>On Mon, Oct 20, 2014 at 11:21:36PM +0200, Lars Ellenberg wrote:
>> On Mon, Oct 20, 2014 at 03:04:31PM -0600, Alan Robertson wrote:
>> > On 10/20/2014 02:52 PM, Alan Robertson wrote:
>> > > For the Assimilation code I use the full pathname of the binary from
>> > > /proc to tell if it's "one of mine". That's not perfect if you're using
>> > > an interpreted language. It works quite well for compiled languages.
>>
>> It works just as well (or as bad) from interpreted languages:
>> readlink /proc/$pid/exe
>> (very old linux has a fsid:inode encoding there, but I digress)
>>
>> But that does solve a different subset of problems,
>> has race conditions in itself, and breaks if you have updated the binary
>> since start of that service (which does happen).
Sorry, I lost the original.
Alan then wrote:
> It only breaks if you change the *name* of the binary. Updating the
> binary contents has no effect. Changing the name of the binary is
> pretty unusual - or so it seems to me. Did I miss something?
>
> And if you do, you should stop with the binary with the old version and
> start it with the new one. Very few methods are going to deal well with
> radical changes in the service without stopping it with the old script,
> updating, and starting with the new script.
Well, the "pid starttime" method does...
> I don't believe I see the race condition.
Does not matter.
> It won't loop, and it's not fooled by pid wraparound. What else are you
> looking for? [Guess I missed something else here]
pid + exe is certainly is better than the pid alone.
It may even be "good enough".
But it still has shortcomings.
/proc/pid/exe is not stable,
(changes to "deleted" if the binary is deleted)
could be accounted for.
/proc/pid/exe links to the interpreter (python, bash, java, whatever)
Even if it is a "real" binary, (pid, /proc/pid/exe) is
still NOT unique for pid re-use after wrap around:
think different instances of mysql or whatever.
(yes, it gets increasingly unlikely...)
However, (pid, starttime) *is* unique (for the lifetime of the pidfile,
as long as that is stored on tmpfs resp. cleared after reboot).
(unless you tell me you can eat through pid_max, or at least the
currently unused pids, within the granularity of starttime...)
So that's why I propose to use (pid, starttime) tuple.
If you see problems with (pid, starttime), please speak up.
If you have something *better*, please speak up.
If you just have something "different",
feel free to tell us anyways :-)
Lars
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