Add setting narinfo-cache-meta-ttl

This makes the current hard-coded 7-day `nix-cache-info` TTL
configurable, making `--offline` and `--refresh` do the right thing.

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

@@ -86,6 +86,17 @@ struct NarInfoDiskCacheSettings : public virtual Config
           would prevent trying to pull the path again and failing with a hash
           mismatch if the build isn't reproducible.
         )"};
+
+    Setting<unsigned int> ttlMeta{
+        this,
+        7 * 24 * 3600,
+        "narinfo-cache-meta-ttl",
+        R"(
+          The TTL in seconds for caching binary cache metadata (i.e.
+          `/nix-cache-info`). This determines how long information about a
+          binary cache (such as its store directory, priority, and whether it
+          wants mass queries) is considered valid before being refreshed.
+        )"};
 };
 
 class Settings : public virtual Config,

src/libstore/nar-info-disk-cache.cc

@@ -63,9 +63,6 @@ struct NarInfoDiskCacheImpl : NarInfoDiskCache
     /* How often to purge expired entries from the cache. */
     const int purgeInterval = 24 * 3600;
 
-    /* How long to cache binary cache info (i.e. /nix-cache-info) */
-    const int cacheInfoTtl = 7 * 24 * 3600;
-
     struct Cache
     {
         int id;
@@ -184,7 +181,7 @@ private:
     {
         auto i = state.caches.find(uri);
         if (i == state.caches.end()) {
-            auto queryCache(state.queryCache.use()(uri)(time(0) - cacheInfoTtl));
+            auto queryCache(state.queryCache.use()(uri)(time(0) - settings.ttlMeta));
             if (!queryCache.next())
                 return std::nullopt;
             auto cache = Cache{

src/nix/main.cc

@@ -564,12 +564,16 @@ void mainWrapped(int argc, char ** argv)
             fileTransferSettings.tries = 0;
         if (!fileTransferSettings.connectTimeout.overridden)
             fileTransferSettings.connectTimeout = 1;
+        auto & ttlMeta = settings.getNarInfoDiskCacheSettings().ttlMeta;
+        if (!ttlMeta.overridden)
+            ttlMeta = std::numeric_limits<unsigned int>::max();
     }
 
     if (args.refresh) {
         fetchSettings.tarballTtl = 0;
         settings.getNarInfoDiskCacheSettings().ttlNegative = 0;
         settings.getNarInfoDiskCacheSettings().ttlPositive = 0;
+        settings.getNarInfoDiskCacheSettings().ttlMeta = 0;
     }
 
     if (args.command->second->forceImpureByDefault() && !evalSettings.pureEval.overridden) {

tests/functional/binary-cache.sh

@@ -312,3 +312,24 @@ nix-store --delete "$outPath" "$docPath"
 # -vvv is the level that logs during the loop
 timeout 60 nix-build --no-out-link -E "$expr" --option substituters "file://$cacheDir" \
   --option trusted-binary-caches "file://$cacheDir"  --no-require-sigs
+
+
+# Test that the narinfo-cache-meta-ttl causes nix-cache-info to be cached,
+# and that --refresh overrides it.
+
+# Populate the metadata cache by querying store info over HTTP.
+_NIX_FORCE_HTTP=1 nix store info --store "file://$cacheDir"
+
+# Remove nix-cache-info from the binary cache.
+rm "$cacheDir/nix-cache-info"
+
+# nix store info should still work because the metadata is cached
+# (narinfo-cache-meta-ttl defaults to 7 days).
+_NIX_FORCE_HTTP=1 nix store info --store "file://$cacheDir"
+
+# But with --refresh, it should fail because nix-cache-info is gone
+# and the cached metadata TTL is overridden to 0.
+_NIX_FORCE_HTTP=1 expectStderr 1 nix store info --store "file://$cacheDir" --refresh | grepQuiet "uploading.*is not supported"
+
+# Remove --refresh and it should work again.
+_NIX_FORCE_HTTP=1 nix store info --store "file://$cacheDir"