[systemd-devel] possible bug guessing mainpid when service type=forking..

[Marius Tolzmann]

hi..

On 06/20/11 21:36, Lennart Poettering wrote:
> 
> This very much sounds as if the service does not wait in the top-level
> process that the daemon process is properly forked and finished writing
> the PID file. systemd tries to read the PID file immediately after the
> top-level process exits which means that systemd will race against the
> daemon there.
> 
> So my guess is that ypbind-mt does something like this:
> 
>    fork() --> fork() --> write_pid_file()
>    exit()     exit()     run_main_loop()

thats it..

> but it should be doing:
> 
>    fork() ------------> fork() --> write_pid_file()
>    wait_for_notify()    exit()     notify_grandparent()
>    exit()                          run_main_loop()
>                                    ... 

the run-in-foreground patch works fine.. and as far as i understood
Type=forking is just good for compatibility and daemons should not fork
away anymore!?

> i.e. the main process should not exit before it hasn't ensure that the
> PID file of the daemon process has been written. Otherwise any usage of
> the PID file is necessarily racy.

but it doesn't work without setting PIDFile= either - so the PID itself
is racy, too.

>> BTW a patch for ypbind-mt adding a -foreground commandline switch will
>> be sent to the maintainers soon..
> 
> Nice, this is good to hear, Type=simple is definitely the ncier solution.

may be it could be mentioned in the manpage where Type=forking is
explained that there could be a race if the daemon is forking twice or
if it behaves odd in another way.. - GuessMainPID= already states that
it may not work reliable..

there was nothing written to the logs that pointed in that direction and
increasing verbosity changed everything 8) tricky situation and i think
systemd should not be blamed for this.. ever 8)

bye marius..


-- 
Dipl.-Inf. Marius Tolzmann <marius.tolzmann at molgen.mpg.de>
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