os: handle trailing slashes in os.RemoveDir on Windows

CL 661575 inadvertently caused os.RemoveDir on Windows to
fail when given a path with a trailing / or \, due to the
splitPath function not correctly stripping trailing
separators.

Fixes #73317

Change-Id: I21977b94bb08ff1e563de6f5f16a4bdf5024a15e
Reviewed-on: https://go-review.googlesource.com/c/go/+/664715
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/os/export_windows_test.go

@@ -11,4 +11,5 @@ var (
 	NewConsoleFile     = newConsoleFile
 	CommandLineToArgv  = commandLineToArgv
 	AllowReadDirFileID = &allowReadDirFileID
+	SplitPath          = splitPath
 )

src/os/os_windows_test.go

@@ -2074,3 +2074,39 @@ func TestFileAssociatedWithExternalIOCP(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestSplitPath(t *testing.T) {
+	t.Parallel()
+	for _, tt := range []struct{ path, wantDir, wantBase string }{
+		{`a`, `.`, `a`},
+		{`a\`, `.`, `a`},
+		{`a\\`, `.`, `a`},
+		{`a\b`, `a`, `b`},
+		{`a\\b`, `a`, `b`},
+		{`a\b\`, `a`, `b`},
+		{`a\b\c`, `a\b`, `c`},
+		{`\a`, `\`, `a`},
+		{`\a\`, `\`, `a`},
+		{`\a\b`, `\a`, `b`},
+		{`\a\b\`, `\a`, `b`},
+		{`\a\b\c`, `\a\b`, `c`},
+		{`\\a`, `\\a`, `.`},
+		{`\\a\`, `\\a\`, `.`},
+		{`\\\a`, `\\\a`, `.`},
+		{`\\\a\`, `\\\a`, `.`},
+		{`\\a\b\c`, `\\a\b`, `c`},
+		{`c:`, `c:`, `.`},
+		{`c:\`, `c:\`, `.`},
+		{`c:\a`, `c:\`, `a`},
+		{`c:a`, `c:`, `a`},
+		{`c:a\b\`, `c:a`, `b`},
+		{`c:base`, `c:`, `base`},
+		{`a/b/c`, `a/b`, `c`},
+		{`a/b/c/`, `a/b`, `c`},
+		{`\\?\c:\a`, `\\?\c:\`, `a`},
+	} {
+		if dir, base := os.SplitPath(tt.path); dir != tt.wantDir || base != tt.wantBase {
+			t.Errorf("splitPath(%q) = %q, %q, want %q, %q", tt.path, dir, base, tt.wantDir, tt.wantBase)
+		}
+	}
+}

src/os/path_windows.go

@@ -23,11 +23,51 @@ func IsPathSeparator(c uint8) bool {
 
 // splitPath returns the base name and parent directory.
 func splitPath(path string) (string, string) {
-	dirname, basename := filepathlite.Split(path)
-	volnamelen := filepathlite.VolumeNameLen(dirname)
-	for len(dirname) > volnamelen && IsPathSeparator(dirname[len(dirname)-1]) {
-		dirname = dirname[:len(dirname)-1]
+	if path == "" {
+		return ".", "."
 	}
+
+	// The first prefixlen bytes are part of the parent directory.
+	// The prefix consists of the volume name (if any) and the first \ (if significant).
+	prefixlen := filepathlite.VolumeNameLen(path)
+	if len(path) > prefixlen && IsPathSeparator(path[prefixlen]) {
+		if prefixlen == 0 {
+			// This is a path relative to the current volume, like \foo.
+			// Include the initial \ in the prefix.
+			prefixlen = 1
+		} else if path[prefixlen-1] == ':' {
+			// This is an absolute path on a named drive, like c:\foo.
+			// Include the initial \ in the prefix.
+			prefixlen++
+		}
+	}
+
+	i := len(path) - 1
+
+	// Remove trailing slashes.
+	for i >= prefixlen && IsPathSeparator(path[i]) {
+		i--
+	}
+	path = path[:i+1]
+
+	// Find the last path separator. The basename is what follows.
+	for i >= prefixlen && !IsPathSeparator(path[i]) {
+		i--
+	}
+	basename := path[i+1:]
+	if basename == "" {
+		basename = "."
+	}
+
+	// Remove trailing slashes. The remainder is dirname.
+	for i >= prefixlen && IsPathSeparator(path[i]) {
+		i--
+	}
+	dirname := path[:i+1]
+	if dirname == "" {
+		dirname = "."
+	}
+
 	return dirname, basename
 }
 

src/os/removeall_test.go

@@ -471,6 +471,27 @@ func TestRemoveAllNoFcntl(t *testing.T) {
 	}
 }
 
+func TestRemoveAllTrailingSlash(t *testing.T) {
+	slashes := []string{"/"}
+	if runtime.GOOS == "windows" {
+		slashes = append(slashes, `\`)
+	}
+	for _, slash := range slashes {
+		dir := makefs(t, []string{
+			"dir/a/file1",
+			"dir/a/file2",
+			"dir/file3",
+		})
+		path := dir + "/dir"
+		if err := RemoveAll(path + slash); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := Stat(path); !IsNotExist(err) {
+			t.Errorf("after RemoveAll(%q), directory still exists", path+slash)
+		}
+	}
+}
+
 func BenchmarkRemoveAll(b *testing.B) {
 	tmpDir := filepath.Join(b.TempDir(), "target")
 	b.ReportAllocs()