Add gclient installhooks to add pre-commit hook

With submodules, users can accidentally stage and commit gitlink
changes. Add a new gclient command to install a pre-commit hook to
automatically drop gitlink changes that don't correspond to a DEPS
change.

Dropping gitlinks can be bypassed by setting
SKIP_GITLINK_PRECOMMIT=1.

Bug: 1481266
Change-Id: Idd8b273e7d8e37d52627964e8ed6004d068b6b7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4863221
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

hooks/pre-commit.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+# Copyright (c) 2023 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""A git pre-commit hook to drop staged gitlink changes.
+
+To bypass this hook, set SKIP_GITLINK_PRECOMMIT=1.
+"""
+
+import os
+import sys
+
+ROOT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+sys.path.insert(0, ROOT_DIR)
+
+import git_common
+from gclient_eval import SYNC
+
+SKIP_VAR = 'SKIP_GITLINK_PRECOMMIT'
+
+
+def main():
+    if os.getenv(SKIP_VAR) == '1':
+        print(f'{SKIP_VAR} is set. Committing gitlinks, if any.')
+        exit(0)
+
+    has_deps_diff = False
+    staged_gitlinks = []
+    diff = git_common.run('diff-index', '--cached', 'HEAD')
+    for line in diff.splitlines():
+        path = line.split()[-1]
+        if path == 'DEPS':
+            has_deps_diff = True
+            continue
+        if line.startswith(':160000 160000'):
+            staged_gitlinks.append(path)
+
+    if not staged_gitlinks or has_deps_diff:
+        exit(0)
+
+    # There are staged gitlinks and DEPS wasn't changed. Get git_dependencies
+    # migration state in DEPS.
+    state = None
+    try:
+        with open('DEPS', 'r') as f:
+            for l in f.readlines():
+                if l.startswith('git_dependencies'):
+                    state = l.split()[-1].strip(' "\'')
+                    break
+    except OSError:
+        # Don't abort the commit if DEPS wasn't found.
+        exit(0)
+
+    if state != SYNC:
+        # DEPS only has to be in sync with gitlinks when state is SYNC.
+        exit(0)
+
+    print(f'Found no change to DEPS, unstaging {len(staged_gitlinks)} '
+          f'staged gitlink(s) found in diff:\n{diff}')
+    git_common.run('restore', '--staged', '--', *staged_gitlinks)
+
+    disable_msg = f'To disable this hook, set {SKIP_VAR}=1'
+    if len(staged_gitlinks) == len(diff.splitlines()):
+        print('Found no changes after unstaging gitlinks, aborting commit.')
+        print(disable_msg)
+        exit(1)
+    print(disable_msg)
+
+
+if __name__ == "__main__":
+    main()