Implement check-useless-excludes meta hook

pre_commit/meta_hooks/check_useless_excludes.py

@@ -0,0 +1,41 @@
+import re
+import sys
+
+import pre_commit.constants as C
+from pre_commit.clientlib import load_config
+from pre_commit.git import get_all_files
+
+
+def exclude_matches_any(filenames, include, exclude):
+    include_re, exclude_re = re.compile(include), re.compile(exclude)
+    for filename in filenames:
+        if include_re.search(filename) and exclude_re.search(filename):
+            return True
+    return False
+
+
+def check_useless_excludes(config_file=None):
+    config = load_config(config_file or C.CONFIG_FILE)
+    files = get_all_files()
+    useless_excludes = False
+
+    exclude = config.get('exclude')
+    if exclude != '^$' and not exclude_matches_any(files, '', exclude):
+        print('The global exclude pattern does not match any files')
+        useless_excludes = True
+
+    for repo in config['repos']:
+        for hook in repo['hooks']:
+            include, exclude = hook.get('files', ''), hook.get('exclude')
+            if exclude and not exclude_matches_any(files, include, exclude):
+                print(
+                    'The exclude pattern for {} does not match any files'
+                    .format(hook['id'])
+                )
+                useless_excludes = True
+
+    return useless_excludes
+
+
+if __name__ == '__main__':
+    sys.exit(check_useless_excludes())

pre_commit/repository.py

@@ -4,6 +4,7 @@ import io
 import json
 import logging
 import os
+import pipes
 import shutil
 import sys
 from collections import defaultdict
@@ -247,13 +248,16 @@ class LocalRepository(Repository):
 
 
 class MetaRepository(LocalRepository):
+    # Note: the hook `entry` is passed through `shlex.split()` by the command
+    # runner, so to prevent issues with spaces and backslashes (on Windows) it
+    # must be quoted here.
     meta_hooks = {
-        'test-hook': {
-            'name': 'Test Hook',
-            'files': '',
+        'check-useless-excludes': {
+            'name': 'Check for useless excludes',
+            'files': '.pre-commit-config.yaml',
             'language': 'system',
-            'entry': 'echo "Hello World!"',
-            'always_run': True,
+            'entry': pipes.quote(sys.executable),
+            'args': ['-m', 'pre_commit.meta_hooks.check_useless_excludes'],
         },
     }