Restarting trell_master results in zombie process of old trell_master

[MrGaribaldi]

When restarting trell_master through the web interface, the currently running process will not exit nicely, but becomes a zombie process.

Cleric needed to turn undead and exorcise #67 to stop it from becoming a powerful necromancer spawning undead processes at the press of a button.

[cdyk]

A process (child) is always spawned by another process (parent). When a process terminates, the parent should be able to retrieve the exit code of its child.

So a zombie is a process that has terminated, but the parent hasn't retrieved the exit code yet. Then the process lingers around in the process table, even though it is terminated.

So: to wipe a zombie process, the parent process can specify explicitly that it doesn't care about the exit code for any of its children, or it can retrieve the exit code, or one can terminate the parent process (so that the child gets adopted by init, which will read its exit code).

The problem with trell_master is that it is spawned by apache, and apache has a host of processes running. So, first of all, we cannot be sure if the process that termitates the process is the same that spawned it, and hence the terminator cannot retrieve the exit code (currently, if you're lucky and these two actions has been done by the same process, the zombie issue is avoided).

If you're unlucky, then the only way is to mess with the signal-handlers of the apache process, which I'm pretty sure that apache already has a use for.

So, as a workaround, restrarting apache should wipe all the zombie processes (as this will terminate the parent processes, and the children will be adopted by init).

In the long term, a better solution is to let an init-script start and stop trell_master the same way apache is started and stopped.

[cdyk]

This might be resolved by using a wrapper-process between the apache process and the master job. The wrapper process is started instead of the master job, and the wrapper process starts the master process and immediately terminates. Our apache module will wait on the wrapper, letting it terminate cleanly. The master process is orphaned, and will be adopted by init, which avoids it becomming a zombie.