Introduce git merge driver for the blink reformatting

clang_format_merge_driver.py

Index: clang_format_merge_driver.py
diff --git a/clang_format_merge_driver.py b/clang_format_merge_driver.py
new file mode 100755
index 0000000000000000000000000000000000000000..aee7f7d78183e438d5e42039b5e992b3637045b8
--- /dev/null
+++ b/clang_format_merge_driver.py
@@ -0,0 +1,40 @@
+#!/usr/bin/env python
+# Copyright 2016 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.
+"""Clang-format 3-way merge driver."""

[Nico, 2016/09/20 21:54:26]

maybe add some more here (what does this do, how does one use it)

[dcheng, 2016/09/20 22:11:19]

Done.

+
+import subprocess
+import sys
+
+import clang_format
+
+
+def main():
+  if len(sys.argv) < 5:
+    print('usage: %s <base> <current> <others> <path in the tree>' %
+          sys.argv[0])
+    sys.exit(1)
+
+  base, current, others, file_name_in_tree = sys.argv[1:5]
+  print '\nRunning clang-format 3-way merge driver on ' + file_name_in_tree
+
+  try:
+    tool = clang_format.FindClangFormatToolInChromiumTree()
+    for fpath in base, current, others:
+      with open(fpath, 'rb') as input_file:

[dcheng, 2016/09/20 21:41:06]

Merge always appear to occur relative to the repo root. In order for
clang-format to pick up the .clang-format, we use --assume-filename to fake the
path name. To do this, we also need to use stdin / stdout instead though, so
I've hoisted reading / writing the file into the py as well.

[Nico, 2016/09/20 21:54:26]

add a comment summarizing this

[dcheng, 2016/09/20 22:11:19]

Done.

+        output = subprocess.check_output(
+            [tool, '--assume-filename=%s' % file_name_in_tree, '--style=file'],
+            stdin=input_file)
+      with open(fpath, 'wb') as output_file:
+        output_file.write(output)
+  except clang_format.NotFoundError, e:
+    print e
+    print 'Failed to find clang-format. Falling-back on standard 3-way merge'
+
+  return subprocess.call(['git', 'merge-file', '-Lcurrent', '-Lbase', '-Lother',
+                          current, base, others])
+
+
+if __name__ == '__main__':
+  sys.exit(main())