Only write gyp-mac-tool when needed

build/toolchain/mac/setup_toolchain.py

 # Copyright (c) 2013 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.
 
 import os
 import stat
 import sys
 
 def CopyTool(source_path):
   """Copies the given tool to the current directory, including a warning not
   to edit it."""
   with open(source_path) as source_file:
     tool_source = source_file.readlines()
 
   # Add header and write it out to the current directory (which should be the
-  # root build dir).
+  # root build dir). Don't write the file if a matching file already exists
+  # because that causes a cascade of unnecessary rebuilds.
+  match = False
+  contents = ''.join([tool_source[0],
+                      '# Generated by setup_toolchain.py do not edit.\n']
+                     + tool_source[1:])
   out_path = 'gyp-mac-tool'
-  with open(out_path, 'w') as tool_file:
-    tool_file.write(''.join([tool_source[0],
-                             '# Generated by setup_toolchain.py do not edit.\n']
-                            + tool_source[1:]))
+  try:
+    with open(out_path, 'rb') as read_tool_file:
+      existing_contents = read_tool_file.read()
+    if existing_contents == contents:
+      match = True
+  except:
+    pass
+  if not match:
+    with open(out_path, 'wb') as write_tool_file:
+      write_tool_file.write(contents)
   st = os.stat(out_path)
-  os.chmod(out_path, st.st_mode | stat.S_IEXEC)
+  if (st.st_mode & stat.S_IEXEC) == 0:
+    # Only chmod when necessary.
+    os.chmod(out_path, st.st_mode | stat.S_IEXEC)
 
 # Find the tool source, it's the first argument, and copy it.
 if len(sys.argv) != 2:
   print "Need one argument (mac_tool source path)."
   sys.exit(1)
 CopyTool(sys.argv[1])