Comment 6 for bug 1457575

The issue here is there is a window for the log file to be appended to during the copy of the contents into the backup tar. This is a problem because before the copy starts, the go tar implementation stats the file in order to get the size and write that in the header. Then during write, if too much data is given error is returned. Seems easier to hit with large log files or slow disk.

I have a simple fix that limits the copy length to that at the time of the stat. So, additional messages may not be backed up, but the process continues sanely.

See irclog for more info:

<https://irclogs.ubuntu.com/2016/07/29/%23juju-dev.html#t13:32>