child_process: add hasRef to ChildProcess

[daeyeon]

Refs: #42091

The issue above reported a specific use case about hasRef().
After reading the issue, I started thinking users may naturally expect
hasRef method if ref() and unref() exist on an object they use.
As of now, however, hasRef() exists on Timer only.

This commit suggests adding hasRef method to ChildProcess first.
There are more similar cases. (fs.FSWatcher, fs.StatWatcher, net.Socket,
and so on)

[jasnell]

Suggested change

	return this._handle ? this._handle.hasRef() : false;
	return !!this._handle?.hasRef()

[daeyeon]

applied the suggestion.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/43528/

[RaisinTen]

cc @nodejs/child_process

[RaisinTen]

Actually, I don't think we should expose hasRef() from the ChildProcess JS object because jest uses async_hooks to collect open handles and the init() callback gets called with resource set to the underlying PROCESSWRAP handle object (this._handle) which already contains the hasRef() method.

To be able to correctly report open ChildProcess handles correctly, jest needs to change https://github.com/facebook/jest/blob/defbc0535a46119d4682f97bf8a13b5562c1445b/packages/jest-core/src/collectHandles.ts#L96:

• to also include PROCESSWRAP OR
• just accept all types of async resources

The reason why Timer objects have the hasRef() method is that it isn't backed up by a C++ HandleWrap instance like the other cases. Essentially, the resource variable that the async_hooks init() callback gets called with is the same as the return value of setTimeout().

[daeyeon]

I haven't gotten a bug report in Jest about them, so I have no opinion.
On a more serious note, I think it makes sense for ref, unref and hasRef to always exist together.

In the issue above, although SimenB had no opinion requiring ChildProcess.hasRef() for Jest, I actually paid more attention to his next words regarding consistency across APIs. But now I understand that exposing ref()/unref() pair is common and Timeout.hasRef() is an exceptional case. Thanks for the review!