os/exec: always try appropriate command extensions during Cmd.Start on windows

Update #7362
Fixes #7377
Fixes #7570

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/83020043

src/cmd/pack/pack_test.go

@@ -218,7 +218,7 @@ func TestHello(t *testing.T) {
 		t.Fatal("cannot find GOCHAR in 'go env' output:\n", out)
 	}
 	char := fields[1]
-	run("go", "build", "-o", "pack", "cmd/pack") // writes pack binary to dir
+	run("go", "build", "cmd/pack") // writes pack binary to dir
 	run("go", "tool", char+"g", "hello.go")
 	run("./pack", "grc", "hello.a", "hello."+char)
 	run("go", "tool", char+"l", "-o", "a.out", "hello.a")

src/pkg/os/exec/exec.go

@@ -13,7 +13,9 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
+	"strings"
 	"sync"
 	"syscall"
 )
@@ -237,6 +239,32 @@ func (c *Cmd) Run() error {
 	return c.Wait()
 }
 
+// lookExtensions finds windows executable by its dir and path.
+// It uses LookPath to try appropriate extensions.
+// lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
+func lookExtensions(path, dir string) (string, error) {
+	if filepath.Base(path) == path {
+		path = filepath.Join(".", path)
+	}
+	if dir == "" {
+		return LookPath(path)
+	}
+	if filepath.VolumeName(path) != "" {
+		return LookPath(path)
+	}
+	if len(path) > 1 && os.IsPathSeparator(path[0]) {
+		return LookPath(path)
+	}
+	dirandpath := filepath.Join(dir, path)
+	// We assume that LookPath will only add file extension.
+	lp, err := LookPath(dirandpath)
+	if err != nil {
+		return "", err
+	}
+	ext := strings.TrimPrefix(lp, dirandpath)
+	return path + ext, nil
+}
+
 // Start starts the specified command but does not wait for it to complete.
 //
 // The Wait method will return the exit code and release associated resources
@@ -247,6 +275,15 @@ func (c *Cmd) Start() error {
 		c.closeDescriptors(c.closeAfterWait)
 		return c.lookPathErr
 	}
+	if runtime.GOOS == "windows" {
+		lp, err := lookExtensions(c.Path, c.Dir)
+		if err != nil {
+			c.closeDescriptors(c.closeAfterStart)
+			c.closeDescriptors(c.closeAfterWait)
+			return err
+		}
+		c.Path = lp
+	}
 	if c.Process != nil {
 		return errors.New("exec: already started")
 	}

src/pkg/os/exec/exec_test.go

@@ -682,6 +682,24 @@ func TestHelperProcess(*testing.T) {
 		}
 		fmt.Fprintf(os.Stderr, "child: %s", response)
 		os.Exit(0)
+	case "exec":
+		cmd := exec.Command(args[1])
+		cmd.Dir = args[0]
+		output, err := cmd.CombinedOutput()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Child: %s %s", err, string(output))
+			os.Exit(1)
+		}
+		fmt.Printf("%s", string(output))
+		os.Exit(0)
+	case "lookpath":
+		p, err := exec.LookPath(args[0])
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "LookPath failed: %v\n", err)
+			os.Exit(1)
+		}
+		fmt.Print(p)
+		os.Exit(0)
 	default:
 		fmt.Fprintf(os.Stderr, "Unknown command %q\n", cmd)
 		os.Exit(2)