local: fix hang listing the root of a Windows network share - fixes #10280

Since v1.75.0 using a bare UNC share such as \\server\share as the
root of a local remote hung forever on Windows.

This was caused by the check that names stay inside the root. This
calls filepath.Rel which never returns on Windows when the base is a
bare UNC share and the target is the same share with a trailing
separator, which is what filepath.Join makes of the root and "".

This fixes it by treating the root and the root with a trailing
separator as the root itself without calling filepath.Rel.

backend/local/local.go

@@ -887,13 +887,25 @@ func (f *Fs) nativePath(name string) string {
 // happen depending on the encoding.
 func (f *Fs) localPath(name string) (string, error) {
 	localPath := filepath.Join(f.root, f.nativePath(name))
-	rel, err := filepath.Rel(f.root, localPath)
+	rel, err := f.relToRoot(localPath)
 	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
 		return "", fserrors.NoRetryError(fmt.Errorf("%q: %w", name, errPathEscapes))
 	}
 	return localPath, nil
 }
 
+// relToRoot returns localPath relative to f.root.
+func (f *Fs) relToRoot(localPath string) (string, error) {
+	// On Windows filepath.Rel loops forever if the base is a bare UNC
+	// share and the target is the same share with a trailing
+	// separator, eg `\\server\share` and `\\server\share\`, which is
+	// what filepath.Join makes of f.root and "".
+	if localPath == f.root || localPath == f.root+string(filepath.Separator) {
+		return ".", nil
+	}
+	return filepath.Rel(f.root, localPath)
+}
+
 // Put the Object to the local filesystem
 func (f *Fs) Put(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
 	// Temporary Object under construction - info filled in by Update()
@@ -1594,7 +1606,7 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 // f.root is the trusted destination the user chose, so it is created (and
 // followed if it is itself a symlink) before being opened.
 func (f *Fs) osRoot(localPath string) (root *os.Root, rel string, err error) {
-	rel, err = filepath.Rel(f.root, localPath)
+	rel, err = f.relToRoot(localPath)
 	if err != nil {
 		return nil, "", err
 	}

backend/local/local_internal_windows_test.go

@@ -8,6 +8,7 @@ import (
 	"runtime"
 	"syscall"
 	"testing"
+	"time"
 
 	"github.com/rclone/rclone/fs/operations"
 	"github.com/rclone/rclone/fstest"
@@ -38,3 +39,36 @@ func TestRmdirWindows(t *testing.T) {
 	err = operations.Rmdir(context.Background(), r.Flocal, "testdir")
 	assert.NoError(t, err)
 }
+
+// TestLocalPathUNCShareRoot checks localPath works when the root is a
+// bare UNC share, where filepath.Rel would never return.
+func TestLocalPathUNCShareRoot(t *testing.T) {
+	for _, root := range []string{`\\server\share`, `\\?\UNC\server\share`} {
+		f := &Fs{root: root}
+		for _, test := range []struct {
+			name string
+			want string
+		}{
+			{name: "", want: root + `\`},
+			{name: "file.txt", want: root + `\file.txt`},
+			{name: "sub/file.txt", want: root + `\sub\file.txt`},
+		} {
+			type result struct {
+				path string
+				err  error
+			}
+			done := make(chan result, 1)
+			go func() {
+				path, err := f.localPath(test.name)
+				done <- result{path, err}
+			}()
+			select {
+			case res := <-done:
+				require.NoError(t, res.err, root, test.name)
+				assert.Equal(t, test.want, res.path, root, test.name)
+			case <-time.After(10 * time.Second):
+				t.Fatalf("localPath(%q) with root %q did not return", test.name, root)
+			}
+		}
+	}
+}