Re: Stale pakfire lock-file causing pakfire to no longer work

[Bernhard Bitsch <bbitsch@ipfire.org>]

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Hi Robin,


Am 15.09.2022 um 21:43 schrieb Robin Roevens:
> Hi Bernhard
> 
> Bernhard Bitsch schreef op do 15-09-2022 om 13:48 [+0200]:
>> Hi all,
>>
>> as an 'old real time programmer' this reminds me deeply at
>> Dijkstra/Hoare's "Dining philosophers problem".
>>
>> The check for presence of the lockfile and the generation of it are
>> not
>> 'atomic'. Means two programs can run in parallel.
> Indeed..
> In a shell script, a more atomic approach would be instead of using a
> lockfile, a lock-directory:
> 'mkdir' creates a directory only if it not already exists and if it
> does already exist, it will return an exit code. So here we have both
> checking and generating in one atomic operation.
> This is better explained here:
> https://wiki.bash-hackers.org/howto/mutex
> 
> Not sure if this can be translated to Perl in an atomic way..
> I did find this perl code snippet however:
> ---
> use strict;
> use warnings;
> use Fcntl ':flock';
> 
> flock(DATA, LOCK_EX|LOCK_NB) or die "There can be only one! [$0]";
> 
> 
> # mandatory line, flocking depends on DATA file handle
> __DATA__
> ---
> Which could be a possible solution, I think.
> 

Looks promising. Will look into this.

> I also found this, which seems quiet promising:
> https://metacpan.org/pod/Script::Singleton
> to perform locking by using shared memory.
> 
>>
>> I'll investigate this further. But the deletion of the lock should
>> happen anyways, as far I've seen till now.
> True, it should be deleted always and as said before, I could not
> reproduce this manually .. but my Zabbix agent seems to be able to
> trigger this problem at least once every 24h on my IPFire mini
> appliance, only by executing pakfire every 10 minutes. That is why I'm
> suspecting the abnormal termination of pakfire, leaving the lockfile in
> place, is actually caused by sudo.
> 
> On the other hand.. this can also happen when pakfire is running and
> suddenly the power is cut.. then the lockfile will still be present
> when the machine is back up.. So I think, if we stay with the lockfile,
> we at least need some check for a stale lockfile, like checking if the
> process that created the lockfile still exists or not and removing it
> if not.
> 

Because the lockfile is located in /tmp, I don't think it survives a reboot.

Regards
Bernhard

> Regards
> Robin
> 
>>
>> Regards,
>> Bernhard
>>
>