cmd/go/internal/modfetch: allow branch name as rev identifier

If someone runs "vgo get x@rev" or adds "require x rev" to go.mod
and rev is a commit hash or git tag, vgo automatically replaces it
with the pseudo-version for the underlying commit.
This CL makes vgo do the same for branch names:
the head commit of the branch at that moment is recorded.
This allows running

	vgo get github.com/gobuffalo/buffalo@development

to sync with the current buffalo development branch.

Fixes golang/go#24045.

Change-Id: I8a58ed3a574e5ee839d37ad452436f80ac8081dc
Reviewed-on: https://go-review.googlesource.com/105216
Reviewed-by: Bryan C. Mills <bcmills@google.com>

vendor/cmd/go/internal/modfetch/coderepo_test.go

@@ -298,6 +298,24 @@ var codeRepoTests = []struct {
 		time:    time.Date(2016, 12, 8, 18, 13, 25, 0, time.UTC),
 		gomod:   "//vgo 0.0.4\n\nmodule gopkg.in/check.v1\n",
 	},
+	{
+		path:    "gopkg.in/yaml.v2",
+		rev:     "v2",
+		version: "v0.0.0-20180328195020-5420a8b6744d",
+		name:    "5420a8b6744d3b0345ab293f6fcba19c978f1183",
+		short:   "5420a8b6744d",
+		time:    time.Date(2018, 3, 28, 19, 50, 20, 0, time.UTC),
+		gomod:   "module \"gopkg.in/yaml.v2\"\n\nrequire (\n\t\"gopkg.in/check.v1\" v0.0.0-20161208181325-20d25e280405\n)\n",
+	},
+	{
+		path:    "github.com/gobuffalo/buffalo",
+		rev:     "development",
+		version: "v0.0.0-20180406185414-59b4005674b6",
+		name:    "59b4005674b633728e2bfc3bb09cc204f7c2d6f5",
+		short:   "59b4005674b6",
+		time:    time.Date(2018, 4, 6, 18, 54, 14, 0, time.UTC),
+		gomod:   "//vgo 0.0.4\n\nmodule github.com/gobuffalo/buffalo\n",
+	},
 }
 
 func TestCodeRepo(t *testing.T) {

vendor/cmd/go/internal/modfetch/github/fetch.go

@@ -117,6 +117,8 @@ func (r *repo) LatestAt(t time.Time, branch string) (*codehost.RevInfo, error) {
 	return info, nil
 }
 
+var refKinds = []string{"tags", "heads"}
+
 func (r *repo) Stat(rev string) (*codehost.RevInfo, error) {
 	var tag string
 	if !codehost.AllHex(rev) {
@@ -130,41 +132,52 @@ func (r *repo) Stat(rev string) (*codehost.RevInfo, error) {
 				URL  string
 			}
 		}
-		err := web.Get(
-			"https://api.github.com/repos/"+url.PathEscape(r.owner)+"/"+url.PathEscape(r.repo)+"/git/refs/tags/"+tag,
-			web.DecodeJSON(&ref),
-		)
-		if err != nil {
-			return nil, err
-		}
-		switch ref.Object.Type {
-		default:
-			return nil, fmt.Errorf("invalid tag %q: not a commit or tag (%q)", tag, ref.Object.Type)
 
-		case "commit":
-			rev = ref.Object.SHA
-
-		case "tag":
-			var info struct {
-				Object struct {
-					SHA  string
-					Type string
-				}
-			}
-			err = web.Get(
-				ref.Object.URL,
-				web.DecodeJSON(&info),
+		var firstErr error
+		for _, kind := range refKinds {
+			err := web.Get(
+				"https://api.github.com/repos/"+url.PathEscape(r.owner)+"/"+url.PathEscape(r.repo)+"/git/refs/"+kind+"/"+tag,
+				web.DecodeJSON(&ref),
 			)
 			if err != nil {
-				return nil, err
+				if firstErr == nil {
+					firstErr = err
+				}
+				continue
 			}
-			if info.Object.Type != "commit" {
-				return nil, fmt.Errorf("invalid annotated tag %q: not a commit (%q)", tag, info.Object.Type)
+			switch ref.Object.Type {
+			default:
+				return nil, fmt.Errorf("invalid ref %q: not a commit or tag (%q)", tag, ref.Object.Type)
+
+			case "commit":
+				rev = ref.Object.SHA
+
+			case "tag":
+				var info struct {
+					Object struct {
+						SHA  string
+						Type string
+					}
+				}
+				err = web.Get(
+					ref.Object.URL,
+					web.DecodeJSON(&info),
+				)
+				if err != nil {
+					return nil, err
+				}
+				if info.Object.Type != "commit" {
+					return nil, fmt.Errorf("invalid annotated tag %q: not a commit (%q)", tag, info.Object.Type)
+				}
+				rev = info.Object.SHA
 			}
-			rev = info.Object.SHA
+			if rev == "" {
+				return nil, fmt.Errorf("invalid ref %q: missing SHA in GitHub response", tag)
+			}
+			break
 		}
 		if rev == "" {
-			return nil, fmt.Errorf("invalid tag %q: missing SHA in GitHub response", tag)
+			return nil, fmt.Errorf("unknown ref %q (%v)", tag, firstErr)
 		}
 	}
 

vendor/cmd/go/internal/modfetch/query.go

@@ -35,6 +35,8 @@ func Query(path, vers string, allowed func(module.Version) bool) (*RevInfo, erro
 	}
 
 	if strings.HasPrefix(vers, "v") && semver.IsValid(vers) {
+		// TODO: This turns query for "v2" into Stat "v2.0.0",
+		// but probably it should allow checking for a branch named "v2".
 		return repo.Stat(semver.Canonical(vers))
 	}
 	if strings.HasPrefix(vers, ">") || strings.HasPrefix(vers, "<") || vers == "latest" {