// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors // Licensed under the MIT License: // // Permission is hereby granted, free of charge, to any person obtaining a copy // of this software and associated documentation files (the "Software"), to deal // in the Software without restriction, including without limitation the rights // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell // copies of the Software, and to permit persons to whom the Software is // furnished to do so, subject to the following conditions: // // The above copyright notice and this permission notice shall be included in // all copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN // THE SOFTWARE. #ifndef KJ_ASYNC_H_ #define KJ_ASYNC_H_ #if defined(__GNUC__) && !KJ_HEADER_WARNINGS #pragma GCC system_header #endif #include "async-prelude.h" #include "exception.h" #include "refcount.h" #include "tuple.h" namespace kj { class EventLoop; class WaitScope; template <typename T> class Promise; template <typename T> class ForkedPromise; template <typename T> class PromiseFulfiller; template <typename T> struct PromiseFulfillerPair; template <typename Func, typename T> using PromiseForResult = Promise<_::JoinPromises<_::ReturnType<Func, T>>>; // Evaluates to the type of Promise for the result of calling functor type Func with parameter type // T. If T is void, then the promise is for the result of calling Func with no arguments. If // Func itself returns a promise, the promises are joined, so you never get Promise<Promise<T>>. // ======================================================================================= // Promises template <typename T> class Promise: protected _::PromiseBase { // The basic primitive of asynchronous computation in KJ. Similar to "futures", but designed // specifically for event loop concurrency. Similar to E promises and JavaScript Promises/A. // // A Promise represents a promise to produce a value of type T some time in the future. Once // that value has been produced, the promise is "fulfilled". Alternatively, a promise can be // "broken", with an Exception describing what went wrong. You may implicitly convert a value of // type T to an already-fulfilled Promise<T>. You may implicitly convert the constant // `kj::READY_NOW` to an already-fulfilled Promise<void>. You may also implicitly convert a // `kj::Exception` to an already-broken promise of any type. // // Promises are linear types -- they are moveable but not copyable. If a Promise is destroyed // or goes out of scope (without being moved elsewhere), any ongoing asynchronous operations // meant to fulfill the promise will be canceled if possible. All methods of `Promise` (unless // otherwise noted) actually consume the promise in the sense of move semantics. (Arguably they // should be rvalue-qualified, but at the time this interface was created compilers didn't widely // support that yet and anyway it would be pretty ugly typing kj::mv(promise).whatever().) If // you want to use one Promise in two different places, you must fork it with `fork()`. // // To use the result of a Promise, you must call `then()` and supply a callback function to // call with the result. `then()` returns another promise, for the result of the callback. // Any time that this would result in Promise<Promise<T>>, the promises are collapsed into a // simple Promise<T> that first waits for the outer promise, then the inner. Example: // // // Open a remote file, read the content, and then count the // // number of lines of text. // // Note that none of the calls here block. `file`, `content` // // and `lineCount` are all initialized immediately before any // // asynchronous operations occur. The lambda callbacks are // // called later. // Promise<Own<File>> file = openFtp("ftp://host/foo/bar"); // Promise<String> content = file.then( // [](Own<File> file) -> Promise<String> { // return file.readAll(); // }); // Promise<int> lineCount = content.then( // [](String text) -> int { // uint count = 0; // for (char c: text) count += (c == '\n'); // return count; // }); // // For `then()` to work, the current thread must have an active `EventLoop`. Each callback // is scheduled to execute in that loop. Since `then()` schedules callbacks only on the current // thread's event loop, you do not need to worry about two callbacks running at the same time. // You will need to set up at least one `EventLoop` at the top level of your program before you // can use promises. // // To adapt a non-Promise-based asynchronous API to promises, use `newAdaptedPromise()`. // // Systems using promises should consider supporting the concept of "pipelining". Pipelining // means allowing a caller to start issuing method calls against a promised object before the // promise has actually been fulfilled. This is particularly useful if the promise is for a // remote object living across a network, as this can avoid round trips when chaining a series // of calls. It is suggested that any class T which supports pipelining implement a subclass of // Promise<T> which adds "eventual send" methods -- methods which, when called, say "please // invoke the corresponding method on the promised value once it is available". These methods // should in turn return promises for the eventual results of said invocations. Cap'n Proto, // for example, implements the type `RemotePromise` which supports pipelining RPC requests -- see // `capnp/capability.h`. // // KJ Promises are based on E promises: // http://wiki.erights.org/wiki/Walnut/Distributed_Computing#Promises // // KJ Promises are also inspired in part by the evolving standards for JavaScript/ECMAScript // promises, which are themselves influenced by E promises: // http://promisesaplus.com/ // https://github.com/domenic/promises-unwrapping public: Promise(_::FixVoid<T> value); // Construct an already-fulfilled Promise from a value of type T. For non-void promises, the // parameter type is simply T. So, e.g., in a function that returns `Promise<int>`, you can // say `return 123;` to return a promise that is already fulfilled to 123. // // For void promises, use `kj::READY_NOW` as the value, e.g. `return kj::READY_NOW`. Promise(kj::Exception&& e); // Construct an already-broken Promise. inline Promise(decltype(nullptr)) {} template <typename Func, typename ErrorFunc = _::PropagateException> PromiseForResult<Func, T> then(Func&& func, ErrorFunc&& errorHandler = _::PropagateException()) KJ_WARN_UNUSED_RESULT; // Register a continuation function to be executed when the promise completes. The continuation // (`func`) takes the promised value (an rvalue of type `T`) as its parameter. The continuation // may return a new value; `then()` itself returns a promise for the continuation's eventual // result. If the continuation itself returns a `Promise<U>`, then `then()` shall also return // a `Promise<U>` which first waits for the original promise, then executes the continuation, // then waits for the inner promise (i.e. it automatically "unwraps" the promise). // // In all cases, `then()` returns immediately. The continuation is executed later. The // continuation is always executed on the same EventLoop (and, therefore, the same thread) which // called `then()`, therefore no synchronization is necessary on state shared by the continuation // and the surrounding scope. If no EventLoop is running on the current thread, `then()` throws // an exception. // // You may also specify an error handler continuation as the second parameter. `errorHandler` // must be a functor taking a parameter of type `kj::Exception&&`. It must return the same // type as `func` returns (except when `func` returns `Promise<U>`, in which case `errorHandler` // may return either `Promise<U>` or just `U`). The default error handler simply propagates the // exception to the returned promise. // // Either `func` or `errorHandler` may, of course, throw an exception, in which case the promise // is broken. When compiled with -fno-exceptions, the framework will still detect when a // recoverable exception was thrown inside of a continuation and will consider the promise // broken even though a (presumably garbage) result was returned. // // If the returned promise is destroyed before the callback runs, the callback will be canceled // (it will never run). // // Note that `then()` -- like all other Promise methods -- consumes the promise on which it is // called, in the sense of move semantics. After returning, the original promise is no longer // valid, but `then()` returns a new promise. // // *Advanced implementation tips:* Most users will never need to worry about the below, but // it is good to be aware of. // // As an optimization, if the callback function `func` does _not_ return another promise, then // execution of `func` itself may be delayed until its result is known to be needed. The // expectation here is that `func` is just doing some transformation on the results, not // scheduling any other actions, therefore the system doesn't need to be proactive about // evaluating it. This way, a chain of trivial then() transformations can be executed all at // once without repeatedly re-scheduling through the event loop. Use the `eagerlyEvaluate()` // method to suppress this behavior. // // On the other hand, if `func` _does_ return another promise, then the system evaluates `func` // as soon as possible, because the promise it returns might be for a newly-scheduled // long-running asynchronous task. // // As another optimization, when a callback function registered with `then()` is actually // scheduled, it is scheduled to occur immediately, preempting other work in the event queue. // This allows a long chain of `then`s to execute all at once, improving cache locality by // clustering operations on the same data. However, this implies that starvation can occur // if a chain of `then()`s takes a very long time to execute without ever stopping to wait for // actual I/O. To solve this, use `kj::evalLater()` to yield control; this way, all other events // in the queue will get a chance to run before your callback is executed. T wait(WaitScope& waitScope); // Run the event loop until the promise is fulfilled, then return its result. If the promise // is rejected, throw an exception. // // wait() is primarily useful at the top level of a program -- typically, within the function // that allocated the EventLoop. For example, a program that performs one or two RPCs and then // exits would likely use wait() in its main() function to wait on each RPC. On the other hand, // server-side code generally cannot use wait(), because it has to be able to accept multiple // requests at once. // // If the promise is rejected, `wait()` throws an exception. If the program was compiled without // exceptions (-fno-exceptions), this will usually abort. In this case you really should first // use `then()` to set an appropriate handler for the exception case, so that the promise you // actually wait on never throws. // // `waitScope` is an object proving that the caller is in a scope where wait() is allowed. By // convention, any function which might call wait(), or which might call another function which // might call wait(), must take `WaitScope&` as one of its parameters. This is needed for two // reasons: // * `wait()` is not allowed during an event callback, because event callbacks are themselves // called during some other `wait()`, and such recursive `wait()`s would only be able to // complete in LIFO order, which might mean that the outer `wait()` ends up waiting longer // than it is supposed to. To prevent this, a `WaitScope` cannot be constructed or used during // an event callback. // * Since `wait()` runs the event loop, unrelated event callbacks may execute before `wait()` // returns. This means that anyone calling `wait()` must be reentrant -- state may change // around them in arbitrary ways. Therefore, callers really need to know if a function they // are calling might wait(), and the `WaitScope&` parameter makes this clear. // // TODO(someday): Implement fibers, and let them call wait() even when they are handling an // event. ForkedPromise<T> fork() KJ_WARN_UNUSED_RESULT; // Forks the promise, so that multiple different clients can independently wait on the result. // `T` must be copy-constructable for this to work. Or, in the special case where `T` is // `Own<U>`, `U` must have a method `Own<U> addRef()` which returns a new reference to the same // (or an equivalent) object (probably implemented via reference counting). Promise<T> exclusiveJoin(Promise<T>&& other) KJ_WARN_UNUSED_RESULT; // Return a new promise that resolves when either the original promise resolves or `other` // resolves (whichever comes first). The promise that didn't resolve first is canceled. // TODO(someday): inclusiveJoin(), or perhaps just join(), which waits for both completions // and produces a tuple? template <typename... Attachments> Promise<T> attach(Attachments&&... attachments) KJ_WARN_UNUSED_RESULT; // "Attaches" one or more movable objects (often, Own<T>s) to the promise, such that they will // be destroyed when the promise resolves. This is useful when a promise's callback contains // pointers into some object and you want to make sure the object still exists when the callback // runs -- after calling then(), use attach() to add necessary objects to the result. template <typename ErrorFunc> Promise<T> eagerlyEvaluate(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT; Promise<T> eagerlyEvaluate(decltype(nullptr)) KJ_WARN_UNUSED_RESULT; // Force eager evaluation of this promise. Use this if you are going to hold on to the promise // for awhile without consuming the result, but you want to make sure that the system actually // processes it. // // `errorHandler` is a function that takes `kj::Exception&&`, like the second parameter to // `then()`, except that it must return void. We make you specify this because otherwise it's // easy to forget to handle errors in a promise that you never use. You may specify nullptr for // the error handler if you are sure that ignoring errors is fine, or if you know that you'll // eventually wait on the promise somewhere. template <typename ErrorFunc> void detach(ErrorFunc&& errorHandler); // Allows the promise to continue running in the background until it completes or the // `EventLoop` is destroyed. Be careful when using this: since you can no longer cancel this // promise, you need to make sure that the promise owns all the objects it touches or make sure // those objects outlive the EventLoop. // // `errorHandler` is a function that takes `kj::Exception&&`, like the second parameter to // `then()`, except that it must return void. // // This function exists mainly to implement the Cap'n Proto requirement that RPC calls cannot be // canceled unless the callee explicitly permits it. kj::String trace(); // Returns a dump of debug info about this promise. Not for production use. Requires RTTI. // This method does NOT consume the promise as other methods do. private: Promise(bool, Own<_::PromiseNode>&& node): PromiseBase(kj::mv(node)) {} // Second parameter prevent ambiguity with immediate-value constructor. template <typename> friend class Promise; friend class EventLoop; template <typename U, typename Adapter, typename... Params> friend Promise<U> newAdaptedPromise(Params&&... adapterConstructorParams); template <typename U> friend PromiseFulfillerPair<U> newPromiseAndFulfiller(); template <typename> friend class _::ForkHub; friend class _::TaskSetImpl; friend Promise<void> _::yield(); friend class _::NeverDone; template <typename U> friend Promise<Array<U>> joinPromises(Array<Promise<U>>&& promises); friend Promise<void> joinPromises(Array<Promise<void>>&& promises); }; template <typename T> class ForkedPromise { // The result of `Promise::fork()` and `EventLoop::fork()`. Allows branches to be created. // Like `Promise<T>`, this is a pass-by-move type. public: inline ForkedPromise(decltype(nullptr)) {} Promise<T> addBranch(); // Add a new branch to the fork. The branch is equivalent to the original promise. private: Own<_::ForkHub<_::FixVoid<T>>> hub; inline ForkedPromise(bool, Own<_::ForkHub<_::FixVoid<T>>>&& hub): hub(kj::mv(hub)) {} friend class Promise<T>; friend class EventLoop; }; constexpr _::Void READY_NOW = _::Void(); // Use this when you need a Promise<void> that is already fulfilled -- this value can be implicitly // cast to `Promise<void>`. constexpr _::NeverDone NEVER_DONE = _::NeverDone(); // The opposite of `READY_NOW`, return this when the promise should never resolve. This can be // implicitly converted to any promise type. You may also call `NEVER_DONE.wait()` to wait // forever (useful for servers). template <typename Func> PromiseForResult<Func, void> evalLater(Func&& func); // Schedule for the given zero-parameter function to be executed in the event loop at some // point in the near future. Returns a Promise for its result -- or, if `func()` itself returns // a promise, `evalLater()` returns a Promise for the result of resolving that promise. // // Example usage: // Promise<int> x = evalLater([]() { return 123; }); // // The above is exactly equivalent to: // Promise<int> x = Promise<void>(READY_NOW).then([]() { return 123; }); // // If the returned promise is destroyed before the callback runs, the callback will be canceled // (never called). // // If you schedule several evaluations with `evalLater` during the same callback, they are // guaranteed to be executed in order. template <typename T> Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises); // Join an array of promises into a promise for an array. // ======================================================================================= // Hack for creating a lambda that holds an owned pointer. template <typename Func, typename MovedParam> class CaptureByMove { public: inline CaptureByMove(Func&& func, MovedParam&& param) : func(kj::mv(func)), param(kj::mv(param)) {} template <typename... Params> inline auto operator()(Params&&... params) -> decltype(kj::instance<Func>()(kj::instance<MovedParam&&>(), kj::fwd<Params>(params)...)) { return func(kj::mv(param), kj::fwd<Params>(params)...); } private: Func func; MovedParam param; }; template <typename Func, typename MovedParam> inline CaptureByMove<Func, Decay<MovedParam>> mvCapture(MovedParam&& param, Func&& func) { // Hack to create a "lambda" which captures a variable by moving it rather than copying or // referencing. C++14 generalized captures should make this obsolete, but for now in C++11 this // is commonly needed for Promise continuations that own their state. Example usage: // // Own<Foo> ptr = makeFoo(); // Promise<int> promise = callRpc(); // promise.then(mvCapture(ptr, [](Own<Foo>&& ptr, int result) { // return ptr->finish(result); // })); return CaptureByMove<Func, Decay<MovedParam>>(kj::fwd<Func>(func), kj::mv(param)); } // ======================================================================================= // Advanced promise construction template <typename T> class PromiseFulfiller { // A callback which can be used to fulfill a promise. Only the first call to fulfill() or // reject() matters; subsequent calls are ignored. public: virtual void fulfill(T&& value) = 0; // Fulfill the promise with the given value. virtual void reject(Exception&& exception) = 0; // Reject the promise with an error. virtual bool isWaiting() = 0; // Returns true if the promise is still unfulfilled and someone is potentially waiting for it. // Returns false if fulfill()/reject() has already been called *or* if the promise to be // fulfilled has been discarded and therefore the result will never be used anyway. template <typename Func> bool rejectIfThrows(Func&& func); // Call the function (with no arguments) and return true. If an exception is thrown, call // `fulfiller.reject()` and then return false. When compiled with exceptions disabled, // non-fatal exceptions are still detected and handled correctly. }; template <> class PromiseFulfiller<void> { // Specialization of PromiseFulfiller for void promises. See PromiseFulfiller<T>. public: virtual void fulfill(_::Void&& value = _::Void()) = 0; // Call with zero parameters. The parameter is a dummy that only exists so that subclasses don't // have to specialize for <void>. virtual void reject(Exception&& exception) = 0; virtual bool isWaiting() = 0; template <typename Func> bool rejectIfThrows(Func&& func); }; template <typename T, typename Adapter, typename... Params> Promise<T> newAdaptedPromise(Params&&... adapterConstructorParams); // Creates a new promise which owns an instance of `Adapter` which encapsulates the operation // that will eventually fulfill the promise. This is primarily useful for adapting non-KJ // asynchronous APIs to use promises. // // An instance of `Adapter` will be allocated and owned by the returned `Promise`. A // `PromiseFulfiller<T>&` will be passed as the first parameter to the adapter's constructor, // and `adapterConstructorParams` will be forwarded as the subsequent parameters. The adapter // is expected to perform some asynchronous operation and call the `PromiseFulfiller<T>` once // it is finished. // // The adapter is destroyed when its owning Promise is destroyed. This may occur before the // Promise has been fulfilled. In this case, the adapter's destructor should cancel the // asynchronous operation. Once the adapter is destroyed, the fulfillment callback cannot be // called. // // An adapter implementation should be carefully written to ensure that it cannot accidentally // be left unfulfilled permanently because of an exception. Consider making liberal use of // `PromiseFulfiller<T>::rejectIfThrows()`. template <typename T> struct PromiseFulfillerPair { Promise<_::JoinPromises<T>> promise; Own<PromiseFulfiller<T>> fulfiller; }; template <typename T> PromiseFulfillerPair<T> newPromiseAndFulfiller(); // Construct a Promise and a separate PromiseFulfiller which can be used to fulfill the promise. // If the PromiseFulfiller is destroyed before either of its methods are called, the Promise is // implicitly rejected. // // Although this function is easier to use than `newAdaptedPromise()`, it has the serious drawback // that there is no way to handle cancellation (i.e. detect when the Promise is discarded). // // You can arrange to fulfill a promise with another promise by using a promise type for T. E.g. // `newPromiseAndFulfiller<Promise<U>>()` will produce a promise of type `Promise<U>` but the // fulfiller will be of type `PromiseFulfiller<Promise<U>>`. Thus you pass a `Promise<U>` to the // `fulfill()` callback, and the promises are chained. // ======================================================================================= // TaskSet class TaskSet { // Holds a collection of Promise<void>s and ensures that each executes to completion. Memory // associated with each promise is automatically freed when the promise completes. Destroying // the TaskSet itself automatically cancels all unfinished promises. // // This is useful for "daemon" objects that perform background tasks which aren't intended to // fulfill any particular external promise, but which may need to be canceled (and thus can't // use `Promise::detach()`). The daemon object holds a TaskSet to collect these tasks it is // working on. This way, if the daemon itself is destroyed, the TaskSet is detroyed as well, // and everything the daemon is doing is canceled. public: class ErrorHandler { public: virtual void taskFailed(kj::Exception&& exception) = 0; }; TaskSet(ErrorHandler& errorHandler); // `loop` will be used to wait on promises. `errorHandler` will be executed any time a task // throws an exception, and will execute within the given EventLoop. ~TaskSet() noexcept(false); void add(Promise<void>&& promise); kj::String trace(); // Return debug info about all promises currently in the TaskSet. private: Own<_::TaskSetImpl> impl; }; // ======================================================================================= // The EventLoop class class EventPort { // Interfaces between an `EventLoop` and events originating from outside of the loop's thread. // All such events come in through the `EventPort` implementation. // // An `EventPort` implementation may interface with low-level operating system APIs and/or other // threads. You can also write an `EventPort` which wraps some other (non-KJ) event loop // framework, allowing the two to coexist in a single thread. public: virtual bool wait() = 0; // Wait for an external event to arrive, sleeping if necessary. Once at least one event has // arrived, queue it to the event loop (e.g. by fulfilling a promise) and return. // // This is called during `Promise::wait()` whenever the event queue becomes empty, in order to // wait for new events to populate the queue. // // It is safe to return even if nothing has actually been queued, so long as calling `wait()` in // a loop will eventually sleep. (That is to say, false positives are fine.) // // Returns true if wake() has been called from another thread. (Precisely, returns true if // no previous call to wait `wait()` nor `poll()` has returned true since `wake()` was last // called.) virtual bool poll() = 0; // Check if any external events have arrived, but do not sleep. If any events have arrived, // add them to the event queue (e.g. by fulfilling promises) before returning. // // This may be called during `Promise::wait()` when the EventLoop has been executing for a while // without a break but is still non-empty. // // Returns true if wake() has been called from another thread. (Precisely, returns true if // no previous call to wait `wait()` nor `poll()` has returned true since `wake()` was last // called.) virtual void setRunnable(bool runnable); // Called to notify the `EventPort` when the `EventLoop` has work to do; specifically when it // transitions from empty -> runnable or runnable -> empty. This is typically useful when // integrating with an external event loop; if the loop is currently runnable then you should // arrange to call run() on it soon. The default implementation does nothing. virtual void wake() const; // Wake up the EventPort's thread from another thread. // // Unlike all other methods on this interface, `wake()` may be called from another thread, hence // it is `const`. // // Technically speaking, `wake()` causes the target thread to cease sleeping and not to sleep // again until `wait()` or `poll()` has returned true at least once. // // The default implementation throws an UNIMPLEMENTED exception. }; class EventLoop { // Represents a queue of events being executed in a loop. Most code won't interact with // EventLoop directly, but instead use `Promise`s to interact with it indirectly. See the // documentation for `Promise`. // // Each thread can have at most one current EventLoop. To make an `EventLoop` current for // the thread, create a `WaitScope`. Async APIs require that the thread has a current EventLoop, // or they will throw exceptions. APIs that use `Promise::wait()` additionally must explicitly // be passed a reference to the `WaitScope` to make the caller aware that they might block. // // Generally, you will want to construct an `EventLoop` at the top level of your program, e.g. // in the main() function, or in the start function of a thread. You can then use it to // construct some promises and wait on the result. Example: // // int main() { // // `loop` becomes the official EventLoop for the thread. // MyEventPort eventPort; // EventLoop loop(eventPort); // // // Now we can call an async function. // Promise<String> textPromise = getHttp("http://example.com"); // // // And we can wait for the promise to complete. Note that you can only use `wait()` // // from the top level, not from inside a promise callback. // String text = textPromise.wait(); // print(text); // return 0; // } // // Most applications that do I/O will prefer to use `setupIoEventLoop()` from `async-io.h` rather // than allocate an `EventLoop` directly. public: EventLoop(); // Construct an `EventLoop` which does not receive external events at all. explicit EventLoop(EventPort& port); // Construct an `EventLoop` which receives external events through the given `EventPort`. ~EventLoop() noexcept(false); void run(uint maxTurnCount = maxValue); // Run the event loop for `maxTurnCount` turns or until there is nothing left to be done, // whichever comes first. This never calls the `EventPort`'s `sleep()` or `poll()`. It will // call the `EventPort`'s `setRunnable(false)` if the queue becomes empty. bool isRunnable(); // Returns true if run() would currently do anything, or false if the queue is empty. private: EventPort& port; bool running = false; // True while looping -- wait() is then not allowed. bool lastRunnableState = false; // What did we last pass to port.setRunnable()? _::Event* head = nullptr; _::Event** tail = &head; _::Event** depthFirstInsertPoint = &head; Own<_::TaskSetImpl> daemons; bool turn(); void setRunnable(bool runnable); void enterScope(); void leaveScope(); friend void _::detach(kj::Promise<void>&& promise); friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, WaitScope& waitScope); friend class _::Event; friend class WaitScope; }; class WaitScope { // Represents a scope in which asynchronous programming can occur. A `WaitScope` should usually // be allocated on the stack and serves two purposes: // * While the `WaitScope` exists, its `EventLoop` is registered as the current loop for the // thread. Most operations dealing with `Promise` (including all of its methods) do not work // unless the thread has a current `EventLoop`. // * `WaitScope` may be passed to `Promise::wait()` to synchronously wait for a particular // promise to complete. See `Promise::wait()` for an extended discussion. public: inline explicit WaitScope(EventLoop& loop): loop(loop) { loop.enterScope(); } inline ~WaitScope() { loop.leaveScope(); } KJ_DISALLOW_COPY(WaitScope); private: EventLoop& loop; friend class EventLoop; friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, WaitScope& waitScope); }; } // namespace kj #include "async-inl.h" #endif // KJ_ASYNC_H_