Fixing cygwin specific assumptions in gyp diff writer.

pylib/gyp/common.py

 #!/usr/bin/python
 
 import errno
 import filecmp
 import os.path
 import re
 import tempfile
+import sys
 
 
 def BuildFileAndTarget(build_file, target):
   # NOTE: If you just want to split up target into a build_file and target,
   # and you know that target already has a build_file that's been produced by
   # this function, pass '' for build_file.
 
   # NOTE: rsplit is used to disambiguate the Windows drive letter separator.
   target_split = target.rsplit(':', 1)
   if len(target_split) == 2:
@@ skipping 234 matching lines @@
           # tempfile.mkstemp uses an overly restrictive mode, resulting in a
           # file that can only be read by the owner, regardless of the umask.
           # There's no reason to not respect the umask here, which means that
           # an extra hoop is required to fetch it and reset the new file's mode.
           #
           # No way to get the umask without setting a new one?  Set a safe one
           # and then set it back to the old value.
           umask = os.umask(077)
           os.umask(umask)
           os.chmod(self.tmp_path, 0666 & ~umask)
+          if sys.platform == 'win32':
+            # NOTE: on windows (but not cygwin) rename will not replace an
+            # existing file, so it must be preceded with a remove. Sadly there
+            # is no way to make the switch atomic.
+            os.remove(filename)
           os.rename(self.tmp_path, filename)
       except Exception:
         # Don't leave turds behind.
         os.unlink(self.tmp_path)
         raise
 
   return Writer()