From: Eric Wong <normalperson@yhbt.net>
To: unicorn list <mongrel-unicorn@rubyforge.org>
Subject: Re: pid file handling issue
Date: Thu, 24 Oct 2013 18:21:37 +0000 [thread overview]
Message-ID: <20131024182137.GA25770@dcvr.yhbt.net> (raw)
In-Reply-To: <CABHxtY4W=rMZb+bv_RqqH0uQy76stHrPY63phz91u+gutaJC=Q@mail.gmail.com>
Michael Fischer <mfischer@zendesk.com> wrote:
> On Wed, Oct 23, 2013 at 7:03 PM, Eric Wong <normalperson@yhbt.net> 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.
Right, we looked at using rename last year but I didn't think it's possible
given we need to write the pid file before binding new listen sockets
http://mid.gmane.org/20121127215146.GA23452@dcvr.yhbt.net
But perhaps we can drop the pid file late iff ENV["UNICORN_FD"] is
detected. I'll see if that can be done w/o breaking compatibility.
> >> > 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?
It doesn't mean the process didn't exit/crash right after writing the PID.
> > 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
> concerned.
But NTP syncs early in the boot process before most processes (including
unicorn) are started. It shouldn't matter, then, right?
> > 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.
OK. I assume you do the same for nginx?
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next prev parent reply other threads:[~2013-10-24 18:21 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-23 22:55 pid file handling issue Michael Fischer
2013-10-24 0:53 ` Eric Wong
2013-10-24 1:01 ` Michael Fischer
2013-10-24 2:03 ` Eric Wong
2013-10-24 17:51 ` Michael Fischer
2013-10-24 18:21 ` Eric Wong [this message]
2013-10-24 19:57 ` Michael Fischer
2013-10-24 20:27 ` Eric Wong
2013-10-24 22:58 ` Eric Wong
2013-10-25 7:33 ` Hongli Lai
2013-10-25 20:31 ` Eric Wong
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