Handling SIGCHLD signal in Java

Question

I have a Java process, that (re)starts a Linux process, which spawns two daemonised child processes and dies. (It's a wrapper to HAProxy, which is configured as a daemon with 2 Processes)

On every re-start, the process table has two more Zombie processes. To prevent these Zombies, I have implemented the following:

final static SignalHandler _signalHandler = new SignalHandler() {

    @Override
    public void handle(Signal signal) {
        LOG.info("Received signal: {}",signal.getName());
    }
};

public HaproxyWrapper() {
    Signal.handle(new Signal("CHLD"), _signalHandler);
    LOG.info("Registered SIGCHLD signal handler");
}

I see the 'Registered SIGCHLD' log in the output, but never the 'Received signal: SIGCHLD'.

Am I doing this wrong?

Alternatively, 'the simplest thing that could possibly work' - I could create a shell script called start_haproxy.sh which will both call haproxy and handle the SIGCHLD. How do I handle SIGCHLD in bash? (handle -> ignore)

Thanks.

When you receive this, the child has exited. But why should this help you - the child processes exist as demons. — laune
laune, can you please elaborate? the Child (haproxy) has two children. So we have a grand-parent-parent-child situation: haproxyWrapper (alive) --> haproxy (dies) --> haproxy daemon 1,2 (alive). — JRun
The haproxWrapper would receive a SIGCHLD. If the daemons really are daemons the shouldn't end up as zombies, and knowing (in the wrapper) that haproxy dies doesn't help anyway. — laune
@laune That's just how HAProxy works, in daemon mode: Main process brings up two workers and dies. — JRun

Peter Lawrey Peter Lawrey · Accepted Answer · 2015-01-11T10:17:42

I am pretty sure that Java already ignores the SIGCHLD which is why you can't trap it. Only a running process would get this signal and if you Process.waitFor() you will be notified that a child process has died.

Zombie processes are ones which cannot be killed, i.e. they live on after they should have died, usually due to a problem in the OS. I suspect you are concerned about orphaned processes, i.e. when you have a process whose parent had died.

The simplest way to prevent that is to make your parent wait for it's children to die.

BTW daemon processes often run as orphaned processes. Are you sure this is really a problem?

Handling SIGCHLD signal in Java

2 Answers