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

exarkun at twistedmatrix.com exarkun at twistedmatrix.com
Tue Mar 16 07:31:05 EDT 2010

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 
>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, 
>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.
>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.


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