Only write .in outputs if they have changed.

Source/build/scripts/in_generator.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 37 matching lines @@
             self.in_file = InFile.load_from_files(in_files, self.defaults, self.valid_values, self.default_parameters)
         else:
             self.in_file = None
         self._outputs = {}  # file_name -> generator
 
     def wrap_with_condition(self, string, condition):
         if not condition:
             return string
         return "#if ENABLE(%(condition)s)\n%(string)s\n#endif" % { 'condition' : condition, 'string' : string }
 
-    def _forcibly_create_text_file_at_path_with_contents(self, file_path, contents):
-        # FIXME: This method can be made less force-full anytime after 6/1/2013.
-        # A gyp error was briefly checked into the tree, causing
-        # a directory to have been generated in place of one of
-        # our output files.  Clean up after that error so that
-        # all users don't need to clobber their output directories.
-        shutil.rmtree(file_path, ignore_errors=True)
+    def _write_file_if_changed(self, output_dir, contents, file_name):
+        path = os.path.join(output_dir, file_name)
+
         # The build system should ensure our output directory exists, but just in case.
-        directory = os.path.dirname(file_path)
+        directory = os.path.dirname(path)
         if not os.path.exists(directory):
             os.makedirs(directory)
 
-        with open(file_path, "w") as file_to_write:
-            file_to_write.write(contents)
-
-    def _write_file(self, output_dir, contents, file_name):
-        path = os.path.join(output_dir, file_name)
-        self._forcibly_create_text_file_at_path_with_contents(path, contents)
+        # Only write the file if the contents have changed. This allows ninja to
+        # skip rebuilding targets which depend on the output.
+        with open(path, "a+") as output_file:
+            output_file.seek(0)
+            if output_file.read() != contents:
+                output_file.truncate(0)
+                output_file.write(contents)
 
     def write_files(self, output_dir):
         for file_name, generator in self._outputs.items():
-            self._write_file(output_dir, generator(), file_name)
+            self._write_file_if_changed(output_dir, generator(), file_name)
 
     def set_gperf_path(self, gperf_path):
         self.gperf_path = gperf_path
 
 
 class Maker(object):
     def __init__(self, writer_class):
         self._writer_class = writer_class
 
     def main(self, argv):
         script_name = os.path.basename(argv[0])
         args = argv[1:]
         if len(args) < 1:
             print "USAGE: %s INPUT_FILES" % script_name
             exit(1)
 
         parser = optparse.OptionParser()
         parser.add_option("--gperf", default="gperf")
         parser.add_option("--output_dir", default=os.getcwd())
         options, args = parser.parse_args()
 
         writer = self._writer_class(args)
         writer.set_gperf_path(options.gperf)
         writer.write_files(options.output_dir)