[Twisted-Python] My twisted application could not start after reboot be cause of "twistd.pid"

[Alexandre Quessy]

Hello everyone,


2010/3/16  <exarkun at twistedmatrix.com>:
> On 07:27 am, tim at commsecure.com.au wrote:
>>On 03/16/2010 06:07 PM, Peter Cai wrote:
>>>I think an init.d script could resolve this problem.
>>>But I am curious about why "turning off power without shutdown twisted
>>>server before" could cause such a problem.
>>>
>>>Is it a bug or it has some more fundamental reason?
>>
>>When twistd starts up, it creates a .pid file as a signal that it's
>>running. If you try and start it a second time, it notices that the
>>.pid
>>file exists, and refuses to run a second time. When twistd is shut down
>>cleanly, it removes the .pid file so that it can be run again.
>>
>>Unfortunately, this means that if twistd is shutdown *uncleanly* (by a
>>power failure, or SIGKILL, or some other means), the .pid file must be
>>removed before twistd will start again.
>>
>>This isn't really a bug in twistd as such; almost all Unix daemons use
>>the same fundamental system. Try running "ls /var/run/*.pid" on a Linux
>>system, for example.
>>
>>It's possible to add refinements to the basic .pid system, to make it
>>more resistant to this kind of problem (off the top of my head,
>>checking
>>that the named PID exists,
>
> Actually it does this already.  But this is a particularly ineffective
> check, especially for processes launched at boot time, because of the
> very high possibility of collisions.

It is possible to retrieve the command line that was used to start a
process. This would be a simple way to check for matches. The command
line used to start the twistd process could even be stored as well for
comparison.

>>
>>that it's a process of the right kind,
>>checking for stale .pid files with exclusive file locks), but all of
>>them require more code to implement, reduce portability, and still have
>>corner-cases that would require an admin to manually delete the .pid
>>file anyway.
>
> Jean-Paul
>
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Best,
-- 
Alexandre Quessy
http://alexandre.quessy.net/