os: adjust TempDir for Z:\

If TMP environment variable is set to Z:\, TempDir returns Z:.
But Z: refers to current directory on Z:, while Z:\ refers to root
directory on Z:. Adjust TempDir to return Z:\.

Fixes #29291

Change-Id: If04d0c7977a8ac2d9d558307502e81beb68776ef
Reviewed-on: https://go-review.googlesource.com/c/154384
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file_windows.go

@@ -325,7 +325,10 @@ func tempDir() string {
 		if n > uint32(len(b)) {
 			continue
 		}
-		if n > 0 && b[n-1] == '\\' {
+		if n == 3 && b[1] == ':' && b[2] == '\\' {
+			// Do nothing for path, like C:\.
+		} else if n > 0 && b[n-1] == '\\' {
+			// Otherwise remove terminating \.
 			n--
 		}
 		return string(utf16.Decode(b[:n]))

src/os/os_windows_test.go

@@ -5,6 +5,7 @@
 package os_test
 
 import (
+	"errors"
 	"fmt"
 	"internal/poll"
 	"internal/syscall/windows"
@@ -1003,3 +1004,46 @@ func TestStatOfInvalidName(t *testing.T) {
 		t.Fatal(`os.Stat("*.go") unexpectedly succeeded`)
 	}
 }
+
+// findUnusedDriveLetter searches mounted drive list on the system
+// (starting from Z: and ending at D:) for unused drive letter.
+// It returns path to the found drive root directory (like Z:\) or error.
+func findUnusedDriveLetter() (string, error) {
+	// Do not use A: and B:, because they are reserved for floppy drive.
+	// Do not use C:, becasue it is normally used for main drive.
+	for l := 'Z'; l >= 'D'; l-- {
+		p := string(l) + `:\`
+		_, err := os.Stat(p)
+		if os.IsNotExist(err) {
+			return p, nil
+		}
+	}
+	return "", errors.New("Could not find unused drive letter.")
+}
+
+func TestRootDirAsTemp(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		fmt.Print(os.TempDir())
+		os.Exit(0)
+	}
+
+	newtmp, err := findUnusedDriveLetter()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := osexec.Command(os.Args[0], "-test.run=TestRootDirAsTemp")
+	cmd.Env = os.Environ()
+	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
+	cmd.Env = append(cmd.Env, "TMP="+newtmp)
+	cmd.Env = append(cmd.Env, "TEMP="+newtmp)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Failed to spawn child process: %v %q", err, string(output))
+	}
+	if want, have := newtmp, string(output); have != want {
+		t.Fatalf("unexpected child process output %q, want %q", have, want)
+	}
+}