This cleans up some of the use of the filepoller which makes reading
significantly more robust and gives fewer changes to fallback to the
polling based watcher.
In a lot of cases, if the file was being rotated while we were adding it
to the watcher, it would return an error that the file doesn't exist and
would fallback.
In some cases this fallback could be triggered multiple times even if we
were already on the fallback/poll-based watcher.
It also fixes an open file leak caused by not closing files properly on
rotate, as well as not closing files that were read via the `tail`
function until after the log reader is completed.
Prior to the above changes, it was relatively simple to cause the log
reader to error out by having quick rotations, for example:
```
$ docker run --name test --log-opt max-size=10b --log-opt max-files=10
-d busybox sh -c 'while true; do usleep 500000; echo hello; done'
$ docker logs -f test
```
After these changes I can run this forever without error.
Another fix removes 2 `os.Stat` calls when rotating files. The stat
calls are not needed since we are just calling `os.Rename` anyway, which
will in turn also just produce the same error that `Stat` would.
These `Stat` calls were also quite expensive.
Removing these stat calls also seemed to resolve an issue causing slow
memory growth on the daemon.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
