Michael Fischer <mfisc...@zendesk.com> wrote: > On Wed, Oct 23, 2013 at 7:03 PM, Eric Wong <normalper...@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.ga23...@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? _______________________________________________ Unicorn mailing list - mongrel-unicorn@rubyforge.org http://rubyforge.org/mailman/listinfo/mongrel-unicorn Do not quote signatures (like this one) or top post when replying
