Fix ./mach test-tidy --faster issue

 issue number: 9778

python/servo/testing_commands.py

@@ -267,7 +267,8 @@ def test_content(self):
              description='Run the source code tidiness check',
              category='testing')
     @CommandArgument('--faster', default=False, action="store_true",
-                     help="Only check changed files and skip the WPT lint in tidy")
+                     help="Only check changed files and skip the WPT lint in tidy, "
+                          "if there are no changes in the WPT files")
     @CommandArgument('--no-progress', default=False, action="store_true",
                      help="Don't show progress for tidy")
     def test_tidy(self, faster, no_progress):

python/tidy.py

@@ -68,6 +68,14 @@
 ]
 
 
+def is_iter_empty(iterator):
+    try:
+        obj = iterator.next()
+        return True, itertools.chain((obj,), iterator)
+    except StopIteration:
+        return False, iterator
+
+
 # A simple wrapper for iterators to show progress (note that it's inefficient for giant iterators)
 def progress_wrapper(iterator):
     list_of_stuff = list(iterator)
@@ -90,6 +98,9 @@ def filter_file(file_name):
 
 def filter_files(start_dir, faster, progress):
     file_iter = get_file_list(start_dir, faster, ignored_dirs)
+    (has_element, file_iter) = is_iter_empty(file_iter)
+    if not has_element:
+        raise StopIteration
     if progress:
         file_iter = progress_wrapper(file_iter)
     for file_name in file_iter:
@@ -100,6 +111,7 @@ def filter_files(start_dir, faster, progress):
             continue
         yield file_name
 
+
 EMACS_HEADER = "/* -*- Mode:"
 VIM_HEADER = "/* vim:"
 MAX_LICENSE_LINESPAN = max(len(license.splitlines()) for license in licenses)
@@ -554,7 +566,10 @@ def check_spec(file_name, lines):
 
 
 def collect_errors_for_files(files_to_check, checking_functions, line_checking_functions):
-    print 'Checking files for tidiness...'
+    (has_element, files_to_check) = is_iter_empty(files_to_check)
+    if not has_element:
+        raise StopIteration
+    print '\rChecking files for tidiness...'
     for filename in files_to_check:
         with open(filename, "r") as f:
             contents = f.read()
@@ -569,9 +584,12 @@ def collect_errors_for_files(files_to_check, checking_functions, line_checking_f
 
 
 def get_wpt_files(only_changed_files, progress):
-    print '\nRunning the WPT lint...'
     wpt_dir = os.path.join(".", "tests", "wpt", "web-platform-tests" + os.sep)
     file_iter = get_file_list(os.path.join(wpt_dir), only_changed_files)
+    (has_element, file_iter) = is_iter_empty(file_iter)
+    if not has_element:
+        raise StopIteration
+    print '\nRunning the WPT lint...'
     if progress:
         file_iter = progress_wrapper(file_iter)
     for f in file_iter:
@@ -618,12 +636,10 @@ def scan(faster=False, progress=True):
     checking_functions = (check_flake8, check_lock, check_webidl_spec, check_json)
     line_checking_functions = (check_license, check_by_line, check_toml, check_rust, check_spec)
     errors = collect_errors_for_files(files_to_check, checking_functions, line_checking_functions)
-
     # wpt lint checks
     wpt_lint_errors = check_wpt_lint_errors(get_wpt_files(faster, progress))
     # collect errors
     errors = itertools.chain(errors, wpt_lint_errors)
-
     error = None
     for error in errors:
         print "\r\033[94m{}\033[0m:\033[93m{}\033[0m: \033[91m{}\033[0m".format(*error)