os: change Open(`C:`) to open current directory on C:

Open(`C:`) currently opens root directory on C:. Change that to open
current directory on C:. Just like cmd.exe's "dir C:" command does.
Just like FindFirstFile("C:*") Windows API does. It is also consistent
with what filepath.Join("C:", "a") currently does.

Fixes #13763

Change-Id: I60b6e7d80215d110bbbb6265c9f32717401638c6
Reviewed-on: https://go-review.googlesource.com/18184Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>

src/os/file_windows.go

@@ -90,7 +90,13 @@ func openFile(name string, flag int, perm FileMode) (file *File, err error) {
 }
 
 func openDir(name string) (file *File, err error) {
-	maskp, e := syscall.UTF16PtrFromString(name + `\*`)
+	var mask string
+	if len(name) == 2 && name[1] == ':' { // it is a drive letter, like C:
+		mask = name + `*`
+	} else {
+		mask = name + `\*`
+	}
+	maskp, e := syscall.UTF16PtrFromString(mask)
 	if e != nil {
 		return nil, e
 	}

src/os/os_windows_test.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	osexec "os/exec"
 	"path/filepath"
+	"sort"
 	"strings"
 	"syscall"
 	"testing"
@@ -179,3 +180,46 @@ func TestStatDir(t *testing.T) {
 		t.Fatal("race condition occured")
 	}
 }
+
+func TestOpenVolumeName(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "TestOpenVolumeName")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Chdir(tmpdir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(wd)
+
+	want := []string{"file1", "file2", "file3", "gopher.txt"}
+	sort.Strings(want)
+	for _, name := range want {
+		err := ioutil.WriteFile(filepath.Join(tmpdir, name), nil, 0777)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	f, err := os.Open(filepath.VolumeName(tmpdir))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	have, err := f.Readdirnames(-1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sort.Strings(have)
+
+	if strings.Join(want, "/") != strings.Join(have, "/") {
+		t.Fatalf("unexpected file list %q, want %q", have, want)
+	}
+}