cmd/go/internal/modfetch: simplify handling of weird version tags

This fixes an obscure bug in 'go list -versions' if the repo contains
a tag with an explicit "+incompatible" suffix. However, I've never
seen such a repo in the wild; mostly it's an attempt to wrap my brain
around the code and simplify things a bit for the future.

Updates #51324
Updates #51312

Change-Id: I1b078b5db36470cf61aaa85b5244c99b5ee2c842
Reviewed-on: https://go-review.googlesource.com/c/go/+/387917
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

2 files changed, 5 insertions, 9 deletions

diff --git a/src/cmd/go/internal/modfetch/codehost/codehost.go b/src/cmd/go/internal/modfetch/codehost/codehost.go
index 31dc811752..e08a84b32c 100644
--- a/src/cmd/go/internal/modfetch/codehost/codehost.go
+++ b/src/cmd/go/internal/modfetch/codehost/codehost.go
@@ -65,7 +65,7 @@ type Repo interface {
 
 	// RecentTag returns the most recent tag on rev or one of its predecessors
 	// with the given prefix. allowed may be used to filter out unwanted versions.
-	RecentTag(rev, prefix string, allowed func(string) bool) (tag string, err error)
+	RecentTag(rev, prefix string, allowed func(tag string) bool) (tag string, err error)
 
 	// DescendsFrom reports whether rev or any of its ancestors has the given tag.
 	//
diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index 9c8fd42833..853d43bc5b 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -523,7 +523,7 @@ func (r *gitRepo) ReadFile(rev, file string, maxSize int64) ([]byte, error) {
 	return out, nil
 }
 
-func (r *gitRepo) RecentTag(rev, prefix string, allowed func(string) bool) (tag string, err error) {
+func (r *gitRepo) RecentTag(rev, prefix string, allowed func(tag string) bool) (tag string, err error) {
 	info, err := r.Stat(rev)
 	if err != nil {
 		return "", err
@@ -553,15 +553,11 @@ func (r *gitRepo) RecentTag(rev, prefix string, allowed func(string) bool) (tag
 			if !strings.HasPrefix(line, prefix) {
 				continue
 			}
-
-			semtag := line[len(prefix):]
-			// Consider only tags that are valid and complete (not just major.minor prefixes).
-			// NOTE: Do not replace the call to semver.Compare with semver.Max.
-			// We want to return the actual tag, not a canonicalized version of it,
-			// and semver.Max currently canonicalizes (see golang.org/issue/32700).
-			if c := semver.Canonical(semtag); c == "" || !strings.HasPrefix(semtag, c) || !allowed(semtag) {
+			if !allowed(line) {
 				continue
 			}
+
+			semtag := line[len(prefix):]
 			if semver.Compare(semtag, highest) > 0 {
 				highest = semtag
 			}