src,async_hooks,n-api: refactor async callback handling

[addaleax]

• Refactor how asynchronous code is executed from within Node
• Merge the two almost-but-not-quite identical MakeCallback() implementations
• Provide a public CallbackScope class for embedders in order to enable MakeCallback()-like behaviour without tying that to calling a JS function
• Enable combining N-API async work with async-hooks

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

/cc @nodejs/async_hooks
/cc @nodejs/n-api (Who may or may not only want to review the N-API commit; this is a breaking N-API change)

[addaleax]

CI: https://ci.nodejs.org/job/node-test-commit/11632/
CI: https://ci.nodejs.org/job/node-test-commit/11633/
CI: https://ci.nodejs.org/job/node-test-commit/11634/
CI: https://ci.nodejs.org/job/node-test-commit/11635/
CI: https://ci.nodejs.org/job/node-test-commit/11636/
CI: https://ci.nodejs.org/job/node-test-commit/11637/

[jasongin]

N-API parts look good.

I'm still planning to add an async context parameter to napi_make_callback() to support other async scenarios that don't use the N-API async work APIs.

If I'm understanding correctly, the change in this PR means that when the N-API async work APIs are used, during the napi_async_complete_callback invocation the current async context is already correct. Usually that callback will then invoke napi_make_callback(). So the additional async context parameter to napi_make_callback() can be optional, and the current async context used if another context is not specified.

[addaleax]

@jasongin This PR would mean that napi_make_callback would be unnecessary for most completion C callbacks; N-API users who use the async work API wouldn’t need to bother with any of the async stuff and could just use a plain function call.

[jasongin]

Oh, so the async work callback should just call napi_call_function() instead? OK, that's simpler.

Then the only need for napi_make_callback() (updated with async context parameter) will be for async code that doesn't use the async work API.

[jasongin]

In that case should test/addons-napi/test_async/test_async.cc be updated to call napi_call_function() instead of napi_make_callback() (in 2 places)?

[addaleax]

@jasongin Exactly 👍

In that case should test/addons-napi/test_async/test_async.cc be updated to call napi_call_function() instead of napi_make_callback() (in 2 places)?

Right. Done!

[addaleax]

Rebased … maybe somebody of @AndreasMadsen@trevnorris@refack@TimothyGu@tniessen@bnoordhuis@jasnell could be reviewer (if only for the yet-unreviewed non-N-API part)?

CI: https://ci.nodejs.org/job/node-test-commit/11769/

[thefourtheye]

Nit: exposed?

[addaleax]

Right, fixed.