cmd/go: handle error when git remote origin doesn't exist

- Let runOutput return the error message
- When `git config ...` returns empty buffer, it means the config key is
  correct, but there is no corresponding value.
- Return the correct error when the url of remote origin is not found.
- Update error message

Fixes: #10922

Change-Id: I3f8880f6717a4f079b840d1249174378d36bca1b
Reviewed-on: https://go-review.googlesource.com/10475Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/get.go

@@ -300,22 +300,23 @@ func downloadPackage(p *Package) error {
 		// Double-check where it came from.
 		if *getU && vcs.remoteRepo != nil {
 			dir := filepath.Join(p.build.SrcRoot, rootPath)
-			if remote, err := vcs.remoteRepo(vcs, dir); err == nil {
-				repo = remote
-
-				if !*getF {
-					if rr, err := repoRootForImportPath(p.ImportPath, security); err == nil {
-						repo := rr.repo
-						if rr.vcs.resolveRepo != nil {
-							resolved, err := rr.vcs.resolveRepo(rr.vcs, dir, repo)
-							if err == nil {
-								repo = resolved
-							}
-						}
-						if remote != repo {
-							return fmt.Errorf("%s is a custom import path for %s, but %s is checked out from %s", rr.root, repo, dir, remote)
+			remote, err := vcs.remoteRepo(vcs, dir)
+			if err != nil {
+				return err
+			}
+			repo = remote
+			if !*getF {
+				if rr, err := repoRootForImportPath(p.ImportPath, security); err == nil {
+					repo := rr.repo
+					if rr.vcs.resolveRepo != nil {
+						resolved, err := rr.vcs.resolveRepo(rr.vcs, dir, repo)
+						if err == nil {
+							repo = resolved
 						}
 					}
+					if remote != repo {
+						return fmt.Errorf("%s is a custom import path for %s, but %s is checked out from %s", rr.root, repo, dir, remote)
+					}
 				}
 			}
 		}

src/cmd/go/vcs.go

@@ -147,8 +147,14 @@ var vcsGit = &vcsCmd{
 func gitRemoteRepo(vcsGit *vcsCmd, rootDir string) (remoteRepo string, err error) {
 	cmd := "config remote.origin.url"
 	errParse := errors.New("unable to parse output of git " + cmd)
-	outb, err := vcsGit.runOutput(rootDir, cmd)
+	errRemoteOriginNotFound := errors.New("remote origin not found")
+	outb, err := vcsGit.run1(rootDir, cmd, nil, false)
 	if err != nil {
+		// if it doesn't output any message, it means the config argument is correct,
+		// but the config value itself doesn't exist
+		if outb != nil && len(outb) == 0 {
+			return "", errRemoteOriginNotFound
+		}
 		return "", err
 	}
 	repoURL, err := url.Parse(strings.TrimSpace(string(outb)))
@@ -333,7 +339,7 @@ func (v *vcsCmd) run1(dir string, cmdline string, keyval []string, verbose bool)
 			fmt.Fprintf(os.Stderr, "# cd %s; %s %s\n", dir, v.cmd, strings.Join(args, " "))
 			os.Stderr.Write(out)
 		}
-		return nil, err
+		return out, err
 	}
 	return out, nil
 }