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* Kotlin version 1.3.70, using HMPP. * Provides newSingleThreadedContext. * Provides Dispatchers.Main on iOS, Dispatchers.Default everywhere. * Coroutine references (Job) and all kinds of channels are shareable across workers. * Each individual coroutine is confined to a single worker. Known problems: * There are some memory leaks.
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| # Sharing and background threads on Kotlin/Native | ||
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| ## Preview disclaimer | ||
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| This is a preview release of sharing and backgrounds threads for coroutines on Kotlin/Native. | ||
| Details of this implementation will change in the future. See also [Known Problems](#known-problems) | ||
| at the end of this document. | ||
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| ## Introduction | ||
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| Kotlin/Native provides an automated memory management that works with mutable data objects separately | ||
| and independently in each thread that uses Kotlin/Native runtime. Sharing data between threads is limited: | ||
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| * Objects to be shared between threads can be [frozen](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/freeze.html). | ||
| This makes the whole object graph deeply immutable and allows to share it between threads. | ||
| * Mutable objects can be wrapped into [DetachedObjectGraph](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/-detached-object-graph/index.html) | ||
| on one thread and later reattached onto the different thread. | ||
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| This introduces several differences between Kotlin/JVM and Kotlin/Native in terms of coroutines that must | ||
| be accounted for when writing cross-platform applications. | ||
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| ## Threads and dispatchers | ||
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| An active coroutine has a mutable state. It cannot migrate from thread to thread. A coroutine in Kotlin/Native | ||
| is always bound to a specific thread. Coroutines that are detached from a thread are currently not supported. | ||
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| `kotlinx.coroutines` provides ability to create single-threaded dispatchers for background work | ||
| via [newSingleThreadContext] function that is available for both Kotlin/JVM and Kotlin/Native. It is not | ||
| recommended shutting down such a dispatcher on Kotlin/Native via [SingleThreadDispatcher.close] function | ||
| while the application still working unless you are absolutely sure all coroutines running in this | ||
| dispatcher have completed. Unlike Kotlin/JVM, there is no backup default thread that might | ||
| execute cleanup code for coroutines that might have been still working in this dispatcher. | ||
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| For interoperability with code that is using Kotlin/Native | ||
| [Worker](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/-worker/index.html) | ||
| API you can get a reference to single-threaded dispacher's worker using its [SingleThreadDispatcher.worker] property. | ||
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| A [Default][Dispatchers.Default] dispatcher on Kotlin/Native contains a single background thread. | ||
| This is the dispatcher that is used by default in [GlobalScope]. | ||
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| > This limitation may be lifted in the future with the default dispatcher becoming multi-threaded and/or | ||
| > its coroutines becoming isolated from each other, so please do not assume that different coroutines running | ||
| > in the default dispatcher can share mutable data between themselves. | ||
| A [Main][Dispatchers.Main] dispatcher is | ||
| properly defined for all Darwin (Apple) targets, refers to the main thread, and integrates | ||
| with Core Foundation main event loop. | ||
| On Linux and Windows there is no platform-defined main thread, so [Main][Dispatchers.Main] simply refers | ||
| to the current thread that must have been either created with `newSingleThreadContext` or running | ||
| inside [runBlocking] function. | ||
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| The main thread of application has two options on using coroutines. | ||
| A backend application's main thread shall use [runBlocking]. | ||
| A UI application running on one Apple's Darwin OSes shall run | ||
| its main queue event loop using `NSRunLoopRun`, `UIApplicationMain`, or ` NSApplicationMain`. | ||
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| ## Switching threads | ||
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| You switch from one dispatcher to another using a regular [withContext] function. For example, a code running | ||
| on the main thread might do: | ||
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| ```kotlin | ||
| // in the main thead | ||
| val result = withContext(Dispatcher.Default) { | ||
| // now executing in background thread | ||
| } | ||
| // now back to the main thread | ||
| result // use result here | ||
| ``` | ||
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| If you capture a reference to any object that is defined in the main thread outside of `withContext` into the | ||
| block inside `withContext` then it gets automatically frozen for transfer from the main thread to the | ||
| background thread. Freezing is recursive, so you might accidentally freeze unrelated objects that are part of | ||
| main thread's mutable state and get | ||
| [InvalidMutabilityException](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/-invalid-mutability-exception/index.html) | ||
| later in unrelated parts of your code. | ||
| The easiest way to trouble-shoot it is to mark the objects that should not have been frozen using | ||
| [ensureNeverFrozen](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/ensure-never-frozen.html) | ||
| function so that you get exception in the very place they were frozen that would pinpoint the corresponding | ||
| `withContext` call in your code. | ||
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| The `result` of `withContext` call can be used after `withContext` call. It gets automatically frozen | ||
| for transfer from background to the main thread, too. | ||
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| A disciplined use of threads in Kotlin/Native is to transfer only immutable data between the threads. | ||
| Such code works equally well both on Kotlin/JVM and Kotlin/Native. | ||
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| > Note: freezing only happens when `withContext` changes from one thread to another. If you call | ||
| > `withContext` and execution stays in the same thread, then there is not freezing and mutable data | ||
| > can be captured and operated on as usual. | ||
| The same rule on freezing applies to coroutines launched with any builder like [launch], [async], [produce], etc. | ||
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| ## Communication objects | ||
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| All core communication and synchronization objects in `kotlin.coroutines` such as | ||
| [Job], [Deferred], [Channel], [BroadcastChannel], [Mutex], and [Semaphore] are _shareable_. | ||
| It means that they can be frozen for sharing with another thread and still continue to operate normally. | ||
| Any object that is transferred via a frozen (shared) [Deferred] or any [Channel] is also automatically frozen. | ||
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| Similar rules apply to [Flow]. When an instance of a [Flow] itself is shared (frozen), then all the references that | ||
| are captured in to the lambdas in this flow operators are frozen. Regardless of whether the flow instance itself | ||
| was frozen, by default, the whole flow operates in a single thread, so mutable data can freely travel down the | ||
| flow from emitter to collector. However, when [flowOn] operator is used to change the thread, then | ||
| objects crossing the thread boundary get frozen. | ||
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| Note, that if you protect any piece of mutable data with a [Mutex] or a [Semaphore] then it does not | ||
| automatically become shareable. In order to share mutable data you have to either | ||
| wrap it into [DetachedObjectGraph](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/-detached-object-graph/index.html) | ||
| or use atomic classes ([AtomicInt](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.native.concurrent/-atomic-int/index.html), etc). | ||
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| ## Cyclic garbage | ||
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| Code working in a single thread on Kotlin/Native enjoys fully automatic memory management. Any object graph that | ||
| is not referenced anymore is automatically reclaimed even if it contains cyclic chains of references. This does | ||
| not extend to shared objects, though. Frozen immutable objects can be freely shared, even if then can contain | ||
| reference cycles, but shareable [communication objects](#communication-objects) leak if a reference cycle | ||
| to them appears. The easiest way to demonstrate it is to return a reference to a [async] coroutine as its result, | ||
| so that the resulting [Deferred] contains a reference to itself: | ||
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| ```kotlin | ||
| // from the main thread call coroutine in a background thread or otherwise share it | ||
| val result = GlobalScope.async { | ||
| coroutineContext // return its coroutine context that contains a self-reference | ||
| } | ||
| // now result will not be reclaimed -- memory leak | ||
| ``` | ||
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| A disciplined use of communication objects to transfer immutable data between coroutines does not | ||
| result in any memory reclamation problems. | ||
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| ## Shared channels are resources | ||
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| All kinds of [Channel] and [BroadcastChannel] implementations become _resources_ on Kotlin/Native when shared. | ||
| They must be closed and fully consumed in order for their memory to be reclaimed. When they are not shared, they | ||
| can be dropped in any state and will be reclaimed by memory manager, but a shared channel generally will not be reclaimed | ||
| unless closed and consumed. | ||
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| This does not affect [Flow], because it is a cold abstraction. Even though [Flow] internally uses channels to transfer | ||
| data between threads, it always properly closes these channels when completing collection of data. | ||
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| ## Known problems | ||
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| The current implementation is tested and works for all kinds of single-threaded cases and simple scenarios that | ||
| transfer data between two thread like shown in [Switching Threads](#switching-threads) section. However, it is known | ||
| to leak memory in scenarios involving concurrency under load, for example when multiple children coroutines running | ||
| in different threads are simultaneously cancelled. | ||
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| <!--- MODULE kotlinx-coroutines-core --> | ||
| <!--- INDEX kotlinx.coroutines --> | ||
| [newSingleThreadContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/new-single-thread-context.html | ||
| [SingleThreadDispatcher.close]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-single-thread-dispatcher/-single-thread-dispatcher/close.html | ||
| [SingleThreadDispatcher.worker]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-single-thread-dispatcher/-single-thread-dispatcher/worker.html | ||
| [Dispatchers.Default]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-default.html | ||
| [GlobalScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-global-scope/index.html | ||
| [Dispatchers.Main]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-main.html | ||
| [runBlocking]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-blocking.html | ||
| [withContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html | ||
| [launch]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html | ||
| [async]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html | ||
| [Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html | ||
| [Deferred]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/index.html | ||
| <!--- INDEX kotlinx.coroutines.flow --> | ||
| [Flow]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html | ||
| [flowOn]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/flow-on.html | ||
| <!--- INDEX kotlinx.coroutines.channels --> | ||
| [produce]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/produce.html | ||
| [Channel]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-channel/index.html | ||
| [BroadcastChannel]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-broadcast-channel/index.html | ||
| <!--- INDEX kotlinx.coroutines.selects --> | ||
| <!--- INDEX kotlinx.coroutines.sync --> | ||
| [Mutex]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.sync/-mutex/index.html | ||
| [Semaphore]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.sync/-semaphore/index.html | ||
| <!--- END --> | ||
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