cmd/go/internal/modfetch/codehost: disable fetch of server-resolved commit hash

We cannot rely on the server to filter out the refs we don't want
(we only want refs/heads/* and refs/tags/*), so do not give it
the full hash.

Fixes #31191.

Change-Id: If1208c35954228aa6e8734f8d5f1725d0ec79c87
Reviewed-on: https://go-review.googlesource.com/c/go/+/174517
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index 2cb6637aae..5273e633b5 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -32,7 +32,7 @@ func LocalGitRepo(remote string) (Repo, error) {
 	return newGitRepoCached(remote, true)
 }
 
-const gitWorkDirType = "git2"
+const gitWorkDirType = "git3"
 
 var gitRepoCache par.Cache
 
@@ -339,8 +339,14 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 		}
 	}
 
-	// If we know a specific commit we need, fetch it.
-	if r.fetchLevel <= fetchSome && hash != "" && !r.local {
+	// If we know a specific commit we need and its ref, fetch it.
+	// We do NOT fetch arbitrary hashes (when we don't know the ref)
+	// because we want to avoid ever importing a commit that isn't
+	// reachable from refs/tags/* or refs/heads/* or HEAD.
+	// Both Gerrit and GitHub expose every CL/PR as a named ref,
+	// and we don't want those commits masquerading as being real
+	// pseudo-versions in the main repo.
+	if r.fetchLevel <= fetchSome && ref != "" && hash != "" && !r.local {
 		r.fetchLevel = fetchSome
 		var refspec string
 		if ref != "" && ref != "HEAD" {