What causes "looking up hash: %v" in golang/go?

Retry the go command — transient network errors during lookupRef are common.

looking up hash: %v

Error message

looking up hash: %v

What it means

In the hash-verification branch of CheckReuse (old has Hash but no Ref/RepoSum), lookupRef is called to confirm the hash is still present. If lookupRef itself errors, that error is wrapped here with %v. The underlying cause is typically a network failure, missing ref, or a 'no lookupRef' sentinel for unsupported VCS types.

Source

Thrown at src/cmd/go/internal/modfetch/codehost/vcs.go:398

func (r *vcsRepo) CheckReuse(ctx context.Context, old *Origin, subdir string) error {
	if old == nil {
		return fmt.Errorf("missing origin")
	}
	if old.VCS != r.cmd.vcs || old.URL != r.remote {
		return fmt.Errorf("origin moved from %v %q to %v %q", old.VCS, old.URL, r.cmd.vcs, r.remote)
	}
	if old.Subdir != subdir {
		return fmt.Errorf("origin moved from %v %q %q to %v %q %q", old.VCS, old.URL, old.Subdir, r.cmd.vcs, r.remote, subdir)
	}

	if old.Ref == "" && old.RepoSum == "" && old.Hash != "" {
		// Hash has to remain in repo.
		hash, err := r.lookupRef(ctx, old.Hash)
		if err == nil && hash == old.Hash {
			return nil
		}
		if err != nil {
			return fmt.Errorf("looking up hash: %v", err)
		}
		return fmt.Errorf("hash changed") // weird but maybe they made a tag
	}

	if old.Ref != "" && old.RepoSum == "" {
		hash, err := r.lookupRef(ctx, old.Ref)
		if err == nil && hash != "" && hash == old.Hash {
			return nil
		}
	}

	r.repoSumOnce.Do(func() { r.loadRepoSum(ctx) })
	if r.repoSum != "" {
		if old.RepoSum == "" {
			return fmt.Errorf("non-specific origin")
		}
		if old.RepoSum != r.repoSum {
			return fmt.Errorf("repo changed")

View on GitHub (pinned to b6b368adc5)

Solutions

Retry the go command — transient network errors during lookupRef are common.
If the hash truly no longer exists upstream, the module is no longer available at that commit; pin a tag instead.
For svn/fossil, switch to GOPROXY to avoid the unsupported lookupRef path.
`go clean -modcache` to force re-resolution from scratch.

Defensive patterns

Strategy: retry

Try / catch

hash, err := r.lookupRef(ctx, old.Hash)
if err != nil {
    // transient network or unsupported-VCS error — fall back to full re-download
    return fmt.Errorf("looking up hash: %w", err)
}

Prevention

Pin modules to tags rather than bare hashes so the hash-verification branch is avoided.
Ensure network reachability for the remote before relying on CheckReuse.
Prefer VCS types that implement lookupRef (git, hg).

When it happens

Trigger: CheckReuse with old.Hash set and old.Ref/RepoSum empty; lookupRef fails — VCS command errored, hash not found on remote, or VCS lacks lookupRef support.

Common situations: Network blip during reuse verification; the upstream commit was garbage-collected or force-pushed away; an svn/fossil repo where lookupRef is unimplemented (then the wrapped error is 'no lookupRef').

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/f8959c466aafe1a3. Report an issue: GitHub.