cmd/go/internal/modload: return a module-only result from QueryPattern

This allows a single QueryPattern call to resolve a path that could be
either a package or a module. It is important to be able to make a
single QueryPattern call — rather than a QueryPattern followed by a
Query for the specific module path — to provide appropriate fallback
behavior: if the proxy returns package results but does not contain a
module result, we don't want to fall back to the next proxy to look
for the (probably-nonexistent) module.

For #37438

Change-Id: I419b8bb3ab4565f443bb5cee9a8b206f453b9801
Reviewed-on: https://go-review.googlesource.com/c/go/+/266657
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 6 insertions, 13 deletions

diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 171c070ab3..6d83af710f 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -912,22 +912,15 @@ func getQuery(ctx context.Context, path, vers string, prevM module.Version, forc
 	// If it turns out to only exist as a module, we can detect the resulting
 	// PackageNotInModuleError and avoid a second round-trip through (potentially)
 	// all of the configured proxies.
-	results, err := modload.QueryPattern(ctx, path, vers, modload.Selected, allowed)
+	results, modOnly, err := modload.QueryPattern(ctx, path, vers, modload.Selected, allowed)
 	if err != nil {
-		// If the path doesn't contain a wildcard, check whether it was actually a
-		// module path instead. If so, return that.
-		if !strings.Contains(path, "...") {
-			var modErr *modload.PackageNotInModuleError
-			if errors.As(err, &modErr) && modErr.Mod.Path == path {
-				if modErr.Mod.Version != vers {
-					logOncef("go: %s %s => %s", path, vers, modErr.Mod.Version)
-				}
-				return modErr.Mod, nil
-			}
-		}
-
 		return module.Version{}, err
 	}
+	if len(results) == 0 {
+		// The path doesn't contain a wildcard, but was actually a
+		// module path instead. Return that.
+		return modOnly.Mod, nil
+	}
 
 	m := results[0].Mod
 	if m.Path != path {