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

• [x] 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. => #62

Workaround

• [x] Workaround: calling process.exit() after execution of subset C (very last). => #62

Further Analysis

• [x] ~Closing all the ports manually (instead of letting the on exit callback do it).~ => moved to #76 .
• [x] ~Checking the pending handles and requests in the Event Loop.~ => moved to #76 .

[nunoguedelha]

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

• Closing the ports https://github.com/ami-iit/yarp-openmct/commit/2952072f755cef70c5d97316f8ef66c53e55540d
• checking the pending handles and requests in the Event Loop nodejs/node#1128 https://github.com/thlorenz/active-handles In the last promise handling... https://github.com/ami-iit/yarp-openmct/blob/2952072f755cef70c5d97316f8ef66c53e55540d/iCubTelemVizServer/terminationHandler.js#L22-L29 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:

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

After completing terminationHandler, so, after Subset C... https://github.com/ami-iit/yarp-openmct/blob/b07deb537ee3513b16bd3a14115c469449d38ba9/iCubTelemVizServer/terminationHandler.js#L22-L27

...do:

• [x] 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))
• [x] 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()}))
• [x] Emit again the captured signal signal.