cmd/go: fix handling of gopkg.in/macaroon-bakery.v2-unstable

When we added v2.0.0+incompatible we generalized the API
enough to make it easy to also accepting these gopkg-specific
v2-unstable suffixes. Do that.

Fixes #23989.

Change-Id: Ieabed11a5250c2999d73450c10b20f4c645ad445
Reviewed-on: https://go-review.googlesource.com/128901Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfetch/coderepo.go

@@ -86,6 +86,13 @@ func (r *codeRepo) ModulePath() string {
 }
 
 func (r *codeRepo) Versions(prefix string) ([]string, error) {
+	// Special case: gopkg.in/macaroon-bakery.v2-unstable
+	// does not use the v2 tags (those are for macaroon-bakery.v2).
+	// It has no possible tags at all.
+	if strings.HasPrefix(r.modPath, "gopkg.in/") && strings.HasSuffix(r.modPath, "-unstable") {
+		return nil, nil
+	}
+
 	p := prefix
 	if r.codeDir != "" {
 		p = r.codeDir + "/" + p

src/cmd/go/internal/modfetch/pseudo.go

@@ -49,6 +49,7 @@ func PseudoVersion(major, older string, t time.Time, rev string) string {
 	if major == "" {
 		major = "v0"
 	}
+	major = strings.TrimSuffix(major, "-unstable") // make gopkg.in/macaroon-bakery.v2-unstable use "v2"
 	segment := fmt.Sprintf("%s-%s", t.UTC().Format("20060102150405"), rev)
 	build := semver.Build(older)
 	older = semver.Canonical(older)

src/cmd/go/internal/module/module.go

@@ -143,7 +143,7 @@ func CheckPath(path string) error {
 		}
 	}
 	if _, _, ok := SplitPathVersion(path); !ok {
-		return fmt.Errorf("malformed module path %q: invalid version %s", path, path[strings.LastIndex(path, "/")+1:])
+		return fmt.Errorf("malformed module path %q: invalid version", path)
 	}
 	return nil
 }
@@ -300,6 +300,9 @@ func splitGopkgIn(path string) (prefix, pathMajor string, ok bool) {
 		return path, "", false
 	}
 	i := len(path)
+	if strings.HasSuffix(path, "-unstable") {
+		i -= len("-unstable")
+	}
 	for i > 0 && ('0' <= path[i-1] && path[i-1] <= '9') {
 		i--
 	}
@@ -317,6 +320,9 @@ func splitGopkgIn(path string) (prefix, pathMajor string, ok bool) {
 // MatchPathMajor reports whether the semantic version v
 // matches the path major version pathMajor.
 func MatchPathMajor(v, pathMajor string) bool {
+	if strings.HasPrefix(pathMajor, ".v") && strings.HasSuffix(pathMajor, "-unstable") {
+		pathMajor = strings.TrimSuffix(pathMajor, "-unstable")
+	}
 	if strings.HasPrefix(v, "v0.0.0-") && pathMajor == ".v1" {
 		// Allow old bug in pseudo-versions that generated v0.0.0- pseudoversion for gopkg .v1.
 		// For example, gopkg.in/yaml.v2@v2.2.1's go.mod requires gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405.

src/cmd/go/proxy_test.go

@@ -81,13 +81,13 @@ func readModList() {
 		encPath := strings.Replace(name[:i], "_", "/", -1)
 		path, err := module.DecodePath(encPath)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "go proxy_test: %v", err)
+			fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
 			continue
 		}
 		encVers := name[i+1:]
 		vers, err := module.DecodeVersion(encVers)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "go proxy_test: %v", err)
+			fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
 			continue
 		}
 		modList = append(modList, module.Version{Path: path, Version: vers})
@@ -140,7 +140,7 @@ func proxyHandler(w http.ResponseWriter, r *http.Request) {
 	encVers, ext := file[:i], file[i+1:]
 	vers, err := module.DecodeVersion(encVers)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "go proxy_test: %v", err)
+		fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
 		http.NotFound(w, r)
 		return
 	}

src/cmd/go/testdata/mod/gopkg.in_dummy.v2-unstable_v2.0.0.txt

+gopkg.in/dummy.v2-unstable v2.0.0
+written by hand
+
+-- .mod --
+module gopkg.in/dummy.v2-unstable
+-- .info --
+{"Version":"v2.0.0"}
+-- dummy.go --
+package dummy

src/cmd/go/testdata/script/mod_gopkg_unstable.txt

+env GO111MODULE=on
+
+cp go.mod.empty go.mod
+go get -d gopkg.in/dummy.v2-unstable
+
+cp x.go.txt x.go
+cp go.mod.empty go.mod
+go list
+
+[!net] skip
+
+env GOPROXY=
+go get gopkg.in/macaroon-bakery.v2-unstable/bakery
+go list -m all
+stdout 'gopkg.in/macaroon-bakery.v2-unstable v2.0.0-[0-9]+-[0-9a-f]+$'
+
+-- go.mod.empty --
+module m
+
+-- x.go.txt --
+package x
+import _ "gopkg.in/dummy.v2-unstable"