pid file handling issue

Michael Fischer mfischer at
Thu Oct 24 17:51:08 UTC 2013

On Wed, Oct 23, 2013 at 7:03 PM, Eric Wong <normalperson at> wrote:

>> > I read and stash the value of the pid file before issuing any USR2.
>> > Later, you can issue "kill -0 $old_pid" after sending SIGQUIT
>> > to ensure it's dead.
>> That's inherently racy; another process can claim the old PID in the interim.
> Right, but raciness goes for anything regarding pid files.
> The OS does make an effort to avoid recycling PIDs too often,
> and going through all the PIDs in a system quickly is
> probably rare.  I haven't hit it, at least.

That's not good enough.

The fact that the pid file contains a pid is immaterial to me; I don't
even need to look at it.  I only care about when it was created, or
what its inode number is, so that I can detect whether Unicorn was
last successfully started or restarted.  rename(2) is atomic per POSIX
and is not subject to race conditions.

>> > Checking the mtime of the pidfile is really bizarre...
>> Perhaps (though it's a normative criticism), but on the other hand, it
>> isn't subject to the race above.
> It's still racy in a different way, though (file could change right
> after checking).

If the file's mtime or inode number changes under my proposal, that
means the reload must have been successful.   What race condition are
you referring to that would render this conclusion inaccurate?

> Having the process start time in /proc be unreliable because the server
> has the wrong time is also in the same category of corner cases.

This is absolutely not true.  A significant minority, if not a
majority, of servers will have at least slightly inaccurate wall
clocks on boot.  This is usually corrected during boot by an NTP sync,
but by then the die has already been cast insofar as ps(1) output is

> Also, can you check the inode of the /proc/$pid entry?  Perhaps

That's not portable.

> PID files are horrible, really :<

To reiterate, I'm not using the PID file in this instance to determine
Unicorn's PID.  It could be empty, for all I care.


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