os: in Symlink, stat the correct target path for drive-relative targets on Windows

Previously, when the target (“old”) path passed to os.Symlink was a
“root-relative” Windows path,¹ we would erroneously prepend
destination (“new”) path when determining which path to Stat,
resulting in an invalid path which was then masked by the lack of
error propagation for the Stat call (#39183).

If the link target is a directory (rather than a file), that would
result in the symlink being created without the
SYMBOLIC_LINK_FLAG_DIRECTORY flag, which then fails in os.Open.

¹https://docs.microsoft.com/en-us/windows/win32/fileio/creating-symbolic-links

Updates #39183

Change-Id: I04f179cd2b0c44f984f34ec330acad2408aa3a20
Reviewed-on: https://go-review.googlesource.com/c/go/+/235317
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/file_windows.go

@@ -330,8 +330,18 @@ func Symlink(oldname, newname string) error {
 
 	// need the exact location of the oldname when it's relative to determine if it's a directory
 	destpath := oldname
-	if !isAbs(oldname) {
-		destpath = dirname(newname) + `\` + oldname
+	if v := volumeName(oldname); v == "" {
+		if len(oldname) > 0 && IsPathSeparator(oldname[0]) {
+			// oldname is relative to the volume containing newname.
+			if v = volumeName(newname); v != "" {
+				// Prepend the volume explicitly, because it may be different from the
+				// volume of the current working directory.
+				destpath = v + oldname
+			}
+		} else {
+			// oldname is relative to newname.
+			destpath = dirname(newname) + `\` + oldname
+		}
 	}
 
 	fi, err := Stat(destpath)

src/os/os_windows_test.go

@@ -965,9 +965,10 @@ func TestWindowsDevNullFile(t *testing.T) {
 // works on Windows when developer mode is active.
 // This is supported starting Windows 10 (1703, v10.0.14972).
 func TestSymlinkCreation(t *testing.T) {
-	if !isWindowsDeveloperModeActive() {
+	if !testenv.HasSymlink() && !isWindowsDeveloperModeActive() {
 		t.Skip("Windows developer mode is not active")
 	}
+	t.Parallel()
 
 	temp, err := ioutil.TempDir("", "TestSymlinkCreation")
 	if err != nil {
@@ -1005,6 +1006,122 @@ func isWindowsDeveloperModeActive() bool {
 	return val != 0
 }
 
+// TestRootRelativeDirSymlink verifies that symlinks to paths relative to the
+// drive root (beginning with "\" but no volume name) are created with the
+// correct symlink type.
+// (See https://golang.org/issue/39183#issuecomment-632175728.)
+func TestRootRelativeDirSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+	t.Parallel()
+
+	temp := t.TempDir()
+	dir := filepath.Join(temp, "dir")
+	if err := os.Mkdir(dir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	volumeRelDir := strings.TrimPrefix(dir, filepath.VolumeName(dir)) // leaves leading backslash
+
+	link := filepath.Join(temp, "link")
+	err := os.Symlink(volumeRelDir, link)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Symlink(%#q, %#q)", volumeRelDir, link)
+
+	f, err := os.Open(link)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	if fi, err := f.Stat(); err != nil {
+		t.Fatal(err)
+	} else if !fi.IsDir() {
+		t.Errorf("Open(%#q).Stat().IsDir() = false; want true", f.Name())
+	}
+}
+
+// TestWorkingDirectoryRelativeSymlink verifies that symlinks to paths relative
+// to the current working directory for the drive, such as "C:File.txt", are
+// correctly converted to absolute links of the correct symlink type (per
+// https://docs.microsoft.com/en-us/windows/win32/fileio/creating-symbolic-links).
+func TestWorkingDirectoryRelativeSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	// Construct a directory to be symlinked.
+	temp := t.TempDir()
+	if v := filepath.VolumeName(temp); len(v) < 2 || v[1] != ':' {
+		t.Skipf("Can't test relative symlinks: t.TempDir() (%#q) does not begin with a drive letter.", temp)
+	}
+
+	absDir := filepath.Join(temp, `dir\sub`)
+	if err := os.MkdirAll(absDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Change to the temporary directory and construct a
+	// working-directory-relative symlink.
+	oldwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		if err := os.Chdir(oldwd); err != nil {
+			t.Fatal(err)
+		}
+	}()
+	if err := os.Chdir(temp); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Chdir(%#q)", temp)
+
+	wdRelDir := filepath.VolumeName(temp) + `dir\sub` // no backslash after volume.
+	absLink := filepath.Join(temp, "link")
+	err = os.Symlink(wdRelDir, absLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Symlink(%#q, %#q)", wdRelDir, absLink)
+
+	// Now change back to the original working directory and verify that the
+	// symlink still refers to its original path and is correctly marked as a
+	// directory.
+	if err := os.Chdir(oldwd); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Chdir(%#q)", oldwd)
+
+	resolved, err := os.Readlink(absLink)
+	if err != nil {
+		t.Errorf("Readlink(%#q): %v", absLink, err)
+	} else if resolved != absDir {
+		t.Errorf("Readlink(%#q) = %#q; want %#q", absLink, resolved, absDir)
+	}
+
+	linkFile, err := os.Open(absLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer linkFile.Close()
+
+	linkInfo, err := linkFile.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !linkInfo.IsDir() {
+		t.Errorf("Open(%#q).Stat().IsDir() = false; want true", absLink)
+	}
+
+	absInfo, err := os.Stat(absDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !os.SameFile(absInfo, linkInfo) {
+		t.Errorf("SameFile(Stat(%#q), Open(%#q).Stat()) = false; want true", absDir, absLink)
+	}
+}
+
 // TestStatOfInvalidName is regression test for issue #24999.
 func TestStatOfInvalidName(t *testing.T) {
 	_, err := os.Stat("*.go")