[go] cmd/go: implement -reuse for Mercurial repos

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Gopher Robot (Gerrit)

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Nov 11, 2025, 10:21:09 AM (yesterday) Nov 11

to Russ Cox, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, Go LUCI, Michael Matloob, Michael Matloob, Ian Alexander, Michael Pratt, golang-co...@googlegroups.com

Gopher Robot submitted the change with unreviewed changes

Unreviewed changes

4 is the latest approved patch-set.
The change was submitted with unreviewed changes in the following files:

```
The name of the file: lib/hg/goreposum.py
Insertions: 7, Deletions: 1.

@@ -15,6 +15,12 @@
 # we will avoid re-cloning a server when there have been no
 # changes at all.
 #
+# Separately, this extension also adds a 'golookup' command that
+# returns the hash of a specific reference, like 'default' or a tag.
+# And golookup of a hash confirms that it still exists on the server.
+# We can use that to revalidate that specific versions still exist and
+# have the same meaning they did the last time we checked.
+#
 # Usage:
 #
 #	hg --config "extensions.goreposum=$GOROOT/lib/hg/goreposum.py" goreposum REPOURL
@@ -41,7 +47,7 @@
       h.update(b' ')
       h.update(r)
     h.update(b'\n')
-  if (b'bookmarks' in peer.listkeys(b'namespaces')) or True:
+  if (b'bookmarks' in peer.listkeys(b'namespaces')):
     for name, rev in peer.listkeys(b'bookmarks').items():
       h.update(name)
       h.update(b'=')
```
```
The name of the file: src/cmd/go/internal/modfetch/codehost/vcs.go
Insertions: 6, Deletions: 8.

@@ -380,17 +380,15 @@
 	return strings.TrimSpace(string(out)), nil
 }
 
-func (r *vcsRepo) origin(ctx context.Context, hash string, repoSum bool) *Origin {
+// repoSumOrigin returns an Origin containing a RepoSum.
+func (r *vcsRepo) repoSumOrigin(ctx context.Context) *Origin {
 	origin := &Origin{
 		VCS:     r.cmd.vcs,
 		URL:     r.remote,
-		Hash:    hash,
 		RepoSum: r.repoSum,
 	}
-	if repoSum {
-		r.repoSumOnce.Do(func() { r.loadRepoSum(ctx) })
-		origin.RepoSum = r.repoSum
-	}
+	r.repoSumOnce.Do(func() { r.loadRepoSum(ctx) })
+	origin.RepoSum = r.repoSum
 	return origin
 }
 
@@ -446,7 +444,7 @@
 
 	r.tagsOnce.Do(func() { r.loadTags(ctx) })
 	tags := &Tags{
-		Origin: r.origin(ctx, "", true),
+		Origin: r.repoSumOrigin(ctx),
 		List:   []Tag{},
 	}
 	for tag := range r.tags {
@@ -513,7 +511,7 @@
 func (r *vcsRepo) statLocal(ctx context.Context, rev string) (*RevInfo, error) {
 	out, err := Run(ctx, r.dir, r.cmd.statLocal(rev, r.remote))
 	if err != nil {
-		info := &RevInfo{Origin: r.origin(ctx, "", true)}
+		info := &RevInfo{Origin: r.repoSumOrigin(ctx)}
 		return info, &UnknownRevisionError{Rev: rev}
 	}
 	info, err := r.cmd.parseStat(rev, string(out))
```

Change information

Commit message:

cmd/go: implement -reuse for Mercurial repos

When we added -reuse in CL 411398, we only handled Git repos.
This was partly because we were focused on Git traffic,
partly because Git is the dominant module VCS, and
partly because I couldn't see how to retrieve the metadata needed
in other version control systems.

This CL adds -reuse support for Mercurial, the second
most popular VCS for modules, now that I see how to
implement it. Although the Mercurial command line does
not have sufficient information, the Mercurial Python API does,
so we ship and invoke a Mercurial extension written in Python
that can compute a hash of the remote repo without downloading
it entirely, as well as resolve a remote name to a hash or check
the continued existence of a hash. Then we can avoid
downloading the repo at all if it hasn't changed since the last check
or if the specific reference we need still resolves or exists.

Fixes #75119.