allow integrating with external event loop

[Guest0x0]

closes #526

moonbitlang/async relies on an event loop to perform asynchronous IO operations internally. However, some applications may have their own event loop as well. For example, GUI frameworks typically has their own event loop. Event loops are non-modular by nature, so to integrate two event loops, the only possible way is to run each loop in a dedicated thread, and use a self-pipe or similar to communicate between the two loops. Some libraries require their event loop being run in the main thread. In that case, the only possible workaround before this PR is to run all MoonBit code in a dedicated thread, which is tedious.

This PR add native support for such external loop integration. A new API, @async.set_external_event_loop, is introduced, which enables external event loop integration by providing an event loop implementation. The implementation is abstracted via a trait @moonbitlang/async/external_loop_integration.ExternalEventLoop. The user-provided loop should provide its wait function with timeout support, and a callback for waking the loop for use by moonbitlang/async. See the documents of that trait for more details on which operations are needed and semantic caveats.

@async.set_external_event_loop can be used with async fn main. In fact it can be called in async fn main as well, as long as it is invoked before any actual async operations. When moonbitlang/async sees that an external event loop is set, it automatically enter external loop integration mode. In this mode, the waiting part of moonbitlang/async event loop will be spawned in a dedicated thread, and the user-provided loop will be run in the main thread instead.

[coveralls]

Coverage Report for CI Build 970

Coverage decreased (-0.007%) to 76.222%

Details

• Coverage decreased (-0.007%) from the base build.
• Patch coverage: 23 uncovered changes across 4 files (79 of 102 lines covered, 77.45%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/internal/event_loop/event_loop.mbt	62	47	75.81%
src/internal/event_loop/event_loop_unix.mbt	15	11	73.33%
src/external_loop_integration/internal/external_loop_test/main.mbt	15	12	80.0%
src/external_loop_integration/internal/external_loop_test/unimplemented.mbt	1	0	0.0%
Total (5 files)	102	79	77.45%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4971
Covered Lines:	3789
Line Coverage:	76.22%
Coverage Strength:	11455.44 hits per line

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💛 - Coveralls