[build] Fix race in v8gen

tools/dev/v8gen.py

 #!/usr/bin/env python
 # Copyright 2016 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Script to generate V8's gn arguments based on common developer defaults
 or builder configurations.
 
 Goma is used by default if detected. The compiler proxy is assumed to run.
 
@@ skipping 247 matching lines @@
   def _append_gn_args(self, type, gn_args_path, more_gn_args):
     """Append extra gn arguments to the generated args.gn file."""
     if not more_gn_args:
       return False
     self.verbose_print_1('Appending """\n%s\n""" to %s.' % (
         more_gn_args, os.path.abspath(gn_args_path)))
     with open(gn_args_path, 'a') as f:
       f.write('\n# Additional %s args:\n' % type)
       f.write(more_gn_args)
       f.write('\n')
+
+    # Artificially increment modification time as our modifications happen too
+    # fast. This makes sure that gn is properly rebuilding the ninja files.
+    mtime = os.path.getmtime(gn_args_path) + 1

[Michael Achenbach, 2016/09/22 09:08:28]

Really needs + 1, smaller increments don't trigger a rebuild. We also need to
open the file once more. Adding this logic to the open block above, doesn't lead
to the desired result either.

+    with open(gn_args_path, 'aw'):
+      os.utime(gn_args_path, (mtime, mtime))
+
     return True
 
   def main(self):
     # Always operate relative to the base directory for better relative-path
     # handling. This script can be used in any v8 checkout.
     workdir = self._find_work_dir(os.getcwd())
     if workdir != os.getcwd():
       self.verbose_print_1('cd ' + workdir)
       os.chdir(workdir)
 
@@ skipping 15 matching lines @@
 
 if __name__ == "__main__":
   gen = GenerateGnArgs(sys.argv[1:])
   try:
     sys.exit(gen.main())
   except Exception:
     if gen._options.verbosity < 2:
       print ('\nHint: You can raise verbosity (-vv) to see the output of '
              'failed commands.\n')
     raise