Terminating zombie child processes forked from socket server

Question

Disclaimer

I am well aware that PHP might not have been the best choice in this case for a socket server. Please refrain from suggesting differen

Answer 1

I promise there is a solution at the end :P

Alright... so here we are, 10 days later and I believe that I have solved this issue. I didn't want to add onto an already longish post so I'll include in this answer some of the things that I tried.

Taking @sym's advice, and reading more into the documentation and the comments on the documentation, the pcntl_waitpid() description states :

If a child as requested by pid has already exited by the time of the call (a so-called
"zombie" process), the function returns immediately. Any system resources used by the child
are freed...

So I setup my pcntl_signal() handler like this -

function sig_handler($signo){ 
    global $childProcesses;
    $pid = pcntl_waitpid(-1, $status, WNOHANG);
    echo "Sound the alarm! ";
    if ($pid != 0){
        if (posix_kill($pid, 9)){
            echo "Child {$pid} has tragically died!".PHP_EOL;
            unset($childProcesses[$pid]);
        }
    }
}
// These define the signal handling
// pcntl_signal(SIGTERM, "sig_handler");
// pcntl_signal(SIGHUP,  "sig_handler");
// pcntl_signal(SIGINT, "sig_handler");
pcntl_signal(SIGCHLD, "sig_handler");

For completion, I'll include the actual code I'm using for forking a child process -

function broadcastData($socketArray, $data){
        global $db,$childProcesses;
        $pid = pcntl_fork();
        if($pid == -1) {
                // Something went wrong (handle errors here)
                // Log error, email the admin, pull emergency stop, etc...
                echo "Could not fork()!!";
        } elseif($pid == 0) {
                // This part is only executed in the child
                foreach($socketArray AS $socket) {
                        // There's more happening here but the essence is this
                        socket_write($socket,$msg,strlen($msg));

                        // TODO : Consider additional forking here for each client. 
                }
                // This is where the signal is fired
                exit(0);
        }

        // If the child process did not exit above, then this code would be
        // executed by both parent and child. In my case, the child will 
        // never reach these commands. 
        $childProcesses[] = $pid;
        // The child process is now occupying the same database 
        // connection as its parent (in my case mysql). We have to
        // reinitialize the parent's DB connection in order to continue using it. 
        $db = dbEngine::factory(_dbEngine); 
}

^{Yea... That's a ratio of 1:1 comments to code :P}

So this was looking great and I saw the echo of :

Sound the alarm! Child 12345 has tragically died!

However when the socket server loop did it's next iteration, the socket_select() function failed throwing this error :

PHP Warning: socket_select(): unable to select [4]: Interrupted system call...

The server would now go into a vegetative state totally oblivious to the world around him, not responding to any requests other than manual kill commands from a root terminal.

---

I'm not going to get into why this was happening or what I did after that to debug it... lets just say it was a frustrating week...

much coffee, sore eyes and 10 days later...

^{Drum roll please}

TL&DR - The Solution :

Mentioned here in a comment from 2007 in the php sockets documentation and in this tutorial on stuporglue (search for "good parenting"), one can simply "ignore" signals comming in from the child processes (SIGCHLD) by passing SIG_IGN to the pcntl_signal() function -

pcntl_signal(SIGCHLD, SIG_IGN);

Quoting from that linked blog post :

If we are ignoring SIGCHLD, the child processes will be reaped automatically upon completion.

Believe it or not - I included that pcntl_signal() line, deleted all the other handlers and things dealing with the children and it worked! There were no more processes left hanging around!

In my case, it really did not interest me to know exactly when a child process died, or who it was, I wasn't interested in them at all - just that they didn't hang around and crash my entire server :P