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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* 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 */
#ifndef vm_OffThreadPromiseRuntimeState_h
#define vm_OffThreadPromiseRuntimeState_h
#include <stddef.h> // size_t
#include "jstypes.h" // JS_PUBLIC_API
#include "ds/Fifo.h" // js::Fifo
#include "js/AllocPolicy.h" // js::SystemAllocPolicy
#include "js/HashTable.h" // js::DefaultHasher, js::HashSet
#include "js/Promise.h" // JS::Dispatchable, JS::Dispatchable::MaybeShuttingDown, JS::DispatchToEventLoopCallback
#include "js/RootingAPI.h" // JS::Handle, JS::PersistentRooted
#include "threading/ConditionVariable.h" // js::ConditionVariable
#include "vm/PromiseObject.h" // js::PromiseObject
struct JS_PUBLIC_API JSContext;
struct JS_PUBLIC_API JSRuntime;
namespace js {
class OffThreadPromiseRuntimeState;
// [SMDOC] OffThreadPromiseTask: an off-main-thread task that resolves a promise
// An OffThreadPromiseTask is an abstract base class holding a JavaScript
// promise that will be resolved (fulfilled or rejected) with the results of a
// task possibly performed by some other thread.
// An OffThreadPromiseTask's lifecycle is as follows:
// - Some JavaScript native wishes to return a promise of the result of some
// computation that might be performed by other threads (say, helper threads
// or the embedding's I/O threads), so it creates a PromiseObject to represent
// the result, and an OffThreadPromiseTask referring to it. After handing the
// OffThreadPromiseTask to the code doing the actual work, the native is free
// to return the PromiseObject to its caller.
// - When the computation is done, successfully or otherwise, it populates the
// OffThreadPromiseTask—which is actually an instance of some concrete
// subclass specific to the task—with the information needed to resolve the
// promise, and calls OffThreadPromiseTask::dispatchResolveAndDestroy. This
// enqueues a runnable on the JavaScript thread to which the promise belongs.
// - When it gets around to the runnable, the JavaScript thread calls the
// OffThreadPromiseTask's `resolve` method, which the concrete subclass has
// overriden to resolve the promise appropriately. This probably enqueues a
// promise reaction job.
// - The JavaScript thread then deletes the OffThreadPromiseTask.
// During shutdown, the process is slightly different. Enqueuing runnables to
// the JavaScript thread begins to fail. JSRuntime shutdown waits for all
// outstanding tasks to call dispatchResolveAndDestroy, and then deletes them on
// the main thread, without calling `resolve`.
// For example, the JavaScript function WebAssembly.compile uses
// OffThreadPromiseTask to manage the result of a helper thread task, accepting
// binary WebAssembly code and returning a promise of a compiled
// WebAssembly.Module. It would like to do this compilation work on a helper
// thread. When called by JavaScript, WebAssembly.compile creates a promise,
// builds a CompileBufferTask (the OffThreadPromiseTask concrete subclass) to
// keep track of it, and then hands that to a helper thread. When the helper
// thread is done, successfully or otherwise, it calls the CompileBufferTask's
// dispatchResolveAndDestroy method, which enqueues a runnable to the JavaScript
// thread to resolve the promise and delete the CompileBufferTask.
// (CompileBufferTask actually implements PromiseHelperTask, which implements
// OffThreadPromiseTask; PromiseHelperTask is what our helper thread scheduler
// requires.)
// OffThreadPromiseTasks are not limited to use with helper threads. For
// example, a function returning a promise of the result of a network operation
// could provide the code collecting the incoming data with an
// OffThreadPromiseTask for the promise, and let the embedding's network I/O
// threads call dispatchResolveAndDestroy.
// OffThreadPromiseTask may also be used purely on the main thread, as a way to
// "queue a task" in HTML terms. Note that a "task" is not the same as a
// "microtask" and there are separate queues for tasks and microtasks that are
// drained at separate times in the browser. The task queue is implemented by
// the browser's main event loop. The microtask queue is implemented
// by JS::JobQueue, used for promises and gets drained before returning to
// the event loop. Thus OffThreadPromiseTask can only be used when the spec
// says "queue a task", as the WebAssembly APIs do.
// An OffThreadPromiseTask has a JSContext, and must be constructed and have its
// 'init' method called on that JSContext's thread. Once initialized, its
// dispatchResolveAndDestroy method may be called from any thread. This is the
// only safe way to destruct an OffThreadPromiseTask; doing so ensures the
// OffThreadPromiseTask's destructor will run on the JSContext's thread, either
// from the event loop or during shutdown.
// OffThreadPromiseTask::dispatchResolveAndDestroy uses the
// JS::DispatchToEventLoopCallback provided by the embedding to enqueue
// runnables on the JavaScript thread. See the comments for
// DispatchToEventLoopCallback for details.
class OffThreadPromiseTask : public JS::Dispatchable {
friend class OffThreadPromiseRuntimeState;
JSRuntime* runtime_;
JS::PersistentRooted<PromiseObject*> promise_;
bool registered_;
void operator=(const OffThreadPromiseTask&) = delete;
OffThreadPromiseTask(const OffThreadPromiseTask&) = delete;
void unregister(OffThreadPromiseRuntimeState& state);
OffThreadPromiseTask(JSContext* cx, JS::Handle<PromiseObject*> promise);
// To be called by OffThreadPromiseTask and implemented by the derived class.
virtual bool resolve(JSContext* cx, JS::Handle<PromiseObject*> promise) = 0;
// JS::Dispatchable implementation. Ends with 'delete this'.
void run(JSContext* cx, MaybeShuttingDown maybeShuttingDown) final;
~OffThreadPromiseTask() override;
// Initializing an OffThreadPromiseTask informs the runtime that it must
// wait on shutdown for this task to rejoin the active JSContext by calling
// dispatchResolveAndDestroy().
bool init(JSContext* cx);
// An initialized OffThreadPromiseTask can be dispatched to an active
// JSContext of its Promise's JSRuntime from any thread. Normally, this will
// lead to resolve() being called on JSContext thread, given the Promise.
// However, if shutdown interrupts, resolve() may not be called, though the
// OffThreadPromiseTask will be destroyed on a JSContext thread.
void dispatchResolveAndDestroy();
void dispatchResolveAndDestroy(const AutoLockHelperThreadState& lock);
using OffThreadPromiseTaskSet =
HashSet<OffThreadPromiseTask*, DefaultHasher<OffThreadPromiseTask*>,
using DispatchableFifo = Fifo<JS::Dispatchable*, 0, SystemAllocPolicy>;
class OffThreadPromiseRuntimeState {
friend class OffThreadPromiseTask;
// These fields are initialized once before any off-thread usage and thus do
// not require a lock.
JS::DispatchToEventLoopCallback dispatchToEventLoopCallback_;
void* dispatchToEventLoopClosure_;
// A set of all OffThreadPromiseTasks that have successfully called 'init'.
// OffThreadPromiseTask's destructor removes them from the set.
HelperThreadLockData<OffThreadPromiseTaskSet> live_;
// The allCanceled_ condition is waited on and notified during engine
// shutdown, communicating when all off-thread tasks in live_ are safe to be
// destroyed from the (shutting down) main thread. This condition is met when
// live_.count() == numCanceled_ where "canceled" means "the
// DispatchToEventLoopCallback failed after this task finished execution".
HelperThreadLockData<ConditionVariable> allCanceled_;
HelperThreadLockData<size_t> numCanceled_;
// The queue of JS::Dispatchables used by the DispatchToEventLoopCallback that
// calling js::UseInternalJobQueues installs.
HelperThreadLockData<DispatchableFifo> internalDispatchQueue_;
HelperThreadLockData<ConditionVariable> internalDispatchQueueAppended_;
HelperThreadLockData<bool> internalDispatchQueueClosed_;
OffThreadPromiseTaskSet& live() { return live_.ref(); }
ConditionVariable& allCanceled() { return allCanceled_.ref(); }
DispatchableFifo& internalDispatchQueue() {
return internalDispatchQueue_.ref();
ConditionVariable& internalDispatchQueueAppended() {
return internalDispatchQueueAppended_.ref();
static bool internalDispatchToEventLoop(void*, JS::Dispatchable*);
bool usingInternalDispatchQueue() const;
void operator=(const OffThreadPromiseRuntimeState&) = delete;
OffThreadPromiseRuntimeState(const OffThreadPromiseRuntimeState&) = delete;
void init(JS::DispatchToEventLoopCallback callback, void* closure);
void initInternalDispatchQueue();
bool initialized() const;
// If initInternalDispatchQueue() was called, internalDrain() can be
// called to periodically drain the dispatch queue before shutdown.
void internalDrain(JSContext* cx);
bool internalHasPending();
// shutdown() must be called by the JSRuntime while the JSRuntime is valid.
void shutdown(JSContext* cx);
} // namespace js
#endif // vm_OffThreadPromiseRuntimeState_h