fspath: Stop empty strings being a valid path - fixes #4239

Before this change you could use "" as a valid remote, so `rclone lsf
""` would work. This was treated as the current directory.

This is unexpected and creates a footgun for scripting when an empty
variable is passed to rclone by accident.

This fix returns the error "can't use empty string as a path" instead
of allowing it.

fs/fspath/path.go

@@ -18,6 +18,7 @@ const (
 
 var (
 	errInvalidCharacters = errors.New("config name contains invalid characters - may only contain 0-9, A-Z ,a-z ,_ , - and space ")
+	errCantBeEmpty       = errors.New("can't use empty string as a path")
 
 	// urlMatcher is a pattern to match an rclone URL
 	// note that this matches invalid remoteNames
@@ -55,8 +56,12 @@ func CheckRemoteName(remoteName string) error {
 //
 // Note that this will turn \ into / in the fsPath on Windows
 //
-// An error may be returned if the remote name has invalid characters in it.
+// An error may be returned if the remote name has invalid characters
+// in it or if the path is empty.
 func Parse(path string) (configName, fsPath string, err error) {
+	if path == "" {
+		return "", "", errCantBeEmpty
+	}
 	parts := urlMatcher.FindStringSubmatch(path)
 	configName, fsPath = "", path
 	if parts != nil && !driveletter.IsDriveLetter(parts[1]) {

fs/fspath/path_test.go

@@ -55,7 +55,7 @@ func TestParse(t *testing.T) {
 		in, wantConfigName, wantFsPath string
 		wantErr                        error
 	}{
-		{"", "", "", nil},
+		{"", "", "", errCantBeEmpty},
 		{":", "", "", errInvalidCharacters},
 		{"::", ":", "", errInvalidCharacters},
 		{":/:", "", "/:", errInvalidCharacters},
@@ -91,7 +91,7 @@ func TestSplit(t *testing.T) {
 		remote, wantParent, wantLeaf string
 		wantErr                      error
 	}{
-		{"", "", "", nil},
+		{"", "", "", errCantBeEmpty},
 
 		{"remote:", "remote:", "", nil},
 		{"remote:potato", "remote:", "potato", nil},