Exit the main process after all closure steps are completed

[nunoguedelha]

The Two Main Issues Addressed Here

• Some modules included in the main script may add listeners to the closure events SIGxxxx, which call process.exit() or any other procedure that we don't want being executed before the terminationHandler of the main application.
• The process should exit automatically if there is no pending queued events in the Node Event Loop. But for some reason, yet to be analyzed, there are pending events preventing the process to exit on its own. A possible workaround is to call process.exit().

References

Event Loop
https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/#setimmediate-vs-settimeout
https://nodejs.dev/learn/the-nodejs-event-loop
https://nodejs.dev/learn/understanding-process-nexttick
https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop

Avoiding a Premature Exit

• Before even adding the listener to the a given closure signal (SIGINT, etc): (a) backup the existing listeners registered for that same signal, (b) remove them, (c) add our own listener terminationHandler, (d) restore the backuped listeners after completing terminationHandler. => Exit the main process after all closure steps are completed #62

Workaround

• Workaround: calling process.exit() after execution of subset C (very last). => Exit the main process after all closure steps are completed #62

Further Analysis

• Closing all the ports manually (instead of letting the on exit callback do it). => moved to Some pending events are preventing the Telemetry Data Server process to exit on its own #76 .
• Checking the pending handles and requests in the Event Loop. => moved to Some pending events are preventing the Telemetry Data Server process to exit on its own #76 .

[nunoguedelha]

Closing all the ports manually, and checking the pending handles and requests in the Event Loop ⌛

• Closing the ports
  2952072
• checking the pending handles and requests in the Event Loop
  nodejs/node#1128
  https://github.com/thlorenz/active-handles
  In the last promise handling...
  

  yarp-openmct/iCubTelemVizServer/terminationHandler.js

  Lines 22 to 29 in 2952072

  	Promise.all([subsetCpromise,subsetApromise.closeServers]).then(
  	function(values) {
  	values.forEach((v) => console.log(v));
  	// console.log(process._getActiveHandles());
  	console.log(process._getActiveRequests());
  	}
  	).catch(console.error);
  	}

  
  we use process._getActiveHandles() and process._getActiveRequests() to get the pending actions in the Event Loop.

=> Moved to #76

[nunoguedelha]

Avoiding a Premature Exit

Before even adding the listener to the a given closure signal (SIGINT, etc), we execute the following a the end of the main script:

• backup the existing listeners registered for that same signal (process.listeners(eventName)),
• remove them (process.removeAllListeners([eventName])),
• add our own listener terminationHandler (process.once).

After completing terminationHandler, so, after Subset C...

yarp-openmct/iCubTelemVizServer/terminationHandler.js

Lines 22 to 27 in b07deb5

	Promise.all([subsetCpromise,subsetApromise.closeServers]).then(
	function(values) {
	values.forEach((v) => console.log(v));
	process.exit();
	}
	).catch(console.error);

...do:

• restore the backed-up listeners (process.on: this restores the 'once' wrapper. For Node v14.17.0 use process.rawListeners(eventName) instead of process.listeners(eventName))
• Add a listener which includes the workaround mentioned in the issue description. The listener is the last after the backuped listeners (process.on(signal,() => {process.exit()}))
• Emit again the captured signal signal.