Support multi-threaded coroutines on Kotlin/Native

You can have multiple threads in Kotlin/Native. ~Each thread can have its own event loop with runBlocking and have number of coroutines running there~. Currently communication between those threads via coroutine primitives (like channels) is not supported. This issue it to track enhancement of Kotlin/Native in kotlinx.coroutines library so that all the following becomes possible:

• Launching coroutines from one thread with a dispatcher on another thread
• Await/join coroutine running on another thread
• Send/Receive elements to/from coroutines on other threads

UPDATE: Currently, coroutines are supported only on the main thread. You cannot have coroutines off the main thread due to the way the library is currently structured.

UPDATE 2: the separate library version that supports Kotlin/Native multithreading is released on a regular basis. For the details and limitations, please follow kotlin-native-sharing.md document. The latest version: 1.5.2-native-mt

[qwwdfsad]

I suggest all users of native-mt builds to read the following announcement: https://blog.jetbrains.com/kotlin/2021/08/try-the-new-kotlin-native-memory-manager-development-preview/

We aim to merge the support of the new MM into coroutines 1.6.0 (https://github.com/Kotlin/kotlinx.coroutines/issues/2914) and then, depending on the stability of the new MM, promote it as the default way to write concurrent code on Native platforms.

native-mt won’t be supported as soon as the new MM is stable.

[elizarov]

📣 UPDATE: Version 1.3.7-native-mt-1.4-M2 is released — the fresh update of native-mt for Kotlin 1.4-M2. For leak-free operation, this version should be used with a specially patched version of Kotlin: 1.4-M2-eap-23 (it differs from stock 1.4-M2 in Kotlin/Native only).

This version includes fixes for additional bugs discovered by native-mt early adopters (thanks a lot for all of you ❤️):

• Fixed awaitAll in multi-threaded scenarios (#2025)
• Made sure that FreezingException does not break coroutine machinery (#2064)

The code is now based on the new features available in Kotlin/Native 1.4 (namely WorkerBoundReference) and there will be no more updates from native-mt branch for Kotlin 1.3. We plan to include the code from native-mt branch into the default kotlinx.coroutines distribution as Kotlin/Native version of the library for upcoming Kotlin 1.4 release.