Meta: Reimplement license checker in python

On my machine, this script took about 3.4 seconds, and was responsible
for essentially all of the time taken by the precommit hook.

The script is a faithful 1:1 reimplementation, even the regexes are
identical. And yet, it takes about 0.02 seconds, making the pre-commit
hook lightning fast again. Apparently python is just faster in this
case.

Fun fact:
- Just reading all ~4000 files took bash about 1.2 seconds
- Checking the license took another 1.8 seconds in total
- Checking for math.h took another 0.4 seconds in total
- Checking for '#pragma once' took another 0.4 seconds in total

The timing is highly load-dependent, so they don't exactly add up to 3.4
seconds. However, it's good enough to determine that bash is no longer
fit for the purpose of this script.

1 files changed, 86 insertions, 0 deletions

diff --git a/Meta/check-style.py b/Meta/check-style.py
new file mode 100755
index 0000000000..8b61db5217
--- /dev/null
+++ b/Meta/check-style.py
@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+
+import os
+import re
+import subprocess
+import sys
+
+# Ensure copyright headers match this format and are followed by a blank line:
+# /*
+#  * Copyright (c) YYYY(-YYYY), Whatever
+#  * ... more of these ...
+#  *
+#  * SPDX-License-Identifier: BSD-2-Clause
+#  */
+GOOD_LICENSE_HEADER_PATTERN = re.compile(
+    '^/\\*\n' +
+    '( \\* Copyright \\(c\\) [0-9]{4}(-[0-9]{4})?, .*\n)+' +
+    ' \\*\n' +
+    ' \\* SPDX-License-Identifier: BSD-2-Clause\n' +
+    ' \\*/\n' +
+    '\n')
+LICENSE_HEADER_CHECK_EXCLUDES = {
+    'AK/Checked.h',
+    'AK/Function.h',
+    'Userland/Libraries/LibC/elf.h',
+    'Userland/DevTools/HackStudio/LanguageServers/Cpp/Tests/',
+    'Userland/Libraries/LibCpp/Tests/parser/',
+    'Userland/Libraries/LibCpp/Tests/preprocessor/'
+}
+
+# We check that "#pragma once" is present
+PRAGMA_ONCE_STRING = '#pragma once'
+
+# We make sure that there's a blank line before and after pragma once
+GOOD_PRAGMA_ONCE_PATTERN = re.compile('(^|\\S\n\n)#pragma once(\n\n\\S.|$)')
+
+# We check that "#include <LibM/math.h>" is not being used
+LIBM_MATH_H_INCLUDE_STRING = '#include <LibM/math.h>'
+
+GIT_LS_FILES = ['git', 'ls-files', '--', '*.cpp', '*.h', ':!:Base', ':!:Kernel/FileSystem/ext2_fs.h']
+
+
+def run():
+    files = subprocess.run(GIT_LS_FILES, check=True, capture_output=True).stdout.decode().strip('\n').split('\n')
+    assert len(files) > 1000
+
+    errors_license = []
+    errors_libm_math_h = []
+    errors_pragma_once_bad = []
+    errors_pragma_once_missing = []
+
+    for filename in files:
+        with open(filename, "r") as f:
+            file_content = f.read()
+        if not any(filename.startswith(forbidden_prefix) for forbidden_prefix in LICENSE_HEADER_CHECK_EXCLUDES):
+            if not GOOD_LICENSE_HEADER_PATTERN.search(file_content):
+                errors_license.append(filename)
+        if LIBM_MATH_H_INCLUDE_STRING in file_content:
+            errors_libm_math_h.append(filename)
+        if filename.endswith('.h'):
+            if GOOD_PRAGMA_ONCE_PATTERN.search(file_content):
+                # Excellent, the formatting is correct.
+                pass
+            elif PRAGMA_ONCE_STRING in file_content:
+                # Bad, the '#pragma once' is present but it's formatted wrong.
+                errors_pragma_once_bad.append(filename)
+            else:
+                # Bad, the '#pragma once' is missing completely.
+                errors_pragma_once_missing.append(filename)
+
+    if errors_license:
+        print("Files with bad licenses:", " ".join(errors_license))
+    if errors_pragma_once_missing:
+        print("Files without #pragma once:", " ".join(errors_pragma_once_missing))
+    if errors_pragma_once_bad:
+        print("Files with a bad #pragma once:", " ".join(errors_pragma_once_bad))
+    if errors_libm_math_h:
+        print("Files including LibM/math.h (include just 'math.h' instead):", " ".join(errors_libm_math_h))
+
+    if errors_license or errors_pragma_once_missing or errors_pragma_once_bad or errors_libm_math_h:
+        sys.exit(1)
+
+
+if __name__ == '__main__':
+    os.chdir(os.path.dirname(__file__) + "/..")
+    run()