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fetchGit: don't resolve HEAD ref when a specific rev is requested #346

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a7a6c4f
Test against nixos/nix#10773
grahamc Feb 11, 2026
98f31ca
fetchGit: don't resolve HEAD ref when a specific rev is requested
grahamc Feb 11, 2026
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11 changes: 8 additions & 3 deletions src/libfetchers/git.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -858,8 +858,13 @@ struct GitInputScheme : InputScheme

auto originalRef = input.getRef();
bool shallow = canDoShallow(input);
auto ref = originalRef ? *originalRef : getDefaultRef(repoInfo, shallow);
input.attrs.insert_or_assign("ref", ref);

/* When a specific rev is requested without an explicit ref, don't
resolve the default ref (which would contact the remote). The
rev can be fetched directly by its hash. */
auto ref = originalRef ? *originalRef : !origRev ? getDefaultRef(repoInfo, shallow) : std::string{};
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@cole-h cole-h Feb 12, 2026

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I think the std::string{} will break things:

The ref is used in a call to resolveRev below, which basically asks git to resolve the string $REF^{commit} (to "peel the ref ultimately down to the underlying commit"). ^{commit} (REF="") will fail to parse, so I think we need to figure something out there.

if (!ref.empty())
input.attrs.insert_or_assign("ref", ref);

std::filesystem::path repoDir;

Expand Down Expand Up @@ -941,7 +946,7 @@ struct GitInputScheme : InputScheme
} catch (Error & e) {
warn("could not update mtime for file %s: %s", localRefFile, e.info().msg);
}
if (!originalRef && !storeCachedHead(repoUrl.to_string(), shallow, ref))
if (!originalRef && !ref.empty() && !storeCachedHead(repoUrl.to_string(), shallow, ref))
warn("could not update cached head '%s' for '%s'", ref, repoInfo.locationToArg());
}

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13 changes: 13 additions & 0 deletions tests/functional/fetchGit.sh
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Expand Up @@ -161,6 +161,19 @@ git -C "$repo" commit -m 'Bla4'
rev3=$(git -C "$repo" rev-parse HEAD)
nix eval --tarball-ttl 3600 --expr "builtins.fetchGit { url = $repo; rev = \"$rev3\"; }" >/dev/null

# Fetching a rev that is already cached should not hit the network,
# even when the cached HEAD ref has expired (NixOS/nix#10773).
export _NIX_FORCE_HTTP=1
nix eval --expr "builtins.fetchGit { url = file://$repo; rev = \"$rev3\"; }" >/dev/null
# Make the repo unavailable so any network access triggers a warning.
mv "$repo" "${repo}"-tmp
path6=$(nix eval --tarball-ttl 0 --raw --expr "(builtins.fetchGit { url = file://$repo; rev = \"$rev3\"; }).outPath" 2>"$TEST_ROOT/stderr-10773")
[[ $(cat "$path6"/hello) = delft ]]
# The old code would try to resolve HEAD here and emit this warning.
(! grep -q "could not get HEAD ref" < "$TEST_ROOT/stderr-10773")
mv "${repo}"-tmp "$repo"
unset _NIX_FORCE_HTTP

# Update 'path' to reflect latest master
path=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$repo).outPath")

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