shouldRollover overwrites backup file in 'w' mode

[Nir Arad]

shouldRollover function calls _shouldRollover() twice.

    if self._shouldRollover(): # First check
           # if some other process already did the rollover we might
           # checked log.1, so we reopen the stream and check again on
           # the right log file
           self.stream.close()
           self._openFile(self.mode)
           return self._shouldRollover()

After the first check, if we should rollover, self._openFile will open the file again with the same mode.
However, if the mode is 'w' then the file is overwritten.

Also, because we overwritten the file, the second call to _shouldRollover will always return false.

[Nir Arad (nira-p)]

I've looked into RotatingFileHandler.__init__() and they added this:

        # If rotation/rollover is wanted, it doesn't make sense to use another
        # mode. If for example 'w' were specified, then if there were multiple
        # runs of the calling application, the logs from previous runs would be
        # lost if the 'w' is respected, because the log file would be truncated
        # on each run.

        if maxBytes > 0:
            mode = 'a'