Only write gyp-win-tool when needed

build/toolchain/win/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.
 #
 # Copies the given "win tool" (which the toolchain uses to wrap compiler
 # invocations) and the environment blocks for the 32-bit and 64-bit builds on
 # Windows to the build directory.
 #
 # The arguments are the visual studio install location and the location of the
 # win tool. The script assumes that the root build directory is the current dir
@@ skipping 129 matching lines @@
   return block
 
 
 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).
-  with open("gyp-win-tool", 'w') as tool_file:
-    tool_file.write(''.join([tool_source[0],
-                             '# Generated by setup_toolchain.py do not edit.\n']
-                            + tool_source[1:]))
+  # 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-win-tool'
+  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)
 
 
 def main():
   if len(sys.argv) != 6:
     print('Usage setup_toolchain.py '
           '<visual studio path> <win tool path> <win sdk path> '
           '<runtime dirs> <target_cpu>')
     sys.exit(2)
   tool_source = sys.argv[2]
   win_sdk_path = sys.argv[3]
@@ skipping 32 matching lines @@
     env_block = _FormatAsEnvironmentBlock(env)
     with open('environment.winrt_' + cpu, 'wb') as f:
         f.write(env_block)
 
   assert vc_bin_dir
   print 'vc_bin_dir = "%s"' % vc_bin_dir
 
 
 if __name__ == '__main__':
   main()