Ability to shutdown Node in-flight

[gireeshpunathil]

• Version: All
• Platform: All
• Subsystem: uv, core

Scope: C++ applications that embed node as a shared library

Issue: Node's life cycle is designed around the event loop method uv_run that assumes exit only when the last handle is drained from the loop, while the embedder may have a larger life cycle in which node workload may be a small subset. For example, the embedder may want to recycle node in response to a shift in workload / configuration, even while the loop is filled with active handles. Current Node / libuv design does not provide such an on-demand exit route.

Proposal: Define a top level API (such as quiesce) that gracefully shuts down the loop and returns from node::Start().

[mhdawson]

FYI @nodejs/delivery-channels

[mhdawson]

@yhwang FYI as this is likely one of the use cases we'd want to include in our extended testing for the shared library support.

[addaleax]

I have written code for Ayo that does basically exactly this. I’ll try to port it over, especially since I’ve been meaning to do that anyway.

[mhdawson]

@addaleax great to hear :)

[addaleax]

Opened #19377 with most of those commits, would be cool if those who are interested could take a look.

[yhwang]

One of the scenario that I tried to handle before is canceling the long-running javascript execution. And it's a tough case because of the resource clean up. For using node.js as shared lib, the mechanism that we provide to shutdown node may also need to handle this case.

[mhdawson]

Looks like #19377 landed, @gireeshpunathil can you confirm we have what is needed for your original request.

[zeke]

TIL

To quiesce is to pause or alter a device or application to achieve a consistent state, usually in preparation for a backup or other maintenance. In software applications that modify information stored on disk, this generally involves flushing any outstanding writes; see buffering.

[addaleax]

I think it’s not quite enough. We basically still want something that exposes uv_stop() and set a flag on the Environment which indicates that the event loop should stop, right?

I have something similar in my upcoming Workers PR, so I can try to make this happen.

It might also make sense to put this in a larger picture and ask what other sensible exiting modes Node.js could use. Rather than introducing a new functions, we maybe want something like this? (API bikeshedding encouraged)

• process.exit({ code, mode: 'immediately' }): Run no further JS code. Basically process.reallyExit().
• process.exit({ code, mode: 'runExitListeners' }): Like immediately, but also runs process.on('exit') listeners. [Current default]
• process.exit({ code, mode: 'graceful' }): Does not actually call exit(3). Rather, we terminate JS execution and return from the main loop. [This is what process.exit() currently does in my upcoming Workers PR.]
• process.exit({ code, mode: 'quiesce' }): Only calls uv_stop() and sets a flag for indicating that we want to return from the main loop, but returns back to JS. Basically graceful without the execution termination.