cmd/go: handle path to cmd directory

Now it works to run 'go install' (no args) in cmd/go.

Fixes #2679.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5543046

src/cmd/go/main.go

@@ -192,7 +192,7 @@ func importPaths(args []string) []string {
 	}
 	var out []string
 	for _, a := range args {
-		if (strings.HasPrefix(a, "./") || strings.HasPrefix(a, "../")) && strings.Contains(a, "...") {
+		if isLocalPath(a) && strings.Contains(a, "...") {
 			out = append(out, allPackagesInFS(a)...)
 			continue
 		}
@@ -246,6 +246,17 @@ func run(cmdargs ...interface{}) {
 	}
 }
 
+func runOut(cmdargs ...interface{}) []byte {
+	cmdline := stringList(cmdargs...)
+	out, err := exec.Command(cmdline[0], cmdline[1:]...).CombinedOutput()
+	if err != nil {
+		os.Stderr.Write(out)
+		errorf("%v", err)
+		out = nil
+	}
+	return out
+}
+
 // matchPattern(pattern)(name) reports whether
 // name matches pattern.  Pattern is a limited glob
 // pattern in which '...' means 'any string' and there
@@ -422,3 +433,10 @@ func stringList(args ...interface{}) []string {
 	}
 	return x
 }
+
+// isLocalPath returns true if arg is an import path denoting
+// a local file system directory.  That is, it returns true if the
+// path begins with ./ or ../ .
+func isLocalPath(arg string) bool {
+	return arg == "." || arg == ".." || strings.HasPrefix(arg, "./") || strings.HasPrefix(arg, "../")
+}

src/cmd/go/pkg.go

@@ -79,7 +79,7 @@ func loadPackage(arg string) (*Package, error) {
 	t, importPath, err := build.FindTree(arg)
 	dir := ""
 	// Maybe it is a standard command.
-	if err != nil && !filepath.IsAbs(arg) && strings.HasPrefix(arg, "cmd/") {
+	if err != nil && strings.HasPrefix(arg, "cmd/") {
 		goroot := build.Path[0]
 		p := filepath.Join(goroot.Path, "src", arg)
 		if st, err1 := os.Stat(p); err1 == nil && st.IsDir() {
@@ -89,6 +89,19 @@ func loadPackage(arg string) (*Package, error) {
 			err = nil
 		}
 	}
+	// Maybe it is a path to a standard command.
+	if err != nil && (filepath.IsAbs(arg) || isLocalPath(arg)) {
+		arg, _ := filepath.Abs(arg)
+		goroot := build.Path[0]
+		cmd := filepath.Join(goroot.Path, "src", "cmd") + string(filepath.Separator)
+		if st, err1 := os.Stat(arg); err1 == nil && st.IsDir() && strings.HasPrefix(arg, cmd) {
+			t = goroot
+			importPath = filepath.FromSlash(arg[len(cmd):])
+			dir = arg
+			err = nil
+		}
+	}
+
 	if err != nil {
 		return nil, err
 	}