dropbox: fix shared folder mount for roots nested more than one level deep

In shared_folders mode NewFs derived the shared folder name with
path.Dir(f.root), which returns the parent path rather than the first
path component. For a root like "SharedFolder/subdir/deeper" this yielded
"SharedFolder/subdir", which findSharedFolder cannot match, so NewFs
failed with ErrorDirNotFound. Use the first path component of the root,
as the shared_folders option documents, so deeply nested roots mount.

Fixes #9705

backend/dropbox/dropbox.go

@@ -634,13 +634,8 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 			return f, nil // our root it empty so we probably want to list shared folders
 		}
 
-		dir := path.Dir(f.root)
-		if dir == "." {
-			dir = f.root
-		}
-
 		// root is not empty so we have find the right shared folder if it exists
-		id, err := f.findSharedFolder(ctx, dir)
+		id, err := f.findSharedFolder(ctx, sharedFolderName(f.root))
 		if err != nil {
 			// if we didn't find the specified shared folder we have to bail out here
 			return nil, err
@@ -948,6 +943,14 @@ func (f *Fs) listSharedFolders(ctx context.Context, callback func(fs.DirEntry) e
 	return nil
 }
 
+// sharedFolderName returns the shared folder name in root, which is its first
+// path component. root must be the trimmed root as produced by setRoot (no
+// leading slash).
+func sharedFolderName(root string) string {
+	name, _, _ := strings.Cut(root, "/")
+	return name
+}
+
 // findSharedFolder find the id for a given shared folder name
 // somewhat annoyingly there is no endpoint to query a shared folder by it's name
 // so our only option is to iterate over all shared folders

backend/dropbox/dropbox_internal_test.go

@@ -188,6 +188,21 @@ func TestInternalCheckPathLength(t *testing.T) {
 	}
 }
 
+func TestInternalSharedFolderName(t *testing.T) {
+	for _, test := range []struct {
+		root string
+		want string
+	}{
+		{root: "", want: ""},
+		{root: "SharedFolder", want: "SharedFolder"},
+		{root: "SharedFolder/subdir", want: "SharedFolder"},
+		{root: "SharedFolder/subdir/deeper", want: "SharedFolder"},
+		{root: "SharedFolder/subdir/deeper/deepest", want: "SharedFolder"},
+	} {
+		assert.Equal(t, test.want, sharedFolderName(test.root), test.root)
+	}
+}
+
 func TestPaperExportRemote(t *testing.T) {
 	ctx := context.Background()
 	info := &files.FileMetadata{