[native-mt] Darwin single thread dispatchers leak ObjC autorelease references

[andersio]

Kotlin: 1.4.10
Coroutines: 1.3.9-native-mt-2

Also filed as https://youtrack.jetbrains.com/issue/KT-42822 for the K/N runtime.

KT-42822 is about K/N Workers not wrapping its work items with an autorelease pool. So when Kotlin code calls into ObjC/Swift on a Worker, any autorelease reference leaks unconditionally, e.g., any id or NSObject* return values.

However, even if KT-42822 is fixed, a fix at the coroutines library is still necessary, since kotlinx-coroutines-core runs its own EventLoop over the one in Worker.

The native single thread CoroutineDispatcher wraps the entire EventLoop with an autorelease pool, but not individual blocks as they get executed. This means autorelease references in these blocks would be pushed to a pool that is never drained until the EventLoop is closed.

So in other words, autorelease references are effectively leaked on any long living dispatchers created by newSingleThreadContext().

kotlinx.coroutines/kotlinx-coroutines-core/native/src/Workers.kt

Lines 48 to 50 in 1b0e196

	workerMain {
	runEventLoop(ThreadLocalEventLoop.eventLoop) { isClosed.value }
	}

This could be solved by either:

• native-mt single-thread dispatchers wrap individual blocks with autoreleasepool before executing them; or

• wrap processNextEvent() with autoreleasepool {} maybe?

  kotlinx.coroutines/kotlinx-coroutines-core/native/src/Builders.kt

  Lines 77 to 83 in 1b0e196

  	eventLoop?.incrementUseCount()
  	val thread = currentThread()
  	while (!isCompleted()) {
  	val parkNanos = eventLoop?.processNextEvent() ?: Long.MAX_VALUE
  	if (isCompleted()) break
  	thread.parkNanos(parkNanos)
  	}

[slavonnet]

maybe native have some issue and park thread forever? (if parkNanos == Long.MAX_VALUE)

#2374

[qwwdfsad]

Fixed in 1.4.3-native-mt