tor-browser

Promise.h (34507B)

---

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* vim: set ts=8 sts=4 et sw=4 tw=99:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_Promise_h
#define js_Promise_h

#include "mozilla/Attributes.h"

#include "jstypes.h"

#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "js/UniquePtr.h"

namespace JS {

class JS_PUBLIC_API AutoDebuggerJobQueueInterruption;

/**
* Abstract base class for an ECMAScript Job Queue:
* https://www.ecma-international.org/ecma-262/9.0/index.html#sec-jobs-and-job-queues
*
* SpiderMonkey doesn't schedule Promise resolution jobs itself; instead, the
* embedding can provide an instance of this class SpiderMonkey can use to do
* that scheduling.
*
* The JavaScript shell includes a simple implementation adequate for running
* tests. Browsers need to augment job handling to meet their own additional
* requirements, so they can provide their own implementation.
*/
class JS_PUBLIC_API JobQueue {
public:
 virtual ~JobQueue() = default;

 /**
  * Ask the embedding for the host defined data.
  *
  * This is the step 5 in
  * https://html.spec.whatwg.org/multipage/webappapis.html#hostmakejobcallback
  *
  * SpiderMonkey doesn't itself have a notion of host defined data as defined
  * by the HTML spec, so we need the embedding to provide this. See
  * dom/script/ScriptSettings.h for details.
  *
  * If the embedding has the host defined data, this method should return the
  * host defined data via the `data` out parameter and return `true`.
  * The object in the `data` out parameter can belong to any compartment.
  * If the embedding doesn't need the host defined data, this method should
  * set the `data` out parameter to `nullptr` and return `true`.
  * If any error happens while generating the host defined data, this method
  * should set a pending exception to `cx` and return `false`.
  */
 virtual bool getHostDefinedData(JSContext* cx,
                                 JS::MutableHandle<JSObject*> data) const = 0;

 /**
  * If the embedding has a host-defined global, return it. This is used when
  * we are able to optimize out the host defined data, as the embedding may
  * still require this when running jobs.
  *
  * In Gecko, this is used for dealing with the incumbent global.
  */
 virtual bool getHostDefinedGlobal(
     JSContext* cx, JS::MutableHandle<JSObject*> data) const = 0;

 /**
  * Enqueue a reaction job `job` for `promise`, which was allocated at
  * `allocationSite`. Provide `hostDefineData` as the host defined data for
  * the reaction job's execution.
  *
  * The `hostDefinedData` value comes from `getHostDefinedData` method.
  * The object is unwrapped, and it can belong to a different compartment
  * than the current compartment. It can be `nullptr` if `getHostDefinedData`
  * returns `nullptr`.
  *
  * `promise` can be null if the promise is optimized out.
  * `promise` is guaranteed not to be optimized out if the promise has
  * non-default user-interaction flag.
  */
 virtual bool enqueuePromiseJob(JSContext* cx, JS::HandleObject promise,
                                JS::HandleObject job,
                                JS::HandleObject allocationSite,
                                JS::HandleObject hostDefinedData) = 0;

 /**
  * Run all jobs in the queue. Running one job may enqueue others; continue to
  * run jobs until the queue is empty.
  *
  * Calling this method at the wrong time can break the web. The HTML spec
  * indicates exactly when the job queue should be drained (in HTML jargon,
  * when it should "perform a microtask checkpoint"), and doing so at other
  * times can incompatibly change the semantics of programs that use promises
  * or other microtask-based features.
  *
  * This method is called only via AutoDebuggerJobQueueInterruption, used by
  * the Debugger API implementation to ensure that the debuggee's job queue is
  * protected from the debugger's own activity. See the comments on
  * AutoDebuggerJobQueueInterruption.
  */
 virtual void runJobs(JSContext* cx) = 0;

 /**
  * Return true if the job queue is empty, false otherwise.
  */
 virtual bool empty() const = 0;

 /**
  * Returns true if the job queue stops draining, which results in `empty()`
  * being false after `runJobs()`.
  */
 virtual bool isDrainingStopped() const = 0;

 /*
  * When enqueueing a job, should the job be enqueued in the debug queue
  * rather than the regular queue?
  */
 virtual bool useDebugQueue(JS::Handle<JSObject*> global) const {
   return false;
 }

 /**
  * Trace hook for non-GCThing microtask values.
  *
  * This hook is called during GC tracing for microtask queue values that are
  * not GC-things. It allows the embedding to provide custom tracing for
  * values stored in the microtask queue, particularly for Private values
  * that represent embedding-specific task representations.
  *
  * If the value is not a GC thing, this method will be called to allow the
  * embedding to trace any GC-reachable data associated with the value.
  *
  * The default implementation does nothing.
  */
 virtual void traceNonGCThingMicroTask(JSTracer* trc, JS::Value* valuePtr) {}

protected:
 friend class AutoDebuggerJobQueueInterruption;

 /**
  * A saved job queue, represented however the JobQueue implementation pleases.
  * Use AutoDebuggerJobQueueInterruption rather than trying to construct one of
  * these directly; see documentation there.
  *
  * Destructing an instance of this class should assert that the current queue
  * is empty, and then restore the queue the instance captured.
  */
 class SavedJobQueue {
  public:
   virtual ~SavedJobQueue() = default;
 };

 /**
  * Capture this JobQueue's current job queue as a SavedJobQueue and return it,
  * leaving the JobQueue's job queue empty. Destroying the returned object
  * should assert that this JobQueue's current job queue is empty, and restore
  * the original queue.
  *
  * On OOM, this should call JS_ReportOutOfMemory on the given JSContext,
  * and return a null UniquePtr.
  */
 virtual js::UniquePtr<SavedJobQueue> saveJobQueue(JSContext*) = 0;
};

/**
* Tell SpiderMonkey to use `queue` to schedule promise reactions.
*
* SpiderMonkey does not take ownership of the queue; it is the embedding's
* responsibility to clean it up after the runtime is destroyed.
*/
extern JS_PUBLIC_API void SetJobQueue(JSContext* cx, JobQueue* queue);

/**
* [SMDOC] Protecting the debuggee's job/microtask queue from debugger activity.
*
* When the JavaScript debugger interrupts the execution of some debuggee code
* (for a breakpoint, for example), the debuggee's execution must be paused
* while the developer takes time to look at it. During this interruption, other
* tabs should remain active and usable. If the debuggee shares a main thread
* with non-debuggee tabs, that means that the thread will have to process
* non-debuggee HTML tasks and microtasks as usual, even as the debuggee's are
* on hold until the debugger lets it continue execution. (Letting debuggee
* microtasks run during the interruption would mean that, from the debuggee's
* point of view, their side effects would take place wherever the breakpoint
* was set - in general, not a place other code should ever run, and a violation
* of the run-to-completion rule.)
*
* This means that, even though the timing and ordering of microtasks is
* carefully specified by the standard - and important to preserve for
* compatibility and predictability - debugger use may, correctly, have the
* effect of reordering microtasks. During the interruption, microtasks enqueued
* by non-debuggee tabs must run immediately alongside their HTML tasks as
* usual, whereas any debuggee microtasks that were in the queue when the
* interruption began must wait for the debuggee to be continued - and thus run
* after microtasks enqueued after they were.
*
* Fortunately, this reordering is visible only at the global level: when
* implemented correctly, it is not detectable by an individual debuggee. Note
* that a debuggee should generally be a complete unit of similar-origin related
* browsing contexts. Since non-debuggee activity falls outside that unit, it
* should never be visible to the debuggee (except via mechanisms that are
* already asynchronous, like events), so the debuggee should be unable to
* detect non-debuggee microtasks running when they normally would not. As long
* as behavior *visible to the debuggee* is unaffected by the interruption, we
* have respected the spirit of the rule.
*
* Of course, even as we accept the general principle that interrupting the
* debuggee should have as little detectable effect as possible, we still permit
* the developer to do things like evaluate expressions at the console that have
* arbitrary effects on the debuggee's state—effects that could never occur
* naturally at that point in the program. But since these are explicitly
* requested by the developer, who presumably knows what they're doing, we
* support this as best we can. If the developer evaluates an expression in the
* console that resolves a promise, it seems most natural for the promise's
* reaction microtasks to run immediately, within the interruption. This is an
* 'unnatural' time for the microtasks to run, but no more unnatural than the
* evaluation that triggered them.
*
* So the overall behavior we need is as follows:
*
* - When the debugger interrupts a debuggee, the debuggee's microtask queue
*   must be saved.
*
* - When debuggee execution resumes, the debuggee's microtask queue must be
*   restored exactly as it was when the interruption occurred.
*
* - Non-debuggee task and microtask execution must take place normally during
*   the interruption.
*
* Since each HTML task begins with an empty microtask queue, and it should not
* be possible for a task to mix debuggee and non-debuggee code, interrupting a
* debuggee should always find a microtask queue containing exclusively debuggee
* microtasks, if any. So saving and restoring the microtask queue should affect
* only the debuggee, not any non-debuggee content.
*
* AutoDebuggerJobQueueInterruption
* --------------------------------
*
* AutoDebuggerJobQueueInterruption is an RAII class, meant for use by the
* Debugger API implementation, that takes care of saving and restoring the
* queue.
*
* Constructing and initializing an instance of AutoDebuggerJobQueueInterruption
* sets aside the given JSContext's job queue, leaving the JSContext's queue
* empty. When the AutoDebuggerJobQueueInterruption instance is destroyed, it
* asserts that the JSContext's current job queue (holding jobs enqueued while
* the AutoDebuggerJobQueueInterruption was alive) is empty, and restores the
* saved queue to the JSContext.
*
* Since the Debugger API's behavior is up to us, we can specify that Debugger
* hooks begin execution with an empty job queue, and that we drain the queue
* after each hook function has run. This drain will be visible to debugger
* hooks, and makes hook calls resemble HTML tasks, with their own automatic
* microtask checkpoint. But, the drain will be invisible to the debuggee, as
* its queue is preserved across the hook invocation.
*
* To protect the debuggee's job queue, Debugger takes care to invoke callback
* functions only within the scope of an AutoDebuggerJobQueueInterruption
* instance.
*
* Why not let the hook functions themselves take care of this?
* ------------------------------------------------------------
*
* Certainly, we could leave responsibility for saving and restoring the job
* queue to the Debugger hook functions themselves.
*
* In fact, early versions of this change tried making the devtools server save
* and restore the queue explicitly, but because hooks are set and changed in
* numerous places, it was hard to be confident that every case had been
* covered, and it seemed that future changes could easily introduce new holes.
*
* Later versions of this change modified the accessor properties on the
* Debugger objects' prototypes to automatically protect the job queue when
* calling hooks, but the effect was essentially a monkeypatch applied to an API
* we defined and control, which doesn't make sense.
*
* In the end, since promises have become such a pervasive part of JavaScript
* programming, almost any imaginable use of Debugger would need to provide some
* kind of protection for the debuggee's job queue, so it makes sense to simply
* handle it once, carefully, in the implementation of Debugger itself.
*/
class MOZ_RAII JS_PUBLIC_API AutoDebuggerJobQueueInterruption {
public:
 explicit AutoDebuggerJobQueueInterruption();
 ~AutoDebuggerJobQueueInterruption();

 bool init(JSContext* cx);
 bool initialized() const { return !!saved; }

 /**
  * Drain the job queue. (In HTML terminology, perform a microtask checkpoint.)
  *
  * To make Debugger hook calls more like HTML tasks or ECMAScript jobs,
  * Debugger promises that each hook begins execution with a clean microtask
  * queue, and that a microtask checkpoint (queue drain) takes place after each
  * hook returns, successfully or otherwise.
  *
  * To ensure these debugger-introduced microtask checkpoints serve only the
  * hook's microtasks, and never affect the debuggee's, the Debugger API
  * implementation uses only this method to perform the checkpoints, thereby
  * statically ensuring that an AutoDebuggerJobQueueInterruption is in scope to
  * protect the debuggee.
  *
  * SavedJobQueue implementations are required to assert that the queue is
  * empty before restoring the debuggee's queue. If the Debugger API ever fails
  * to perform a microtask checkpoint after calling a hook, that assertion will
  * fail, catching the mistake.
  */
 void runJobs();

private:
 JSContext* cx;
 js::UniquePtr<JobQueue::SavedJobQueue> saved;
};

enum class PromiseRejectionHandlingState { Unhandled, Handled };

typedef void (*PromiseRejectionTrackerCallback)(
   JSContext* cx, bool mutedErrors, JS::HandleObject promise,
   JS::PromiseRejectionHandlingState state, void* data);

/**
* Sets the callback that's invoked whenever a Promise is rejected without
* a rejection handler, and when a Promise that was previously rejected
* without a handler gets a handler attached.
*/
extern JS_PUBLIC_API void SetPromiseRejectionTrackerCallback(
   JSContext* cx, PromiseRejectionTrackerCallback callback,
   void* data = nullptr);

/**
* Inform the runtime that the job queue is empty and the embedding is going to
* execute its last promise job. The runtime may now choose to skip creating
* promise jobs for asynchronous execution and instead continue execution
* synchronously. More specifically, this optimization is used to skip the
* standard job queuing behavior for `await` operations in async functions.
*
* This function may be called before executing the last job in the job queue.
* When it was called, JobQueueMayNotBeEmpty must be called in order to restore
* the default job queuing behavior before the embedding enqueues its next job
* into the job queue.
*/
extern JS_PUBLIC_API void JobQueueIsEmpty(JSContext* cx);

/**
* Inform the runtime that job queue is no longer empty. The runtime can now no
* longer skip creating promise jobs for asynchronous execution, because
* pending jobs in the job queue must be executed first to preserve the FIFO
* (first in - first out) property of the queue. This effectively undoes
* JobQueueIsEmpty and re-enables the standard job queuing behavior.
*
* This function must be called whenever enqueuing a job to the job queue when
* JobQueueIsEmpty was called previously.
*/
extern JS_PUBLIC_API void JobQueueMayNotBeEmpty(JSContext* cx);

/**
* Returns a new instance of the Promise builtin class in the current
* compartment, with the right slot layout.
*
* The `executor` can be a `nullptr`. In that case, the only way to resolve or
* reject the returned promise is via the `JS::ResolvePromise` and
* `JS::RejectPromise` JSAPI functions.
*/
extern JS_PUBLIC_API JSObject* NewPromiseObject(JSContext* cx,
                                               JS::HandleObject executor);

/**
* Returns true if the given object is an unwrapped PromiseObject, false
* otherwise.
*/
extern JS_PUBLIC_API bool IsPromiseObject(JS::HandleObject obj);

/**
* Returns the current compartment's original Promise constructor.
*/
extern JS_PUBLIC_API JSObject* GetPromiseConstructor(JSContext* cx);

/**
* Returns the current compartment's original Promise.prototype.
*/
extern JS_PUBLIC_API JSObject* GetPromisePrototype(JSContext* cx);

// Keep this in sync with the PROMISE_STATE defines in SelfHostingDefines.h.
enum class PromiseState { Pending, Fulfilled, Rejected };

/**
* Returns the given Promise's state as a JS::PromiseState enum value.
*
* Returns JS::PromiseState::Pending if the given object is a wrapper that
* can't safely be unwrapped.
*/
extern JS_PUBLIC_API PromiseState GetPromiseState(JS::HandleObject promise);

/**
* Returns the given Promise's process-unique ID.
*/
JS_PUBLIC_API uint64_t GetPromiseID(JS::HandleObject promise);

/**
* Returns the given Promise's result: either the resolution value for
* fulfilled promises, or the rejection reason for rejected ones.
*/
extern JS_PUBLIC_API JS::Value GetPromiseResult(JS::HandleObject promise);

/**
* Returns whether the given promise's rejection is already handled or not.
*
* The caller must check the given promise is rejected before checking it's
* handled or not.
*/
extern JS_PUBLIC_API bool GetPromiseIsHandled(JS::HandleObject promise);

/*
* Given a settled (i.e. fulfilled or rejected, not pending) promise, sets
* |promise.[[PromiseIsHandled]]| to true and removes it from the list of
* unhandled rejected promises.
*/
extern JS_PUBLIC_API bool SetSettledPromiseIsHandled(JSContext* cx,
                                                    JS::HandleObject promise);

/*
* Given a promise (settled or not), sets |promise.[[PromiseIsHandled]]| to true
* and removes it from the list of unhandled rejected promises if it's settled.
*/
[[nodiscard]] extern JS_PUBLIC_API bool SetAnyPromiseIsHandled(
   JSContext* cx, JS::HandleObject promise);

/**
* Returns a js::SavedFrame linked list of the stack that lead to the given
* Promise's allocation.
*
* The promise parameter here should be an unwrapped promise.
*/
extern JS_PUBLIC_API JSObject* GetPromiseAllocationSite(
   JS::HandleObject promise);

/**
* A more flexible version of the above: Handles the cases where
* 1. maybePromise is wrapped
* 2. maybePromise is null
* 3. maybePromise isn't actually a promise.
*
* Returns js::SavedFrame if the passed argument is a
* wrapped promise (that's not a dead object wrapper) and it
* has an allocation site.
*/
extern JS_PUBLIC_API JSObject*
MaybeGetPromiseAllocationSiteFromPossiblyWrappedPromise(
   JS::HandleObject maybePromise);

extern JS_PUBLIC_API JSObject* GetPromiseResolutionSite(
   JS::HandleObject promise);

#ifdef DEBUG
extern JS_PUBLIC_API void DumpPromiseAllocationSite(JSContext* cx,
                                                   JS::HandleObject promise);

extern JS_PUBLIC_API void DumpPromiseResolutionSite(JSContext* cx,
                                                   JS::HandleObject promise);
#endif

/**
* Calls the current compartment's original Promise.resolve on the original
* Promise constructor, with `resolutionValue` passed as an argument.
*/
extern JS_PUBLIC_API JSObject* CallOriginalPromiseResolve(
   JSContext* cx, JS::HandleValue resolutionValue);

/**
* Calls the current compartment's original Promise.reject on the original
* Promise constructor, with `resolutionValue` passed as an argument.
*/
extern JS_PUBLIC_API JSObject* CallOriginalPromiseReject(
   JSContext* cx, JS::HandleValue rejectionValue);

/**
* Resolves the given Promise with the given `resolutionValue`.
*
* Calls the `resolve` function that was passed to the executor function when
* the Promise was created.
*/
extern JS_PUBLIC_API bool ResolvePromise(JSContext* cx,
                                        JS::HandleObject promiseObj,
                                        JS::HandleValue resolutionValue);

/**
* Rejects the given `promise` with the given `rejectionValue`.
*
* Calls the `reject` function that was passed to the executor function when
* the Promise was created.
*/
extern JS_PUBLIC_API bool RejectPromise(JSContext* cx,
                                       JS::HandleObject promiseObj,
                                       JS::HandleValue rejectionValue);

/**
* Create a Promise with the given fulfill/reject handlers, that will be
* fulfilled/rejected with the value/reason that the promise `promise` is
* fulfilled/rejected with.
*
* This function basically acts like `promise.then(onFulfilled, onRejected)`,
* except that its behavior is unaffected by changes to `Promise`,
* `Promise[Symbol.species]`, `Promise.prototype.then`, `promise.constructor`,
* `promise.then`, and so on.
*
* This function throws if `promise` is not a Promise from this or another
* realm.
*
* This function will assert if `onFulfilled` or `onRejected` is non-null and
* also not IsCallable.
*/
extern JS_PUBLIC_API JSObject* CallOriginalPromiseThen(
   JSContext* cx, JS::HandleObject promise, JS::HandleObject onFulfilled,
   JS::HandleObject onRejected);

/**
* Unforgeable, optimized version of the JS builtin Promise.prototype.then.
*
* Takes a Promise instance and nullable `onFulfilled`/`onRejected` callables to
* enqueue as reactions for that promise. In contrast to Promise.prototype.then,
* this doesn't create and return a new Promise instance.
*
* Throws a TypeError if `promise` isn't a Promise (or possibly a different
* error if it's a security wrapper or dead object proxy).
*/
extern JS_PUBLIC_API bool AddPromiseReactions(JSContext* cx,
                                             JS::HandleObject promise,
                                             JS::HandleObject onFulfilled,
                                             JS::HandleObject onRejected);

/**
* Unforgeable, optimized version of the JS builtin Promise.prototype.then.
*
* Takes a Promise instance and nullable `onFulfilled`/`onRejected` callables to
* enqueue as reactions for that promise. In contrast to Promise.prototype.then,
* this doesn't create and return a new Promise instance.
*
* Throws a TypeError if `promise` isn't a Promise (or possibly a different
* error if it's a security wrapper or dead object proxy).
*
* If `onRejected` is null and `promise` is rejected, this function -- unlike
* the function above -- will not report an unhandled rejection.
*/
extern JS_PUBLIC_API bool AddPromiseReactionsIgnoringUnhandledRejection(
   JSContext* cx, JS::HandleObject promise, JS::HandleObject onFulfilled,
   JS::HandleObject onRejected);

// This enum specifies whether a promise is expected to keep track of
// information that is useful for embedders to implement user activation
// behavior handling as specified in the HTML spec:
// https://html.spec.whatwg.org/multipage/interaction.html#triggered-by-user-activation
// By default, promises created by SpiderMonkey do not make any attempt to keep
// track of information about whether an activation behavior was being processed
// when the original promise in a promise chain was created.  If the embedder
// sets either of the HadUserInteractionAtCreation or
// DidntHaveUserInteractionAtCreation flags on a promise after creating it,
// SpiderMonkey will propagate that flag to newly created promises when
// processing Promise#then and will make it possible to query this flag off of a
// promise further down the chain later using the
// GetPromiseUserInputEventHandlingState() API.
enum class PromiseUserInputEventHandlingState {
 // Don't keep track of this state (default for all promises)
 DontCare,
 // Keep track of this state, the original promise in the chain was created
 // while an activation behavior was being processed.
 HadUserInteractionAtCreation,
 // Keep track of this state, the original promise in the chain was created
 // while an activation behavior was not being processed.
 DidntHaveUserInteractionAtCreation
};

/**
* Returns the given Promise's activation behavior state flag per above as a
* JS::PromiseUserInputEventHandlingState value.  All promises are created with
* the DontCare state by default.
*
* Returns JS::PromiseUserInputEventHandlingState::DontCare if the given object
* is a wrapper that can't safely be unwrapped.
*/
extern JS_PUBLIC_API PromiseUserInputEventHandlingState
GetPromiseUserInputEventHandlingState(JS::HandleObject promise);

/**
* Sets the given Promise's activation behavior state flag per above as a
* JS::PromiseUserInputEventHandlingState value.
*
* Returns false if the given object is a wrapper that can't safely be
* unwrapped.
*/
extern JS_PUBLIC_API bool SetPromiseUserInputEventHandlingState(
   JS::HandleObject promise, JS::PromiseUserInputEventHandlingState state);

/**
* Unforgeable version of the JS builtin Promise.all.
*
* Takes a HandleObjectVector of Promise objects and returns a promise that's
* resolved with an array of resolution values when all those promises have
* been resolved, or rejected with the rejection value of the first rejected
* promise.
*
* Asserts that all objects in the `promises` vector are, maybe wrapped,
* instances of `Promise` or a subclass of `Promise`.
*/
extern JS_PUBLIC_API JSObject* GetWaitForAllPromise(
   JSContext* cx, JS::HandleObjectVector promises);

/**
* The Dispatchable interface allows the embedding to call SpiderMonkey
* on a JSContext thread when requested via DispatchToEventLoopCallback.
*/
class JS_PUBLIC_API Dispatchable {
protected:
 // The registered_ flag indicates that this task is counted as part of
 // numRegistered_ of OffThreadPromiseRuntimeState, which will wait until
 // all registered tasks have been run or destroyed.
 bool registered_ = false;

public:
 bool registered() const { return registered_; }

 // Destruction of Dispatchables is public in order to be used with
 // UniquePtrs. Their destruction by SpiderMonkey is enforced by
 // ReleaseFailedTask.
 virtual ~Dispatchable() = default;

 // ShuttingDown indicates that SpiderMonkey should abort async tasks to
 // expedite shutdown.
 enum MaybeShuttingDown { NotShuttingDown, ShuttingDown };

 // Called by the embedding after DispatchToEventLoopCallback succeeds.
 // Used to correctly release the unique ptr and call the run task.
 static void Run(JSContext* cx, js::UniquePtr<Dispatchable>&& task,
                 MaybeShuttingDown maybeShuttingDown);

 // Used to correctly release the unique ptr. Relies on the
 // OffThreadRuntimePromiseState to handle cleanup via iteration over the
 // live() set.
 static void ReleaseFailedTask(js::UniquePtr<Dispatchable>&& task);

protected:
 // Dispatchables are created exclusively by SpiderMonkey.
 Dispatchable() = default;

 // These two methods must be implemented in order to correctly handle
 // success and failure cases for the task.

 // Used to execute the task, run on the owning thread.
 // A subclass should override this method to
 //   1) execute the task as necessary and
 //   2) delete the task.
 virtual void run(JSContext* cx, MaybeShuttingDown maybeShuttingDown) = 0;

 // Used to transfer the task back to the runtime if the embedding, upon
 // taking ownership of the task, fails to dispatch and run it. This allows
 // the runtime to delete the task during shutdown. This method can be called
 // from any thread.
 // Typically, this will be used with a UniquePtr like so:
 //   auto task = myTask.release();
 //   task->transferToRuntime();
 virtual void transferToRuntime() = 0;
};

/**
* Callback to dispatch a JS::Dispatchable to a JSContext's thread's event
* loop.
*
* The DispatchToEventLoopCallback set on a particular JSContext must accept
* JS::Dispatchable instances and arrange for their `run` methods to be called
* eventually on the JSContext's thread. This is used for cross-thread dispatch,
* so the callback itself must be safe to call from any thread. It cannot
* trigger a GC.
*
* The DelayedDispatchToEventLoopCallback in addition takes a delay, and it
* must accept JS::Dispatchable instances and arrange for their `run` methods
* to be called after the delay on the JSContext's thread.
* The embeddings must have its own timeout manager to handle the delay.
* If a timeout manager is not available for given context, it should return
* false, optionally with a warning message printed.
*
* The callback accepts a closure that is the same as the one given by the call
* to JS::InitAsyncTaskCallbacks.
*
* If the callback returns `true`, it must eventually run the given
* Dispatchable; otherwise, SpiderMonkey may leak memory or hang.
*
* The callback may return `false` to indicate that the JSContext's thread is
* shutting down and is no longer accepting runnables. Shutting down is a
* one-way transition: once the callback has rejected a runnable, it must reject
* all subsequently submitted runnables as well.
*/

typedef bool (*DispatchToEventLoopCallback)(
   void* closure, js::UniquePtr<Dispatchable>&& dispatchable);

/**
* Callback to dispatch a JS::Dispatchable to a JSContext's thread's event
* loop with a delay.
*
* The DelayedDispatchToEventLoopCallback set on a particular JSContext must
* accept JS::Dispatchable instances and arrange for their `run` methods to be
* called after the given delay on the JSContext's thread. This is used for
* cross-thread dispatch, so the callback itself must be safe to call from any
* thread. It cannot trigger a GC.
*
* The embeddings must have its own timeout manager to handle the delay.
* If a timeout manager is not available for given context, it should return
* false, optionally with a warning message printed.
*
* The callback accepts a closure that is the same as the one given by the call
* to JS::InitAsyncTaskCallbacks.
*
* If the callback returns `true`, it must eventually run the given
* Dispatchable; otherwise, SpiderMonkey may leak memory or hang.
*
* The callback may return `false` to indicate that the JSContext's thread is
* shutting down and is no longer accepting runnables. Shutting down is a
* one-way transition: once the callback has rejected a runnable, it must reject
* all subsequently submitted runnables as well.
*/

typedef bool (*DelayedDispatchToEventLoopCallback)(
   void* closure, js::UniquePtr<Dispatchable>&& dispatchable, uint32_t delay);

/**
* Callback to notify the embedder that a background task has begun. This can
* be used to keep the JSContext's thread alive if it's subject to garbage
* collection (e.g. as web workers are).
*
* This callback will be called on the same thread as the JSContext, but it
* must not perform any garbage collection or re-enter the engine.
*
* The passed dispatchable can be used to track the async task. Most async
* tasks will finish in a bounded amount of time, but some
* (i.e. Atomics.waitAsync) are not guaranteed to finish. These tasks may be
* canceled using JS::CancelAsyncTasks.
*
* The callback accepts a closure that is the same as the one given by the call
* to JS::InitAsyncTaskCallbacks.
*/
typedef void (*AsyncTaskStartedCallback)(void* closure,
                                        Dispatchable* dispatchable);

/**
* Callback to notify the embedder that a background task has finished. This can
* be used to keep the JSContext's thread alive if it's subject to garbage
* collection (e.g. as web workers are).
*
* This callback will be called on the same thread as the JSContext, but it
* must not perform any garbage collection or re-enter the engine.
*
* The passed dispatchable will be the same as the one given in
* AsyncTaskStartedCallback. See that callback for more information.
*
* The callback accepts a closure that is the same as the one given by the call
* to JS::InitAsyncTaskCallbacks.
*/
typedef void (*AsyncTaskFinishedCallback)(void* closure,
                                         Dispatchable* dispatchable);

/**
* Initialize the async task callbacks. Embedders must use this or else call
* js::UseInternalJobQueues to select a default implementation built into
* SpiderMonkey. This latter depends on the embedding to call js::RunJobs on
* the JavaScript thread to process queued Dispatchables at appropriate times.
*
* The dispatchCallback and delayedDispatchCallback are mandatory.
* asyncTaskStartedCallback and asyncTaskFinishedCallback are optional and may
* be null.
*
*/
extern JS_PUBLIC_API void InitAsyncTaskCallbacks(
   JSContext* cx, DispatchToEventLoopCallback dispatchCallback,
   DelayedDispatchToEventLoopCallback delayedDispatchCallback,
   AsyncTaskStartedCallback asyncTaskStartedCallback,
   AsyncTaskFinishedCallback asyncTaskFinishedCallback, void* closure);

/**
* Cancel all cancellable async tasks. Some tasks may not be cancellable, and
* must be waited on through a call to JS::ShutdownAsyncTasks.
*
* This will fire JSAsyncTaskFinished callbacks for cancelled tasks.
*/
extern JS_PUBLIC_API void CancelAsyncTasks(JSContext* cx);

/**
* Cancel all cancellable async tasks and block until non-cancellable tasks are
* finished.
*
* This will fire JSAsyncTaskFinished callbacks for all tasks, and there should
* be no outstanding tasks after this returns. All roots held by tasks will be
* released.
*
* Destroying a JSRuntime will implicitly perform this. However, this is not
* soon enough for cycle collection, which needs to have roots dropped earlier
* so that the cycle collector can transitively remove roots for a future GC.
* For these and other cases, the set of pending async tasks can be canceled
* with this call earlier than JSRuntime destruction.
*/

extern JS_PUBLIC_API void ShutdownAsyncTasks(JSContext* cx);

}  // namespace JS

#endif  // js_Promise_h