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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
#ifndef vm_AsyncIteration_h
#define vm_AsyncIteration_h
#include "builtin/Promise.h" // js::PromiseHandler
#include "builtin/SelfHostingDefines.h"
#include "js/Class.h"
#include "vm/GeneratorObject.h"
#include "vm/JSObject.h"
#include "vm/List.h"
#include "vm/PromiseObject.h"
// [SMDOC] Async generators
//
// # Start
//
// When an async generator is called, it synchronously runs until the
// JSOp::InitialYield and then suspends, just like a sync generator, and returns
// an async generator object (js::AsyncGeneratorObject).
//
//
// # Request queue
//
// When next/return/throw is called on the async generator object,
// js::AsyncGeneratorEnqueue performs the following:
// * Create a new AsyncGeneratorRequest and enqueue it in the generator
// object's request queue.
// * Resume the generator with the oldest request, if the generator is
// suspended (see "Resume" section below)
// * Return the promise for the request
//
// This is done in js::AsyncGeneratorEnqueue, which corresponds to
// AsyncGeneratorEnqueue in the spec,
// and js::AsyncGeneratorResumeNext corresponds to the following:
// * AsyncGeneratorResolve
// * AsyncGeneratorReject
// * AsyncGeneratorResumeNext
//
// The returned promise is resolved when the resumption for the request
// completes with yield/throw/return, in js::AsyncGeneratorResolve and
// js::AsyncGeneratorReject.
// They correspond to AsyncGeneratorResolve and AsyncGeneratorReject in the
// spec.
//
//
// # Await
//
// Async generator's `await` is implemented differently than async function's
// `await`.
//
// The bytecode is the following:
// (ignoring CanSkipAwait; see the comment in AsyncFunction.h for more details)
//
// ```
// (operand here) # VALUE
// GetAliasedVar ".generator" # VALUE .generator
// Await 0 # RVAL GENERATOR RESUMEKIND
//
// AfterYield # RVAL GENERATOR RESUMEKIND
// CheckResumeKind # RVAL
// ```
//
// Async generators don't use JSOp::AsyncAwait, and that part is handled
// in js::AsyncGeneratorResume, and js::AsyncGeneratorAwait called there.
//
// Both JSOp::Await and JSOp::Yield behave in the exactly same way,
// and js::AsyncGeneratorResume checks the last opcode and branches for
// await/yield/return cases.
//
//
// # Reaction jobs and resume after await
//
// This is almost same as for async functions (see AsyncFunction.h).
//
// The reaction record for the job is marked as "this is for async generator"
// (see js::AsyncGeneratorAwait), and handled specially in
// js::PromiseReactionJob, which calls js::AsyncGeneratorPromiseReactionJob.
//
//
// # Yield
//
// `yield` is implemented with the following bytecode sequence:
// (Ignoring CanSkipAwait for simplicity)
//
// ```
// (operand here) # VALUE
// GetAliasedVar ".generator" # VALUE .generator
// Await 1 # RVAL GENERATOR RESUMEKIND
// AfterYield # RVAL GENERATOR RESUMEKIND
// CheckResumeKind # RVAL
//
// GetAliasedVar ".generator" # RVAL .generator
// Yield 2 # RVAL2 GENERATOR RESUMEKIND
//
// AfterYield # RVAL2 GENERATOR RESUMEKIND
// CheckResumeKind # RVAL2
// ```
//
// The 1st part (JSOp::Await + JSOp::CheckResumeKind) performs an implicit
// `await`, as specified in AsyncGeneratorYield step 5.
//
// AsyncGeneratorYield ( value )
//
// 5. Set value to ? Await(value).
//
// The 2nd part (JSOp::Yield) suspends execution and yields the result of
// `await`, as specified in AsyncGeneratorYield steps 1-4, 6-7, 9-10.
//
// AsyncGeneratorYield ( value )
//
// 1. Let genContext be the running execution context.
// 2. Assert: genContext is the execution context of a generator.
// 3. Let generator be the value of the Generator component of genContext.
// 4. Assert: GetGeneratorKind() is async.
// ..
// 6. Set generator.[[AsyncGeneratorState]] to suspendedYield.
// 7. Remove genContext from the execution context stack and restore the
// execution context that is at the top of the execution context stack as
// the running execution context.
// 8. ...
// 9. Return ! AsyncGeneratorResolve(generator, value, false).
// 10. NOTE: This returns to the evaluation of the operation that had most
// previously resumed evaluation of genContext.
//
// The last part (JSOp::CheckResumeKind) checks the resumption type and
// resumes/throws/returns the execution, as specified in AsyncGeneratorYield
// step 8.
//
// 8. Set the code evaluation state of genContext such that when evaluation is
// resumed with a Completion resumptionValue the following steps will be
// performed:
// a. If resumptionValue.[[Type]] is not return, return
// Completion(resumptionValue).
// b. Let awaited be Await(resumptionValue.[[Value]]).
// c. If awaited.[[Type]] is throw, return Completion(awaited).
// d. Assert: awaited.[[Type]] is normal.
// e. Return Completion { [[Type]]: return, [[Value]]: awaited.[[Value]],
// [[Target]]: empty }.
// f. NOTE: When one of the above steps returns, it returns to the
// evaluation of the YieldExpression production that originally called
// this abstract operation.
//
// Resumption with `AsyncGenerator.prototype.return` is handled differently.
// See "Resumption with return" section below.
//
//
// # Return
//
// `return` with operand is implemented with the following bytecode sequence:
// (Ignoring CanSkipAwait for simplicity)
//
// ```
// (operand here) # VALUE
// GetAliasedVar ".generator" # VALUE .generator
// Await 0 # RVAL GENERATOR RESUMEKIND
// AfterYield # RVAL GENERATOR RESUMEKIND
// CheckResumeKind # RVAL
//
// SetRval #
// GetAliasedVar ".generator" # .generator
// FinalYieldRval #
// ```
//
// The 1st part (JSOp::Await + JSOp::CheckResumeKind) performs implicit
// `await`, as specified in ReturnStatement's Evaluation step 3.
//
// ReturnStatement: return Expression;
//
// 3. If ! GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
//
// And the 2nd part corresponds to AsyncGeneratorStart steps 5.a-e, 5.g.
//
// AsyncGeneratorStart ( generator, generatorBody )
//
// 5. Set the code evaluation state of genContext such that when evaluation
// is resumed for that execution context the following steps will be
// performed:
// a. Let result be the result of evaluating generatorBody.
// b. Assert: If we return here, the async generator either threw an
// exception or performed either an implicit or explicit return.
// c. Remove genContext from the execution context stack and restore the
// execution context that is at the top of the execution context stack
// as the running execution context.
// d. Set generator.[[AsyncGeneratorState]] to completed.
// e. If result is a normal completion, let resultValue be undefined.
// ...
// g. Return ! AsyncGeneratorResolve(generator, resultValue, true).
//
// `return` without operand or implicit return is implicit with the following
// bytecode sequence:
//
// ```
// Undefined # undefined
// SetRval #
// GetAliasedVar ".generator" # .generator
// FinalYieldRval #
// ```
//
// This is also AsyncGeneratorStart steps 5.a-e, 5.g.
//
//
// # Throw
//
// Unlike async function, async generator doesn't use implicit try-catch,
// but the throw completion is handled by js::AsyncGeneratorResume,
// and js::AsyncGeneratorThrown is called there.
//
// 5. ...
// f. Else,
// i. Let resultValue be result.[[Value]].
// ii. If result.[[Type]] is not return, then
// 1. Return ! AsyncGeneratorReject(generator, resultValue).
//
//
// # Resumption with return
//
// Resumption with return completion is handled in js::AsyncGeneratorResumeNext.
//
// If the generator is suspended, it doesn't immediately resume the generator
// script itself, but handles implicit `await` it in
// js::AsyncGeneratorResumeNext.
// (See PromiseHandlerAsyncGeneratorYieldReturnAwaitedFulfilled and
// PromiseHandlerAsyncGeneratorYieldReturnAwaitedRejected), and resumes the
// generator with the result of await.
// And the return completion is finally handled in JSOp::CheckResumeKind
// after JSOp::Yield.
//
// This corresponds to AsyncGeneratorYield step 8.
//
// AsyncGeneratorYield ( value )
//
// 8. Set the code evaluation state of genContext such that when evaluation
// is resumed with a Completion resumptionValue the following steps will
// be performed:
// ..
// b. Let awaited be Await(resumptionValue.[[Value]]).
// c. If awaited.[[Type]] is throw, return Completion(awaited).
// d. Assert: awaited.[[Type]] is normal.
// e. Return Completion { [[Type]]: return, [[Value]]: awaited.[[Value]],
// [[Target]]: empty }.
//
// If the generator is already completed, it awaits on the return value,
// (See PromiseHandlerAsyncGeneratorResumeNextReturnFulfilled and
// PromiseHandlerAsyncGeneratorResumeNextReturnRejected), and resolves the
// request's promise with the value.
//
// It corresponds to AsyncGeneratorResumeNext step 10.b.i.
//
// AsyncGeneratorResumeNext ( generator )
//
// 10. If completion is an abrupt completion, then
// ..
// b. If state is completed, then
// i. If completion.[[Type]] is return, then
// 1. Set generator.[[AsyncGeneratorState]] to awaiting-return.
// 2. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
// 3. Let stepsFulfilled be the algorithm steps defined in
// AsyncGeneratorResumeNext Return Processor Fulfilled Functions.
// 4. Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, «
// [[Generator]] »).
// 5. Set onFulfilled.[[Generator]] to generator.
// 6. Let stepsRejected be the algorithm steps defined in
// AsyncGeneratorResumeNext Return Processor Rejected Functions.
// 7. Let onRejected be ! CreateBuiltinFunction(stepsRejected, «
// [[Generator]] »).
// 8. Set onRejected.[[Generator]] to generator.
// 9. Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).
// 10. Return undefined.
//
namespace js {
class AsyncGeneratorObject;
enum class CompletionKind : uint8_t;
extern const JSClass AsyncGeneratorFunctionClass;
[[nodiscard]] bool AsyncGeneratorPromiseReactionJob(
JSContext* cx, PromiseHandler handler,
Handle<AsyncGeneratorObject*> generator, HandleValue argument);
bool AsyncGeneratorNext(JSContext* cx, unsigned argc, Value* vp);
bool AsyncGeneratorReturn(JSContext* cx, unsigned argc, Value* vp);
bool AsyncGeneratorThrow(JSContext* cx, unsigned argc, Value* vp);
// AsyncGeneratorRequest record in the spec.
// Stores the info from AsyncGenerator#{next,return,throw}.
//
// This object is reused across multiple requests as an optimization, and
// stored in the Slot_CachedRequest slot.
class AsyncGeneratorRequest : public NativeObject {
private:
enum AsyncGeneratorRequestSlots {
// Int32 value with CompletionKind.
// Normal: next
// Return: return
// Throw: throw
Slot_CompletionKind = 0,
// The value passed to AsyncGenerator#{next,return,throw}.
Slot_CompletionValue,
// The promise returned by AsyncGenerator#{next,return,throw}.
Slot_Promise,
Slots,
};
void init(CompletionKind completionKind, const Value& completionValue,
PromiseObject* promise) {
setFixedSlot(Slot_CompletionKind,
Int32Value(static_cast<int32_t>(completionKind)));
setFixedSlot(Slot_CompletionValue, completionValue);
setFixedSlot(Slot_Promise, ObjectValue(*promise));
}
// Clear the request data for reuse.
void clearData() {
setFixedSlot(Slot_CompletionValue, NullValue());
setFixedSlot(Slot_Promise, NullValue());
}
friend AsyncGeneratorObject;
public:
static const JSClass class_;
static AsyncGeneratorRequest* create(JSContext* cx,
CompletionKind completionKind,
HandleValue completionValue,
Handle<PromiseObject*> promise);
CompletionKind completionKind() const {
return static_cast<CompletionKind>(
getFixedSlot(Slot_CompletionKind).toInt32());
}
JS::Value completionValue() const {
return getFixedSlot(Slot_CompletionValue);
}
PromiseObject* promise() const {
return &getFixedSlot(Slot_Promise).toObject().as<PromiseObject>();
}
};
class AsyncGeneratorObject : public AbstractGeneratorObject {
private:
enum AsyncGeneratorObjectSlots {
// Int32 value containing one of the |State| fields from below.
Slot_State = AbstractGeneratorObject::RESERVED_SLOTS,
// * null value if this async generator has no requests
// * AsyncGeneratorRequest if this async generator has only one request
// * list object if this async generator has 2 or more requests
Slot_QueueOrRequest,
// Cached AsyncGeneratorRequest for later use.
// undefined if there's no cache.
Slot_CachedRequest,
Slots
};
public:
enum State {
// "suspendedStart" in the spec.
// Suspended after invocation.
State_SuspendedStart,
// "suspendedYield" in the spec
// Suspended with `yield` expression.
State_SuspendedYield,
// "executing" in the spec.
// Resumed from initial suspend or yield, and either running the script
// or awaiting for `await` expression.
State_Executing,
// Part of "executing" in the spec.
// Awaiting on the value passed by AsyncGenerator#return which is called
// while executing.
State_AwaitingYieldReturn,
// "awaiting-return" in the spec.
// Awaiting on the value passed by AsyncGenerator#return which is called
// after completed.
State_AwaitingReturn,
// "completed" in the spec.
// The generator is completed.
State_Completed
};
State state() const {
return static_cast<State>(getFixedSlot(Slot_State).toInt32());
}
void setState(State state_) { setFixedSlot(Slot_State, Int32Value(state_)); }
private:
// Queue is implemented in 2 ways. If only one request is queued ever,
// request is stored directly to the slot. Once 2 requests are queued, a
// list is created and requests are appended into it, and the list is
// stored to the slot.
bool isSingleQueue() const {
return getFixedSlot(Slot_QueueOrRequest).isNull() ||
getFixedSlot(Slot_QueueOrRequest)
.toObject()
.is<AsyncGeneratorRequest>();
}
bool isSingleQueueEmpty() const {
return getFixedSlot(Slot_QueueOrRequest).isNull();
}
void setSingleQueueRequest(AsyncGeneratorRequest* request) {
setFixedSlot(Slot_QueueOrRequest, ObjectValue(*request));
}
void clearSingleQueueRequest() {
setFixedSlot(Slot_QueueOrRequest, NullValue());
}
AsyncGeneratorRequest* singleQueueRequest() const {
return &getFixedSlot(Slot_QueueOrRequest)
.toObject()
.as<AsyncGeneratorRequest>();
}
ListObject* queue() const {
return &getFixedSlot(Slot_QueueOrRequest).toObject().as<ListObject>();
}
void setQueue(ListObject* queue_) {
setFixedSlot(Slot_QueueOrRequest, ObjectValue(*queue_));
}
public:
static const JSClass class_;
static const JSClassOps classOps_;
static AsyncGeneratorObject* create(JSContext* cx, HandleFunction asyncGen);
bool isSuspendedStart() const { return state() == State_SuspendedStart; }
bool isSuspendedYield() const { return state() == State_SuspendedYield; }
bool isExecuting() const { return state() == State_Executing; }
bool isAwaitingYieldReturn() const {
return state() == State_AwaitingYieldReturn;
}
bool isAwaitingReturn() const { return state() == State_AwaitingReturn; }
bool isCompleted() const { return state() == State_Completed; }
void setSuspendedStart() { setState(State_SuspendedStart); }
void setSuspendedYield() { setState(State_SuspendedYield); }
void setExecuting() { setState(State_Executing); }
void setAwaitingYieldReturn() { setState(State_AwaitingYieldReturn); }
void setAwaitingReturn() { setState(State_AwaitingReturn); }
void setCompleted() { setState(State_Completed); }
[[nodiscard]] static bool enqueueRequest(
JSContext* cx, Handle<AsyncGeneratorObject*> generator,
Handle<AsyncGeneratorRequest*> request);
static AsyncGeneratorRequest* dequeueRequest(
JSContext* cx, Handle<AsyncGeneratorObject*> generator);
static AsyncGeneratorRequest* peekRequest(
Handle<AsyncGeneratorObject*> generator);
bool isQueueEmpty() const {
if (isSingleQueue()) {
return isSingleQueueEmpty();
}
return queue()->getDenseInitializedLength() == 0;
}
// This function does either of the following:
// * return a cached request object with the slots updated
// * create a new request object with the slots set
static AsyncGeneratorRequest* createRequest(
JSContext* cx, Handle<AsyncGeneratorObject*> generator,
CompletionKind completionKind, HandleValue completionValue,
Handle<PromiseObject*> promise);
// Stores the given request to the generator's cache after clearing its data
// slots. The cached request will be reused in the subsequent createRequest
// call.
void cacheRequest(AsyncGeneratorRequest* request) {
if (hasCachedRequest()) {
return;
}
request->clearData();
setFixedSlot(Slot_CachedRequest, ObjectValue(*request));
}
private:
bool hasCachedRequest() const {
return getFixedSlot(Slot_CachedRequest).isObject();
}
AsyncGeneratorRequest* takeCachedRequest() {
auto request = &getFixedSlot(Slot_CachedRequest)
.toObject()
.as<AsyncGeneratorRequest>();
clearCachedRequest();
return request;
}
void clearCachedRequest() { setFixedSlot(Slot_CachedRequest, NullValue()); }
};
JSObject* CreateAsyncFromSyncIterator(JSContext* cx, HandleObject iter,
HandleValue nextMethod);
class AsyncFromSyncIteratorObject : public NativeObject {
private:
enum AsyncFromSyncIteratorObjectSlots {
// Object that implements the sync iterator protocol.
Slot_Iterator = 0,
// The `next` property of the iterator object.
Slot_NextMethod = 1,
Slots
};
void init(JSObject* iterator, const Value& nextMethod) {
setFixedSlot(Slot_Iterator, ObjectValue(*iterator));
setFixedSlot(Slot_NextMethod, nextMethod);
}
public:
static const JSClass class_;
static JSObject* create(JSContext* cx, HandleObject iter,
HandleValue nextMethod);
JSObject* iterator() const { return &getFixedSlot(Slot_Iterator).toObject(); }
const Value& nextMethod() const { return getFixedSlot(Slot_NextMethod); }
};
class AsyncIteratorObject : public NativeObject {
public:
static const JSClass class_;
static const JSClass protoClass_;
};
// Iterator Helpers proposal
class AsyncIteratorHelperObject : public NativeObject {
public:
static const JSClass class_;
enum { GeneratorSlot, SlotCount };
static_assert(GeneratorSlot == ASYNC_ITERATOR_HELPER_GENERATOR_SLOT,
"GeneratorSlot must match self-hosting define for generator "
"object slot.");
};
AsyncIteratorHelperObject* NewAsyncIteratorHelper(JSContext* cx);
} // namespace js
#endif /* vm_AsyncIteration_h */