OffThreadPromiseRuntimeState.h

/* -*- 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 "threading/Mutex.h"              // js::Mutex
#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();
};

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_;

  // All following fields are mutated by any thread and are guarded by mutex_.
  Mutex mutex_;

  // A set of all OffThreadPromiseTasks that have successfully called 'init'.
  // OffThreadPromiseTask's destructor removes them from the set.
  OffThreadPromiseTaskSet live_;

  // The allCancelled_ 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".
  ConditionVariable allCanceled_;
  size_t numCanceled_;

  // The queue of JS::Dispatchables used by the DispatchToEventLoopCallback that
  // calling js::UseInternalJobQueues installs.
  DispatchableFifo internalDispatchQueue_;
  ConditionVariable internalDispatchQueueAppended_;
  bool internalDispatchQueueClosed_;

  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