Avoid clobbering generated files if the content hasn't changed.

tools/gypv8sh.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """This script is used by chrome_tests.gypi's js2webui action to maintain the
 argument lists and to generate inlinable tests.
 """
 
 import json
 import optparse
 import os
 import subprocess
 import sys
 import shutil
 
 
+def HasSameContent(filename, content):
+  '''Returns true if the given file is readable and has the given content.'''
+  try:
+    with open(filename) as file:
+      return file.read() == content
+  except:
+    # Ignore all errors and fall back on a safe bet.
+    return False
+
+
 def main ():
   parser = optparse.OptionParser()
   parser.set_usage(
       "%prog v8_shell mock.js test_api.js js2webui.js "
       "testtype inputfile inputrelfile cxxoutfile jsoutfile")
   parser.add_option('-v', '--verbose', action='store_true')
   parser.add_option('-n', '--impotent', action='store_true',
                     help="don't execute; just print (as if verbose)")
   parser.add_option('--deps_js', action="store",
                     help=("Path to deps.js for dependency resolution, " +
@@ skipping 20 matching lines @@
                cxxoutfile, test_type]
   cmd.extend(['-e', "arguments=" + json.dumps(arguments), mock_js,
          test_api, js2webui])
   if opts.verbose or opts.impotent:
     print cmd
   if not opts.impotent:
     try:
       p = subprocess.Popen(
           cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, bufsize=0)
       out, err = p.communicate()
-      with open(cxxoutfile, 'wb') as f:
-        f.write(out)
+      if not HasSameContent(cxxoutfile, out):
+        with open(cxxoutfile, 'wb') as f:
+          f.write(out)
       shutil.copyfile(inputfile, jsoutfile)
     except Exception, ex:
       if os.path.exists(cxxoutfile):
         os.remove(cxxoutfile)
       if os.path.exists(jsoutfile):
         os.remove(jsoutfile)
       raise
 
 
 if __name__ == '__main__':
- sys.exit(main())
+  sys.exit(main())