Give build a --write-only-new option.

grit/tool/build_unittest.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.
 
 '''Unit tests for the 'grit build' tool.
 '''
 
 import codecs
 import os
@@ skipping 227 matching lines @@
         'IDR_STRUCTURE_IN_FALSE_IF_NOT_WHITELISTED',
         'IDR_INCLUDE_NOT_WHITELISTED',
     ]
     for output_file in (header, map_cc):
       self._verifyWhitelistedOutput(
         output_file,
         whitelisted_ids,
         non_whitelisted_ids,
       )
 
+  def testWriteOnlyNew(self):
+    import time
+    output_dir = tempfile.mkdtemp()
+    builder = build.RcBuilder()
+    class DummyOpts(object):
+      def __init__(self):
+        self.input = util.PathFromRoot('grit/testdata/substitute.grd')
+        self.verbose = False
+        self.extra_verbose = False
+    builder.Run(DummyOpts(), ['-o', output_dir])
+    header = os.path.join(output_dir, 'resource.h')
+    self.failUnless(os.path.exists(header))
+    first_mtime = os.stat(header).st_mtime
+
+    mtime_resolution = 2 if sys.platform == 'win32' else 1
+
+    time.sleep(mtime_resolution)

[newt (away), 2014/12/04 22:09:42]

Is there a way do this without sleeping for 1 or 2 seconds? The entire grit test
suite runs in 1.3 seconds on my machine, so it's a bit sad for this one test to
take 2 or 4 seconds.

Idea: set the mtime of the file before each call to builder.Run(), then check if
the mtime was updated. Or you could try patching the time or datetime module
(http://stackoverflow.com/a/2659472/598094), or you could add a hook in grit.py.

[Nico, 2014/12/04 22:33:07]

Yeah, I was pretty unhappy with this too :-/
That's a great idea!

+    builder.Run(DummyOpts(), ['-o', output_dir])
+    self.failUnless(os.path.exists(header))
+    second_mtime = os.stat(header).st_mtime
+
+    time.sleep(mtime_resolution)
+    builder.Run(DummyOpts(), ['-o', output_dir, '--write-only-new'])
+    self.failUnless(os.path.exists(header))
+    third_mtime = os.stat(header).st_mtime
+
+    self.assertNotEqual(first_mtime, second_mtime)
+    self.assertEqual(second_mtime, third_mtime)
+
 
 if __name__ == '__main__':
   unittest.main()