GN build improvemenst

tools/gn/last_commit_position.py

Index: tools/gn/last_commit_position.py
diff --git a/tools/gn/last_commit_position.py b/tools/gn/last_commit_position.py
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+# Copyright 2014 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.
+
+"""Writes the most recent "Cr-Commit-Position" value on the master branch
+to a C header file.
+
+Usage: last_commit_position.py <dir> <outfile> <headerguard>
+
+  <dir>
+    Some directory inside the repo to check. This will be used as the current
+    directory when running git. It's best to pass the repo toplevel directory.
+
+  <outfile>
+    C header file to write.
+
+  <headerguard>
+    String to use as the header guard for the written file.
+"""
+
+import os
+import re
+import subprocess
+import sys
+
+def RunGitCommand(directory, command):
+  """
+  Launches git subcommand.
+
+  Errors are swallowed.
+
+  Returns:
+    A process object or None.
+  """
+  command = ['git'] + command
+  # Force shell usage under cygwin. This is a workaround for
+  # mysterious loss of cwd while invoking cygwin's git.
+  # We can't just pass shell=True to Popen, as under win32 this will
+  # cause CMD to be used, while we explicitly want a cygwin shell.
+  if sys.platform == 'cygwin':
+    command = ['sh', '-c', ' '.join(command)]
+  try:
+    proc = subprocess.Popen(command,
+                            stdout=subprocess.PIPE,
+                            stderr=subprocess.PIPE,
+                            cwd=directory,
+                            shell=(sys.platform=='win32'))
+    return proc
+  except OSError:
+    return None
+
+
+def FetchCommitPosition(directory):
+  regex = re.compile(r'\s*Cr-Commit-Position: refs/heads/master@\{#(\d+)\}\s*')
+
+  # Search this far backward in the git log. The commit position should be
+  # close to the top. We allow some slop for long commit messages, and maybe
+  # there were some local commits after the last "official" one. Having this
+  # max prevents us from searching all history in the case of an error.
+  max_lines = 2048
+
+  proc = RunGitCommand(directory, ['log'])
+  for i in range(max_lines):
+    line = proc.stdout.readline()
+    if not line:
+      return None
+
+    match = regex.match(line)
+    if match:
+      return match.group(1)
+
+  return None
+
+
+def WriteHeader(header_file, header_guard, value):
+  with open(header_file, 'w') as f:
+    f.write('''/* Generated by last_commit_position.py. */
+
+#ifndef %(guard)s
+#define %(guard)s
+
+#define LAST_COMMIT_POSITION "%(value)s"
+
+#endif
+''' % {'guard': header_guard, 'value': value})
+
+
+if len(sys.argv) != 4:
+  print "Wrong number of arguments"
+  sys.exit(1)
+
+git_directory = sys.argv[1]
+output_file = sys.argv[2]
+header_guard = sys.argv[3]
+
+value = FetchCommitPosition(".")
+if not value:
+  print "Could not get last commit position."
+  sys.exit(1)
+
+WriteHeader(output_file, header_guard, value)