How to find out whether there are still events registered in the event loop (bonus: how many)

Question

I am trying to write a Node.js program to execute and monitor javascript programs. I am looking for a way to find out whether the monitored program is still "running" i.e. doing anything useful.

In my current approach, when receiving code to test, I start a new child process and hand the code to it. The child process instruments the code creates a Sandbox using Contextify and executes the code using this sandbox.

After the sandbox.run(code) call returned I know that the blocking part of the code finished and can show that in the UI. However, I don't now whether the code registered any timers using setTimeouts or created any other event sources that would cause parts of the code to be exited later. So I don't know whether it's really "finished" yet.

Is there a way in Node.js to check whether there are still events on the event loop to be handled (or even better, how many are left)?

I found this other question, but it only talks about how to monitor the event loop to find out whether the performance of node is still fine. But I'm not interested in the performance (I don't care if the executing code is blocking for 10s or only doing something for 1ms every 2 minutes) and I don't want to use outside tools but find out about the state of the event loop from inside node itself. Is that possible?

Answer 1

I solved my problem in a way, although I didn't find a general answer to the question.

The idea here is that the process will exit by itself if it has executed all the code it was started with and no EventEmitters are registered anymore. This was basically what I wanted, since I wanted to be notified when the process was "done" and now I could just listen to the "exit" event of the child_process.

But my process that executed the code didn't exit by itself. This had two reasons:

1. I used a timer to regularly send the data gathered about the execution to the parent process. If such a timer is registered the process won't exit. You could unref the timer but I was afraid that would lead to data loss since the process could quit before the last bit of data was sent out (since it wouldn't wait for the timer to execute). So I changed my code to only schedule a timer if there was data to be sent out instead of regularly checking for data to be sent.
2. I used fork to create the child process. This also creates a communication channel between the parent and child process and since I needed to send the code to execute to the child_process I registered the child process for messages received from the parent using process.on("message", callback). However, now we have another EventEmitter registered which prevents the process from quitting. Luckily, I realized that I only needed one message from the parent process and no further messages so I could remove the event emitter after receiving that message. So instead of process.on() I used process.once() which will execute the callback only once and automatically remove the event emitter after that. Exactly what I wanted. Alternatively you could use process.removeListener().

Now I just wait for the child_process to exit and thus know that everything is finished and can notify the client.

So the solution here is to make sure none of your own EventEmitters keep the process alive and then just wait for it to exit.