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| -rw-r--r-- | js/src/vm/OffThreadPromiseRuntimeState.h | 207 |
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diff --git a/js/src/vm/OffThreadPromiseRuntimeState.h b/js/src/vm/OffThreadPromiseRuntimeState.h new file mode 100644 index 0000000000..f2d24a600b --- /dev/null +++ b/js/src/vm/OffThreadPromiseRuntimeState.h @@ -0,0 +1,207 @@ +/* -*- 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 http://mozilla.org/MPL/2.0/. */ + +#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); + + protected: + 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; + + public: + ~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*>, + SystemAllocPolicy>; + +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; + + public: + OffThreadPromiseRuntimeState(); + ~OffThreadPromiseRuntimeState(); + 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 |