[ClusterLabs Developers] checking all procs on system enough during stop action?

[Jan Pokorný]

On 24/04/17 17:32 +0200, Jehan-Guillaume de Rorthais wrote:
> On Mon, 24 Apr 2017 17:08:15 +0200
> Lars Ellenberg <lars.ellenberg at linbit.com> wrote:
> 
>> On Mon, Apr 24, 2017 at 04:34:07PM +0200, Jehan-Guillaume de Rorthais wrote:
>>> Hi all,
>>> 
>>> In the PostgreSQL Automatic Failover (PAF) project, one of most frequent
>>> negative feedback we got is how difficult it is to experience with it
>>> because of fencing occurring way too frequently. I am currently hunting
>>> this kind of useless fencing to make life easier.
>>> 
>>> It occurs to me, a frequent reason of fencing is because during the stop
>>> action, we check the status of the PostgreSQL instance using our monitor
>>> function before trying to stop the resource. If the function does not return
>>> OCF_NOT_RUNNING, OCF_SUCCESS or OCF_RUNNING_MASTER, we just raise an error,
>>> leading to a fencing. See:
>>> https://github.com/dalibo/PAF/blob/d50d0d783cfdf5566c3b7c8bd7ef70b11e4d1043/script/pgsqlms#L1291-L1301
>>> 
>>> I am considering adding a check to define if the instance is stopped even
>>> if the monitor action returns an error. The idea would be to parse **all**
>>> the local processes looking for at least one pair of
>>> "/proc/<PID>/{comm,cwd}" related to the PostgreSQL instance we want to
>>> stop. If none are found, we consider the instance is not running.
>>> Gracefully or not, we just know it is down and we can return OCF_SUCCESS.
>>> 
>>> Just for completeness, the piece of code would be:
>>> 
>>>    my @pids;
>>>    foreach my $f (glob "/proc/[0-9]*") {
>>>        push @pids => basename($f)
>>>            if -r $f
>>>                and basename( readlink( "$f/exe" ) ) eq "postgres"
>>>                and readlink( "$f/cwd" ) eq $pgdata;
>>>    }
>>> 
>>> I feels safe enough to me.
> 
> [...]
> 
> But anyway, here or there, I would have to add this piece of code looking at
> each processes. According to you, is it safe enough? Do you see some hazard
> with it?

Just for the sake of completeness, there's a race condition, indeed,
in multiple repeated path traversals (without being fixed of particular
entry inode), which can be interleaved with new postgres process being
launched anew (or what not).  But that may happen even before the code
in question is executed -- naturally not having a firm grip on the
process is open to such possible issues, so this is just an aside.

-- 
Jan (Poki)
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