Promise.h

/* -*- 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 "mozilla/GuardObjects.h"

#include "jspubtd.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 incumbent global.
   *
   * SpiderMonkey doesn't itself have a notion of incumbent globals as defined
   * by the HTML spec, so we need the embedding to provide this. See
   * dom/script/ScriptSettings.h for details.
   */
  virtual JSObject* getIncumbentGlobal(JSContext* cx) = 0;

  /**
   * Enqueue a reaction job `job` for `promise`, which was allocated at
   * `allocationSite`. Provide `incumbentGlobal` as the incumbent global for
   * the reaction job's execution.
   */
  virtual bool enqueuePromiseJob(JSContext* cx, JS::HandleObject promise,
                                 JS::HandleObject job,
                                 JS::HandleObject allocationSite,
                                 JS::HandleObject incumbentGlobal) = 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;

 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 olny 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(
      MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM);
  ~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:
  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER;
  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 void SetSettledPromiseIsHandled(JSContext* cx,
                                                     JS::HandleObject promise);

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

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:
  // Dispatchables are created and destroyed by SpiderMonkey.
  Dispatchable() = default;
  virtual ~Dispatchable() = default;

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

  // Called by the embedding after DispatchToEventLoopCallback succeeds.
  virtual void run(JSContext* cx, MaybeShuttingDown maybeShuttingDown) = 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.
 *
 * 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.
 *
 * To establish a DispatchToEventLoopCallback, the embedding may either call
 * InitDispatchToEventLoop to provide its own, or 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.
 */

typedef bool (*DispatchToEventLoopCallback)(void* closure,
                                            Dispatchable* dispatchable);

extern JS_PUBLIC_API void InitDispatchToEventLoop(
    JSContext* cx, DispatchToEventLoopCallback callback, void* closure);

/**
 * When a JSRuntime is destroyed it implicitly cancels all async tasks in
 * progress, releasing any roots held by the task. 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