … while evaluating the attribute 'absolutePath'
+         at /pwd/lang/eval-fail-readDir-nonexistent-1.nix:2:3:
+            1| {
+            2|   absolutePath = builtins.readDir /this/path/really/should/not/exist;
+             |   ^
+            3| }
+
+       … while calling the 'readDir' builtin
+         at /pwd/lang/eval-fail-readDir-nonexistent-1.nix:2:18:
+            1| {
+            2|   absolutePath = builtins.readDir /this/path/really/should/not/exist;
+             |                  ^
+            3| }
+
+       error: path '/this/path/really/should/not/exist' does not exist

tests/functional/lang/eval-fail-readDir-nonexistent-1.nix

@@ -0,0 +1,3 @@
+{
+  absolutePath = builtins.readDir /this/path/really/should/not/exist;
+}

tests/functional/lang/eval-fail-readDir-nonexistent-2.err.exp

@@ -0,0 +1,16 @@
+error:
+       … while evaluating the attribute 'relativePath'
+         at /pwd/lang/eval-fail-readDir-nonexistent-2.nix:2:3:
+            1| {
+            2|   relativePath = builtins.readDir ./this/path/really/should/not/exist;
+             |   ^
+            3| }
+
+       … while calling the 'readDir' builtin
+         at /pwd/lang/eval-fail-readDir-nonexistent-2.nix:2:18:
+            1| {
+            2|   relativePath = builtins.readDir ./this/path/really/should/not/exist;
+             |                  ^
+            3| }
+
+       error: path '/pwd/lang/this/path/really/should/not/exist' does not exist

tests/functional/lang/eval-fail-readDir-nonexistent-2.nix

@@ -0,0 +1,3 @@
+{
+  relativePath = builtins.readDir ./this/path/really/should/not/exist;
+}

tests/functional/lang/eval-fail-readDir-not-a-directory-1.err.exp

@@ -0,0 +1,16 @@
+error:
+       … while evaluating the attribute 'regularFile'
+         at /pwd/lang/eval-fail-readDir-not-a-directory-1.nix:2:3:
+            1| {
+            2|   regularFile = builtins.readDir ./readDir/bar;
+             |   ^
+            3| }
+
+       … while calling the 'readDir' builtin
+         at /pwd/lang/eval-fail-readDir-not-a-directory-1.nix:2:17:
+            1| {
+            2|   regularFile = builtins.readDir ./readDir/bar;
+             |                 ^
+            3| }
+
+       error: cannot read directory "/pwd/lang/readDir/bar": Not a directory

tests/functional/lang/eval-fail-readDir-not-a-directory-1.nix

@@ -0,0 +1,3 @@
+{
+  regularFile = builtins.readDir ./readDir/bar;
+}

tests/functional/lang/eval-fail-readDir-not-a-directory-2.err.exp

@@ -0,0 +1,16 @@
+error:
+       … while evaluating the attribute 'symlinkedRegularFile'
+         at /pwd/lang/eval-fail-readDir-not-a-directory-2.nix:2:3:
+            1| {
+            2|   symlinkedRegularFile = builtins.readDir ./readDir/linked;
+             |   ^
+            3| }
+
+       … while calling the 'readDir' builtin
+         at /pwd/lang/eval-fail-readDir-not-a-directory-2.nix:2:26:
+            1| {
+            2|   symlinkedRegularFile = builtins.readDir ./readDir/linked;
+             |                          ^
+            3| }
+
+       error: cannot read directory "/pwd/lang/readDir/foo/git-hates-directories": Not a directory

tests/functional/lang/eval-fail-readDir-not-a-directory-2.nix

@@ -0,0 +1,3 @@
+{
+  symlinkedRegularFile = builtins.readDir ./readDir/linked;
+}

tests/functional/lang/eval-okay-readDir-symlinked-directory.exp

@@ -0,0 +1 @@
+{ git-hates-directories = "regular"; }

tests/functional/lang/eval-okay-readDir-symlinked-directory.nix

@@ -0,0 +1 @@
+builtins.readDir ./readDir/ldir