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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
package kotlinx.coroutines
import kotlinx.coroutines.internal.*
import kotlin.coroutines.*
import kotlin.coroutines.intrinsics.*
// --------------- cancellable continuations ---------------
/**
* Cancellable continuation. It is _completed_ when resumed or cancelled.
* When the [cancel] function is explicitly invoked, this continuation immediately resumes with a [CancellationException] or
* the specified cancel cause.
*
* An instance of `CancellableContinuation` is created by the [suspendCancellableCoroutine] function.
*
* Cancellable continuation has three states (as subset of [Job] states):
*
* | **State** | [isActive] | [isCompleted] | [isCancelled] |
* | ----------------------------------- | ---------- | ------------- | ------------- |
* | _Active_ (initial state) | `true` | `false` | `false` |
* | _Resumed_ (final _completed_ state) | `false` | `true` | `false` |
* | _Canceled_ (final _completed_ state)| `false` | `true` | `true` |
*
* Invocation of [cancel] transitions this continuation from _active_ to _cancelled_ state, while
* invocation of [Continuation.resume] or [Continuation.resumeWithException] transitions it from _active_ to _resumed_ state.
*
* A [cancelled][isCancelled] continuation implies that it is [completed][isCompleted].
*
* Invocation of [Continuation.resume] or [Continuation.resumeWithException] in _resumed_ state produces an [IllegalStateException],
* but is ignored in _cancelled_ state.
*
* ```
* +-----------+ resume +---------+
* | Active | ----------> | Resumed |
* +-----------+ +---------+
* |
* | cancel
* V
* +-----------+
* | Cancelled |
* +-----------+
* ```
*/
public interface CancellableContinuation : Continuation {
/**
* Returns `true` when this continuation is active -- it has not completed or cancelled yet.
*/
public val isActive: Boolean
/**
* Returns `true` when this continuation has completed for any reason. A cancelled continuation
* is also considered complete.
*/
public val isCompleted: Boolean
/**
* Returns `true` if this continuation was [cancelled][cancel].
*
* It implies that [isActive] is `false` and [isCompleted] is `true`.
*/
public val isCancelled: Boolean
/**
* Tries to resume this continuation with the specified [value] and returns a non-null object token if successful,
* or `null` otherwise (it was already resumed or cancelled). When a non-null object is returned,
* [completeResume] must be invoked with it.
*
* When [idempotent] is not `null`, this function performs an _idempotent_ operation, so that
* further invocations with the same non-null reference produce the same result.
*
* @suppress **This is unstable API and it is subject to change.**
*/
@InternalCoroutinesApi
public fun tryResume(value: T, idempotent: Any? = null): Any?
/**
* Same as [tryResume] but with [onCancellation] handler that called if and only if the value is not
* delivered to the caller because of the dispatch in the process, so that atomicity delivery
* guaranteed can be provided by having a cancellation fallback.
*/
@InternalCoroutinesApi
public fun tryResume(value: T, idempotent: Any?, onCancellation: ((cause: Throwable) -> Unit)?): Any?
/**
* Tries to resume this continuation with the specified [exception] and returns a non-null object token if successful,
* or `null` otherwise (it was already resumed or cancelled). When a non-null object is returned,
* [completeResume] must be invoked with it.
*
* @suppress **This is unstable API and it is subject to change.**
*/
@InternalCoroutinesApi
public fun tryResumeWithException(exception: Throwable): Any?
/**
* Completes the execution of [tryResume] or [tryResumeWithException] on its non-null result.
*
* @suppress **This is unstable API and it is subject to change.**
*/
@InternalCoroutinesApi
public fun completeResume(token: Any)
/**
* Internal function that setups cancellation behavior in [suspendCancellableCoroutine].
* It's illegal to call this function in any non-`kotlinx.coroutines` code and
* such calls lead to undefined behaviour.
* Exposed in our ABI since 1.0.0 withing `suspendCancellableCoroutine` body.
*
* @suppress **This is unstable API and it is subject to change.**
*/
@InternalCoroutinesApi
public fun initCancellability()
/**
* Cancels this continuation with an optional cancellation `cause`. The result is `true` if this continuation was
* cancelled as a result of this invocation, and `false` otherwise.
*/
public fun cancel(cause: Throwable? = null): Boolean
/**
* Registers a [handler] to be **synchronously** invoked on [cancellation][cancel] (regular or exceptional) of this continuation.
* When the continuation is already cancelled, the handler is immediately invoked
* with the cancellation exception. Otherwise, the handler will be invoked as soon as this
* continuation is cancelled.
*
* The installed [handler] should not throw any exceptions.
* If it does, they will get caught, wrapped into a [CompletionHandlerException] and
* processed as an uncaught exception in the context of the current coroutine
* (see [CoroutineExceptionHandler]).
*
* At most one [handler] can be installed on a continuation. Attempt to call `invokeOnCancellation` second
* time produces [IllegalStateException].
*
* This handler is also called when this continuation [resumes][Continuation.resume] normally (with a value) and then
* is cancelled while waiting to be dispatched. More generally speaking, this handler is called whenever
* the caller of [suspendCancellableCoroutine] is getting a [CancellationException].
*
* A typical example for `invokeOnCancellation` usage is given in
* the documentation for the [suspendCancellableCoroutine] function.
*
* **Note**: Implementation of `CompletionHandler` must be fast, non-blocking, and thread-safe.
* This `handler` can be invoked concurrently with the surrounding code.
* There is no guarantee on the execution context in which the `handler` will be invoked.
*/
public fun invokeOnCancellation(handler: CompletionHandler)
/**
* Resumes this continuation with the specified [value] in the invoker thread without going through
* the [dispatch][CoroutineDispatcher.dispatch] function of the [CoroutineDispatcher] in the [context].
* This function is designed to only be used by [CoroutineDispatcher] implementations.
* **It should not be used in general code**.
*
* **Note: This function is experimental.** Its signature general code may be changed in the future.
*/
@ExperimentalCoroutinesApi
public fun CoroutineDispatcher.resumeUndispatched(value: T)
/**
* Resumes this continuation with the specified [exception] in the invoker thread without going through
* the [dispatch][CoroutineDispatcher.dispatch] function of the [CoroutineDispatcher] in the [context].
* This function is designed to only be used by [CoroutineDispatcher] implementations.
* **It should not be used in general code**.
*
* **Note: This function is experimental.** Its signature general code may be changed in the future.
*/
@ExperimentalCoroutinesApi
public fun CoroutineDispatcher.resumeUndispatchedWithException(exception: Throwable)
/**
* Resumes this continuation with the specified `value` and calls the specified `onCancellation`
* handler when either resumed too late (when continuation was already cancelled) or, although resumed
* successfully (before cancellation), the coroutine's job was cancelled before it had a
* chance to run in its dispatcher, so that the suspended function threw an exception
* instead of returning this value.
*
* The installed [onCancellation] handler should not throw any exceptions.
* If it does, they will get caught, wrapped into a [CompletionHandlerException] and
* processed as an uncaught exception in the context of the current coroutine
* (see [CoroutineExceptionHandler]).
*
* This function shall be used when resuming with a resource that must be closed by
* code that called the corresponding suspending function, for example:
*
* ```
* continuation.resume(resource) {
* resource.close()
* }
* ```
*
* A more complete example and further details are given in
* the documentation for the [suspendCancellableCoroutine] function.
*
* **Note**: The [onCancellation] handler must be fast, non-blocking, and thread-safe.
* It can be invoked concurrently with the surrounding code.
* There is no guarantee on the execution context of its invocation.
*/
@ExperimentalCoroutinesApi // since 1.2.0
public fun resume(value: T, onCancellation: ((cause: Throwable) -> Unit)?)
}
/**
* Suspends the coroutine like [suspendCoroutine], but providing a [CancellableContinuation] to
* the [block]. This function throws a [CancellationException] if the [Job] of the coroutine is
* cancelled or completed while it is suspended.
*
* A typical use of this function is to suspend a coroutine while waiting for a result
* from a single-shot callback API and to return the result to the caller.
* For multi-shot callback APIs see [callbackFlow][kotlinx.coroutines.flow.callbackFlow].
*
* ```
* suspend fun awaitCallback(): T = suspendCancellableCoroutine { continuation ->
* val callback = object : Callback { // Implementation of some callback interface
* override fun onCompleted(value: T) {
* // Resume coroutine with a value provided by the callback
* continuation.resume(value)
* }
* override fun onApiError(cause: Throwable) {
* // Resume coroutine with an exception provided by the callback
* continuation.resumeWithException(cause)
* }
* }
* // Register callback with an API
* api.register(callback)
* // Remove callback on cancellation
* continuation.invokeOnCancellation { api.unregister(callback) }
* // At this point the coroutine is suspended by suspendCancellableCoroutine until callback fires
* }
* ```
*
* > The callback `register`/`unregister` methods provided by an external API must be thread-safe, because
* > `invokeOnCancellation` block can be called at any time due to asynchronous nature of cancellation, even
* > concurrently with the call of the callback.
*
* ### Prompt cancellation guarantee
*
* This function provides **prompt cancellation guarantee**.
* If the [Job] of the current coroutine was cancelled while this function was suspended it will not resume
* successfully.
*
* The cancellation of the coroutine's job is generally asynchronous with respect to the suspended coroutine.
* The suspended coroutine is resumed with the call it to its [Continuation.resumeWith] member function or to
* [resume][Continuation.resume] extension function.
* However, when coroutine is resumed, it does not immediately start executing, but is passed to its
* [CoroutineDispatcher] to schedule its execution when dispatcher's resources become available for execution.
* The job's cancellation can happen both before, after, and concurrently with the call to `resume`. In any
* case, prompt cancellation guarantees that the the coroutine will not resume its code successfully.
*
* If the coroutine was resumed with an exception (for example, using [Continuation.resumeWithException] extension
* function) and cancelled, then the resulting exception of the `suspendCancellableCoroutine` function is determined
* by whichever action (exceptional resume or cancellation) that happened first.
*
* ### Returning resources from a suspended coroutine
*
* As a result of a prompt cancellation guarantee, when a closeable resource
* (like open file or a handle to another native resource) is returned from a suspended coroutine as a value
* it can be lost when the coroutine is cancelled. In order to ensure that the resource can be properly closed
* in this case, the [CancellableContinuation] interface provides two functions.
*
* * [invokeOnCancellation][CancellableContinuation.invokeOnCancellation] installs a handler that is called
* whenever a suspend coroutine is being cancelled. In addition to the example at the beginning, it can be
* used to ensure that a resource that was opened before the call to
* `suspendCancellableCoroutine` or in its body is closed in case of cancellation.
*
* ```
* suspendCancellableCoroutine { continuation ->
* val resource = openResource() // Opens some resource
* continuation.invokeOnCancellation {
* resource.close() // Ensures the resource is closed on cancellation
* }
* // ...
* }
* ```
*
* * [resume(value) { ... }][CancellableContinuation.resume] method on a [CancellableContinuation] takes
* an optional `onCancellation` block. It can be used when resuming with a resource that must be closed by
* the code that called the corresponding suspending function.
*
* ```
* suspendCancellableCoroutine { continuation ->
* val callback = object : Callback { // Implementation of some callback interface
* // A callback provides a reference to some closeable resource
* override fun onCompleted(resource: T) {
* // Resume coroutine with a value provided by the callback and ensure the resource is closed in case
* // when the coroutine is cancelled before the caller gets a reference to the resource.
* continuation.resume(resource) {
* resource.close() // Close the resource on cancellation
* }
* }
* // ...
* }
* ```
*
* ### Implementation details and custom continuation interceptors
*
* The prompt cancellation guarantee is the result of a coordinated implementation inside `suspendCancellableCoroutine`
* function and the [CoroutineDispatcher] class. The coroutine dispatcher checks for the status of the [Job] immediately
* before continuing its normal execution and aborts this normal execution, calling all the corresponding
* cancellation handlers, if the job was cancelled.
*
* If a custom implementation of [ContinuationInterceptor] is used in a coroutine's context that does not extend
* [CoroutineDispatcher] class, then there is no prompt cancellation guarantee. A custom continuation interceptor
* can resume execution of a previously suspended coroutine even if its job was already cancelled.
*/
public suspend inline fun suspendCancellableCoroutine(
crossinline block: (CancellableContinuation) -> Unit
): T =
suspendCoroutineUninterceptedOrReturn { uCont ->
val cancellable = CancellableContinuationImpl(uCont.intercepted(), resumeMode = MODE_CANCELLABLE)
/*
* For non-atomic cancellation we setup parent-child relationship immediately
* in case when `block` blocks the current thread (e.g. Rx2 with trampoline scheduler), but
* properly supports cancellation.
*/
cancellable.initCancellability()
block(cancellable)
cancellable.getResult()
}
/**
* Suspends the coroutine similar to [suspendCancellableCoroutine], but an instance of
* [CancellableContinuationImpl] is reused.
*/
internal suspend inline fun suspendCancellableCoroutineReusable(
crossinline block: (CancellableContinuation) -> Unit
): T = suspendCoroutineUninterceptedOrReturn { uCont ->
val cancellable = getOrCreateCancellableContinuation(uCont.intercepted())
block(cancellable)
cancellable.getResult()
}
internal fun getOrCreateCancellableContinuation(delegate: Continuation): CancellableContinuationImpl {
// NOTE: Reuse is not support on Kotlin/Native due to platform peculiarities making it had to properly
// split DispatchedContinuation / CancellableContinuationImpl state across workers.
if (!isReuseSupportedInPlatform() || delegate !is DispatchedContinuation) {
return CancellableContinuationImpl(delegate, MODE_CANCELLABLE).also {
if (delegate is DispatchedContinuation) it.initCancellability()
}
}
/*
* Attempt to claim reusable instance.
*
* suspendCancellableCoroutineReusable { // <- claimed
* // Any asynchronous cancellation is "postponed" while this block
* // is being executed
* } // postponed cancellation is checked here.
*
* Claim can fail for the following reasons:
* 1) Someone tried to make idempotent resume.
* Idempotent resume is internal (used only by us) and is used only in `select`,
* thus leaking CC instance for indefinite time.
* 2) Continuation was cancelled. Then we should prevent any further reuse and bail out.
*/
return delegate.claimReusableCancellableContinuation()?.takeIf { it.resetStateReusable() }
?: return CancellableContinuationImpl(delegate, MODE_CANCELLABLE_REUSABLE)
}
/**
* Removes the specified [node] on cancellation. This function assumes that this node is already
* removed on successful resume and does not try to remove it if the continuation is cancelled during dispatch.
*/
internal fun CancellableContinuation<*>.removeOnCancellation(node: LockFreeLinkedListNode) =
invokeOnCancellation(handler = RemoveOnCancel(node).asHandler)
/**
* Disposes the specified [handle] when this continuation is cancelled.
*
* This is a shortcut for the following code with slightly more efficient implementation (one fewer object created):
* ```
* invokeOnCancellation { handle.dispose() }
* ```
*
* @suppress **This an internal API and should not be used from general code.**
*/
@InternalCoroutinesApi
public fun CancellableContinuation<*>.disposeOnCancellation(handle: DisposableHandle): Unit =
invokeOnCancellation(handler = DisposeOnCancel(handle).asHandler)
// --------------- implementation details ---------------
private class RemoveOnCancel(private val node: LockFreeLinkedListNode) : BeforeResumeCancelHandler() {
override fun invoke(cause: Throwable?) { node.remove() }
override fun toString() = "RemoveOnCancel[$node]"
}
private class DisposeOnCancel(private val handle: DisposableHandle) : CancelHandler() {
override fun invoke(cause: Throwable?) = handle.dispose()
override fun toString(): String = "DisposeOnCancel[$handle]"
}