pid file deleted briefly when doing hot restart

Petteri Räty betelgeuse at gentoo.org
Mon Nov 26 10:39:56 UTC 2012


On 26.11.2012 2.43, Eric Wong wrote:
> Petteri Räty <betelgeuse at gentoo.org> wrote:
>> What follows are all the write actions related to unicorn pid file when
>> doing a hot restart. Seems like a bug to me that unicorn is deleting the
>> pid file before writing the new file. Is there a reason for it? It seems
>> to go against that rename that aims for an atomic replace that would
>> always ensure the pid file is there.
> 
> Unfortunately, pid files are inherently racy.  However, I
> seem to recall a pid file not existing for a brief moment was needed
> to allow some nginx-based scripts to work.
> 

Would you be open for a config switch to avoid this behavior?

Can you say if lib/unicorn/http_server.rb:196 is the correct spot?

The use case here is that with health monitors wouldn't have a window
where a pid file does not exist. With a hot restart it should always be
possible to have a pid file that points to either the old or the new master.

> I think unicorn differs a bit from nginx here:
> 
> nginx uses rename() to clear the way for a new pid file.  Like unicorn,
> this still leaves a window where no pid file exists.
> 

Looking at the inotify log it seems the reason pid file does not exist
is an explicit delete and not due to rename. It happens a couple seconds
earlier also so the window is possible to hit even with a periodic poller.

Regards,
Petteri


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