Handling Exceptions in Gearman Tasks (Even Background Ones)

I recently had some issues with Gearman tasks throwing exceptions and killing the whole Gearman daemon. This made it nearly impossible to trace errors back to their origin, because the logged exception stack trace didn’t provide much useful information, because it just logged where it failed in Gearman – not the actual file and line of code that was doing the work. I dug into the code and started trying things like GearmanClient::setExceptionCallback and running the tasks, but since the tasks were being run with addTaskBackground instead of just addTask, the callbacks were never getting executed, and I still was not able to do anything to handle exceptions for the jobs that were being run (and they were still killing the Gearman daemon). Clearly, I was going to have to get a little more creative.

The only other place to add code that will catch exceptions for all jobs run is in the GearmanWorker::addFunction method. So I looked at the following one-liner for adding named job callbacks:

$worker->addFunction($name, $task);

And replaced it with a closure that uses a try/catch and then logs any exceptions to Exceptional so we can see the full stack trace and exact point of failure for any job – even background jobs:

$worker->addFunction($name, function() use($task) {
     try {
         $result = call_user_func_array($task, func_get_args());
     } catch(\Exception $e) {
         $result = GEARMAN_WORK_EXCEPTION;
         echo "Gearman: CAUGHT EXCEPTION: " . $e->getMessage();
         // Send exception to Exceptional so it can be logged with details
         Exceptional::handle_exception($e, FALSE);

     return $result;

And it works beautifully. Now all the jobs are run, the Gearman daemon is never killed by a PHP process, and all the exceptions are logged with full granular details that makes it easy to troubleshoot and fix any errors.