package kotlinx.coroutines

/**
 * Handler for [Job.invokeOnCompletion] and [CancellableContinuation.invokeOnCancellation].
 *
 * The meaning of `cause` that is passed to the handler is:
 * - It is `null` if the job has completed normally or the continuation was cancelled without a `cause`.
 * - It is an instance of [CancellationException] if the job or the continuation was cancelled _normally_.
 *   **It should not be treated as an error**. In particular, it should not be reported to error logs.
 * - Otherwise, the job or the continuation had _failed_.
 *
 * A function used for this should not throw any exceptions.
 * If it does, they will get caught, wrapped into [CompletionHandlerException], and then either
 * - passed to [handleCoroutineException] for [CancellableContinuation.invokeOnCancellation]
 *   and, for [Job] instances that are coroutines, [Job.invokeOnCompletion], or
 * - for [Job] instances that are not coroutines, simply thrown, potentially crashing unrelated code.
 *
 * Functions used for this 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 function is invoked.
 *
 * **Note**: This type is a part of internal machinery that supports parent-child hierarchies
 * and allows for implementation of suspending functions that wait on the Job's state.
 * This type should not be used in general application code.
 */
// TODO: deprecate. This doesn't seem better than a simple function type.
public typealias CompletionHandler = (cause: Throwable?) -> Unit