Chris@63: // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors Chris@63: // Licensed under the MIT License: Chris@63: // Chris@63: // Permission is hereby granted, free of charge, to any person obtaining a copy Chris@63: // of this software and associated documentation files (the "Software"), to deal Chris@63: // in the Software without restriction, including without limitation the rights Chris@63: // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell Chris@63: // copies of the Software, and to permit persons to whom the Software is Chris@63: // furnished to do so, subject to the following conditions: Chris@63: // Chris@63: // The above copyright notice and this permission notice shall be included in Chris@63: // all copies or substantial portions of the Software. Chris@63: // Chris@63: // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR Chris@63: // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, Chris@63: // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE Chris@63: // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER Chris@63: // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, Chris@63: // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN Chris@63: // THE SOFTWARE. Chris@63: Chris@63: #ifndef KJ_ASYNC_H_ Chris@63: #define KJ_ASYNC_H_ Chris@63: Chris@63: #if defined(__GNUC__) && !KJ_HEADER_WARNINGS Chris@63: #pragma GCC system_header Chris@63: #endif Chris@63: Chris@63: #include "async-prelude.h" Chris@63: #include "exception.h" Chris@63: #include "refcount.h" Chris@63: Chris@63: namespace kj { Chris@63: Chris@63: class EventLoop; Chris@63: class WaitScope; Chris@63: Chris@63: template Chris@63: class Promise; Chris@63: template Chris@63: class ForkedPromise; Chris@63: template Chris@63: class PromiseFulfiller; Chris@63: template Chris@63: struct PromiseFulfillerPair; Chris@63: Chris@63: template Chris@63: using PromiseForResult = Promise<_::JoinPromises<_::ReturnType>>; Chris@63: // Evaluates to the type of Promise for the result of calling functor type Func with parameter type Chris@63: // T. If T is void, then the promise is for the result of calling Func with no arguments. If Chris@63: // Func itself returns a promise, the promises are joined, so you never get Promise>. Chris@63: Chris@63: // ======================================================================================= Chris@63: // Promises Chris@63: Chris@63: template Chris@63: class Promise: protected _::PromiseBase { Chris@63: // The basic primitive of asynchronous computation in KJ. Similar to "futures", but designed Chris@63: // specifically for event loop concurrency. Similar to E promises and JavaScript Promises/A. Chris@63: // Chris@63: // A Promise represents a promise to produce a value of type T some time in the future. Once Chris@63: // that value has been produced, the promise is "fulfilled". Alternatively, a promise can be Chris@63: // "broken", with an Exception describing what went wrong. You may implicitly convert a value of Chris@63: // type T to an already-fulfilled Promise. You may implicitly convert the constant Chris@63: // `kj::READY_NOW` to an already-fulfilled Promise. You may also implicitly convert a Chris@63: // `kj::Exception` to an already-broken promise of any type. Chris@63: // Chris@63: // Promises are linear types -- they are moveable but not copyable. If a Promise is destroyed Chris@63: // or goes out of scope (without being moved elsewhere), any ongoing asynchronous operations Chris@63: // meant to fulfill the promise will be canceled if possible. All methods of `Promise` (unless Chris@63: // otherwise noted) actually consume the promise in the sense of move semantics. (Arguably they Chris@63: // should be rvalue-qualified, but at the time this interface was created compilers didn't widely Chris@63: // support that yet and anyway it would be pretty ugly typing kj::mv(promise).whatever().) If Chris@63: // you want to use one Promise in two different places, you must fork it with `fork()`. Chris@63: // Chris@63: // To use the result of a Promise, you must call `then()` and supply a callback function to Chris@63: // call with the result. `then()` returns another promise, for the result of the callback. Chris@63: // Any time that this would result in Promise>, the promises are collapsed into a Chris@63: // simple Promise that first waits for the outer promise, then the inner. Example: Chris@63: // Chris@63: // // Open a remote file, read the content, and then count the Chris@63: // // number of lines of text. Chris@63: // // Note that none of the calls here block. `file`, `content` Chris@63: // // and `lineCount` are all initialized immediately before any Chris@63: // // asynchronous operations occur. The lambda callbacks are Chris@63: // // called later. Chris@63: // Promise> file = openFtp("ftp://host/foo/bar"); Chris@63: // Promise content = file.then( Chris@63: // [](Own file) -> Promise { Chris@63: // return file.readAll(); Chris@63: // }); Chris@63: // Promise lineCount = content.then( Chris@63: // [](String text) -> int { Chris@63: // uint count = 0; Chris@63: // for (char c: text) count += (c == '\n'); Chris@63: // return count; Chris@63: // }); Chris@63: // Chris@63: // For `then()` to work, the current thread must have an active `EventLoop`. Each callback Chris@63: // is scheduled to execute in that loop. Since `then()` schedules callbacks only on the current Chris@63: // thread's event loop, you do not need to worry about two callbacks running at the same time. Chris@63: // You will need to set up at least one `EventLoop` at the top level of your program before you Chris@63: // can use promises. Chris@63: // Chris@63: // To adapt a non-Promise-based asynchronous API to promises, use `newAdaptedPromise()`. Chris@63: // Chris@63: // Systems using promises should consider supporting the concept of "pipelining". Pipelining Chris@63: // means allowing a caller to start issuing method calls against a promised object before the Chris@63: // promise has actually been fulfilled. This is particularly useful if the promise is for a Chris@63: // remote object living across a network, as this can avoid round trips when chaining a series Chris@63: // of calls. It is suggested that any class T which supports pipelining implement a subclass of Chris@63: // Promise which adds "eventual send" methods -- methods which, when called, say "please Chris@63: // invoke the corresponding method on the promised value once it is available". These methods Chris@63: // should in turn return promises for the eventual results of said invocations. Cap'n Proto, Chris@63: // for example, implements the type `RemotePromise` which supports pipelining RPC requests -- see Chris@63: // `capnp/capability.h`. Chris@63: // Chris@63: // KJ Promises are based on E promises: Chris@63: // http://wiki.erights.org/wiki/Walnut/Distributed_Computing#Promises Chris@63: // Chris@63: // KJ Promises are also inspired in part by the evolving standards for JavaScript/ECMAScript Chris@63: // promises, which are themselves influenced by E promises: Chris@63: // http://promisesaplus.com/ Chris@63: // https://github.com/domenic/promises-unwrapping Chris@63: Chris@63: public: Chris@63: Promise(_::FixVoid value); Chris@63: // Construct an already-fulfilled Promise from a value of type T. For non-void promises, the Chris@63: // parameter type is simply T. So, e.g., in a function that returns `Promise`, you can Chris@63: // say `return 123;` to return a promise that is already fulfilled to 123. Chris@63: // Chris@63: // For void promises, use `kj::READY_NOW` as the value, e.g. `return kj::READY_NOW`. Chris@63: Chris@63: Promise(kj::Exception&& e); Chris@63: // Construct an already-broken Promise. Chris@63: Chris@63: inline Promise(decltype(nullptr)) {} Chris@63: Chris@63: template Chris@63: PromiseForResult then(Func&& func, ErrorFunc&& errorHandler = _::PropagateException()) Chris@63: KJ_WARN_UNUSED_RESULT; Chris@63: // Register a continuation function to be executed when the promise completes. The continuation Chris@63: // (`func`) takes the promised value (an rvalue of type `T`) as its parameter. The continuation Chris@63: // may return a new value; `then()` itself returns a promise for the continuation's eventual Chris@63: // result. If the continuation itself returns a `Promise`, then `then()` shall also return Chris@63: // a `Promise` which first waits for the original promise, then executes the continuation, Chris@63: // then waits for the inner promise (i.e. it automatically "unwraps" the promise). Chris@63: // Chris@63: // In all cases, `then()` returns immediately. The continuation is executed later. The Chris@63: // continuation is always executed on the same EventLoop (and, therefore, the same thread) which Chris@63: // called `then()`, therefore no synchronization is necessary on state shared by the continuation Chris@63: // and the surrounding scope. If no EventLoop is running on the current thread, `then()` throws Chris@63: // an exception. Chris@63: // Chris@63: // You may also specify an error handler continuation as the second parameter. `errorHandler` Chris@63: // must be a functor taking a parameter of type `kj::Exception&&`. It must return the same Chris@63: // type as `func` returns (except when `func` returns `Promise`, in which case `errorHandler` Chris@63: // may return either `Promise` or just `U`). The default error handler simply propagates the Chris@63: // exception to the returned promise. Chris@63: // Chris@63: // Either `func` or `errorHandler` may, of course, throw an exception, in which case the promise Chris@63: // is broken. When compiled with -fno-exceptions, the framework will still detect when a Chris@63: // recoverable exception was thrown inside of a continuation and will consider the promise Chris@63: // broken even though a (presumably garbage) result was returned. Chris@63: // Chris@63: // If the returned promise is destroyed before the callback runs, the callback will be canceled Chris@63: // (it will never run). Chris@63: // Chris@63: // Note that `then()` -- like all other Promise methods -- consumes the promise on which it is Chris@63: // called, in the sense of move semantics. After returning, the original promise is no longer Chris@63: // valid, but `then()` returns a new promise. Chris@63: // Chris@63: // *Advanced implementation tips:* Most users will never need to worry about the below, but Chris@63: // it is good to be aware of. Chris@63: // Chris@63: // As an optimization, if the callback function `func` does _not_ return another promise, then Chris@63: // execution of `func` itself may be delayed until its result is known to be needed. The Chris@63: // expectation here is that `func` is just doing some transformation on the results, not Chris@63: // scheduling any other actions, therefore the system doesn't need to be proactive about Chris@63: // evaluating it. This way, a chain of trivial then() transformations can be executed all at Chris@63: // once without repeatedly re-scheduling through the event loop. Use the `eagerlyEvaluate()` Chris@63: // method to suppress this behavior. Chris@63: // Chris@63: // On the other hand, if `func` _does_ return another promise, then the system evaluates `func` Chris@63: // as soon as possible, because the promise it returns might be for a newly-scheduled Chris@63: // long-running asynchronous task. Chris@63: // Chris@63: // As another optimization, when a callback function registered with `then()` is actually Chris@63: // scheduled, it is scheduled to occur immediately, preempting other work in the event queue. Chris@63: // This allows a long chain of `then`s to execute all at once, improving cache locality by Chris@63: // clustering operations on the same data. However, this implies that starvation can occur Chris@63: // if a chain of `then()`s takes a very long time to execute without ever stopping to wait for Chris@63: // actual I/O. To solve this, use `kj::evalLater()` to yield control; this way, all other events Chris@63: // in the queue will get a chance to run before your callback is executed. Chris@63: Chris@63: Promise ignoreResult() KJ_WARN_UNUSED_RESULT { return then([](T&&) {}); } Chris@63: // Convenience method to convert the promise to a void promise by ignoring the return value. Chris@63: // Chris@63: // You must still wait on the returned promise if you want the task to execute. Chris@63: Chris@63: template Chris@63: Promise catch_(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT; Chris@63: // Equivalent to `.then(identityFunc, errorHandler)`, where `identifyFunc` is a function that Chris@63: // just returns its input. Chris@63: Chris@63: T wait(WaitScope& waitScope); Chris@63: // Run the event loop until the promise is fulfilled, then return its result. If the promise Chris@63: // is rejected, throw an exception. Chris@63: // Chris@63: // wait() is primarily useful at the top level of a program -- typically, within the function Chris@63: // that allocated the EventLoop. For example, a program that performs one or two RPCs and then Chris@63: // exits would likely use wait() in its main() function to wait on each RPC. On the other hand, Chris@63: // server-side code generally cannot use wait(), because it has to be able to accept multiple Chris@63: // requests at once. Chris@63: // Chris@63: // If the promise is rejected, `wait()` throws an exception. If the program was compiled without Chris@63: // exceptions (-fno-exceptions), this will usually abort. In this case you really should first Chris@63: // use `then()` to set an appropriate handler for the exception case, so that the promise you Chris@63: // actually wait on never throws. Chris@63: // Chris@63: // `waitScope` is an object proving that the caller is in a scope where wait() is allowed. By Chris@63: // convention, any function which might call wait(), or which might call another function which Chris@63: // might call wait(), must take `WaitScope&` as one of its parameters. This is needed for two Chris@63: // reasons: Chris@63: // * `wait()` is not allowed during an event callback, because event callbacks are themselves Chris@63: // called during some other `wait()`, and such recursive `wait()`s would only be able to Chris@63: // complete in LIFO order, which might mean that the outer `wait()` ends up waiting longer Chris@63: // than it is supposed to. To prevent this, a `WaitScope` cannot be constructed or used during Chris@63: // an event callback. Chris@63: // * Since `wait()` runs the event loop, unrelated event callbacks may execute before `wait()` Chris@63: // returns. This means that anyone calling `wait()` must be reentrant -- state may change Chris@63: // around them in arbitrary ways. Therefore, callers really need to know if a function they Chris@63: // are calling might wait(), and the `WaitScope&` parameter makes this clear. Chris@63: // Chris@63: // TODO(someday): Implement fibers, and let them call wait() even when they are handling an Chris@63: // event. Chris@63: Chris@63: ForkedPromise fork() KJ_WARN_UNUSED_RESULT; Chris@63: // Forks the promise, so that multiple different clients can independently wait on the result. Chris@63: // `T` must be copy-constructable for this to work. Or, in the special case where `T` is Chris@63: // `Own`, `U` must have a method `Own addRef()` which returns a new reference to the same Chris@63: // (or an equivalent) object (probably implemented via reference counting). Chris@63: Chris@63: _::SplitTuplePromise split(); Chris@63: // Split a promise for a tuple into a tuple of promises. Chris@63: // Chris@63: // E.g. if you have `Promise>`, `split()` returns Chris@63: // `kj::Tuple, Promise>`. Chris@63: Chris@63: Promise exclusiveJoin(Promise&& other) KJ_WARN_UNUSED_RESULT; Chris@63: // Return a new promise that resolves when either the original promise resolves or `other` Chris@63: // resolves (whichever comes first). The promise that didn't resolve first is canceled. Chris@63: Chris@63: // TODO(someday): inclusiveJoin(), or perhaps just join(), which waits for both completions Chris@63: // and produces a tuple? Chris@63: Chris@63: template Chris@63: Promise attach(Attachments&&... attachments) KJ_WARN_UNUSED_RESULT; Chris@63: // "Attaches" one or more movable objects (often, Owns) to the promise, such that they will Chris@63: // be destroyed when the promise resolves. This is useful when a promise's callback contains Chris@63: // pointers into some object and you want to make sure the object still exists when the callback Chris@63: // runs -- after calling then(), use attach() to add necessary objects to the result. Chris@63: Chris@63: template Chris@63: Promise eagerlyEvaluate(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT; Chris@63: Promise eagerlyEvaluate(decltype(nullptr)) KJ_WARN_UNUSED_RESULT; Chris@63: // Force eager evaluation of this promise. Use this if you are going to hold on to the promise Chris@63: // for awhile without consuming the result, but you want to make sure that the system actually Chris@63: // processes it. Chris@63: // Chris@63: // `errorHandler` is a function that takes `kj::Exception&&`, like the second parameter to Chris@63: // `then()`, except that it must return void. We make you specify this because otherwise it's Chris@63: // easy to forget to handle errors in a promise that you never use. You may specify nullptr for Chris@63: // the error handler if you are sure that ignoring errors is fine, or if you know that you'll Chris@63: // eventually wait on the promise somewhere. Chris@63: Chris@63: template Chris@63: void detach(ErrorFunc&& errorHandler); Chris@63: // Allows the promise to continue running in the background until it completes or the Chris@63: // `EventLoop` is destroyed. Be careful when using this: since you can no longer cancel this Chris@63: // promise, you need to make sure that the promise owns all the objects it touches or make sure Chris@63: // those objects outlive the EventLoop. Chris@63: // Chris@63: // `errorHandler` is a function that takes `kj::Exception&&`, like the second parameter to Chris@63: // `then()`, except that it must return void. Chris@63: // Chris@63: // This function exists mainly to implement the Cap'n Proto requirement that RPC calls cannot be Chris@63: // canceled unless the callee explicitly permits it. Chris@63: Chris@63: kj::String trace(); Chris@63: // Returns a dump of debug info about this promise. Not for production use. Requires RTTI. Chris@63: // This method does NOT consume the promise as other methods do. Chris@63: Chris@63: private: Chris@63: Promise(bool, Own<_::PromiseNode>&& node): PromiseBase(kj::mv(node)) {} Chris@63: // Second parameter prevent ambiguity with immediate-value constructor. Chris@63: Chris@63: template Chris@63: friend class Promise; Chris@63: friend class EventLoop; Chris@63: template Chris@63: friend Promise newAdaptedPromise(Params&&... adapterConstructorParams); Chris@63: template Chris@63: friend PromiseFulfillerPair newPromiseAndFulfiller(); Chris@63: template Chris@63: friend class _::ForkHub; Chris@63: friend class _::TaskSetImpl; Chris@63: friend Promise _::yield(); Chris@63: friend class _::NeverDone; Chris@63: template Chris@63: friend Promise> joinPromises(Array>&& promises); Chris@63: friend Promise joinPromises(Array>&& promises); Chris@63: }; Chris@63: Chris@63: template Chris@63: class ForkedPromise { Chris@63: // The result of `Promise::fork()` and `EventLoop::fork()`. Allows branches to be created. Chris@63: // Like `Promise`, this is a pass-by-move type. Chris@63: Chris@63: public: Chris@63: inline ForkedPromise(decltype(nullptr)) {} Chris@63: Chris@63: Promise addBranch(); Chris@63: // Add a new branch to the fork. The branch is equivalent to the original promise. Chris@63: Chris@63: private: Chris@63: Own<_::ForkHub<_::FixVoid>> hub; Chris@63: Chris@63: inline ForkedPromise(bool, Own<_::ForkHub<_::FixVoid>>&& hub): hub(kj::mv(hub)) {} Chris@63: Chris@63: friend class Promise; Chris@63: friend class EventLoop; Chris@63: }; Chris@63: Chris@63: constexpr _::Void READY_NOW = _::Void(); Chris@63: // Use this when you need a Promise that is already fulfilled -- this value can be implicitly Chris@63: // cast to `Promise`. Chris@63: Chris@63: constexpr _::NeverDone NEVER_DONE = _::NeverDone(); Chris@63: // The opposite of `READY_NOW`, return this when the promise should never resolve. This can be Chris@63: // implicitly converted to any promise type. You may also call `NEVER_DONE.wait()` to wait Chris@63: // forever (useful for servers). Chris@63: Chris@63: template Chris@63: PromiseForResult evalLater(Func&& func) KJ_WARN_UNUSED_RESULT; Chris@63: // Schedule for the given zero-parameter function to be executed in the event loop at some Chris@63: // point in the near future. Returns a Promise for its result -- or, if `func()` itself returns Chris@63: // a promise, `evalLater()` returns a Promise for the result of resolving that promise. Chris@63: // Chris@63: // Example usage: Chris@63: // Promise x = evalLater([]() { return 123; }); Chris@63: // Chris@63: // The above is exactly equivalent to: Chris@63: // Promise x = Promise(READY_NOW).then([]() { return 123; }); Chris@63: // Chris@63: // If the returned promise is destroyed before the callback runs, the callback will be canceled Chris@63: // (never called). Chris@63: // Chris@63: // If you schedule several evaluations with `evalLater` during the same callback, they are Chris@63: // guaranteed to be executed in order. Chris@63: Chris@63: template Chris@63: PromiseForResult evalNow(Func&& func) KJ_WARN_UNUSED_RESULT; Chris@63: // Run `func()` and return a promise for its result. `func()` executes before `evalNow()` returns. Chris@63: // If `func()` throws an exception, the exception is caught and wrapped in a promise -- this is the Chris@63: // main reason why `evalNow()` is useful. Chris@63: Chris@63: template Chris@63: Promise> joinPromises(Array>&& promises); Chris@63: // Join an array of promises into a promise for an array. Chris@63: Chris@63: // ======================================================================================= Chris@63: // Hack for creating a lambda that holds an owned pointer. Chris@63: Chris@63: template Chris@63: class CaptureByMove { Chris@63: public: Chris@63: inline CaptureByMove(Func&& func, MovedParam&& param) Chris@63: : func(kj::mv(func)), param(kj::mv(param)) {} Chris@63: Chris@63: template Chris@63: inline auto operator()(Params&&... params) Chris@63: -> decltype(kj::instance()(kj::instance(), kj::fwd(params)...)) { Chris@63: return func(kj::mv(param), kj::fwd(params)...); Chris@63: } Chris@63: Chris@63: private: Chris@63: Func func; Chris@63: MovedParam param; Chris@63: }; Chris@63: Chris@63: template Chris@63: inline CaptureByMove> mvCapture(MovedParam&& param, Func&& func) { Chris@63: // Hack to create a "lambda" which captures a variable by moving it rather than copying or Chris@63: // referencing. C++14 generalized captures should make this obsolete, but for now in C++11 this Chris@63: // is commonly needed for Promise continuations that own their state. Example usage: Chris@63: // Chris@63: // Own ptr = makeFoo(); Chris@63: // Promise promise = callRpc(); Chris@63: // promise.then(mvCapture(ptr, [](Own&& ptr, int result) { Chris@63: // return ptr->finish(result); Chris@63: // })); Chris@63: Chris@63: return CaptureByMove>(kj::fwd(func), kj::mv(param)); Chris@63: } Chris@63: Chris@63: // ======================================================================================= Chris@63: // Advanced promise construction Chris@63: Chris@63: template Chris@63: class PromiseFulfiller { Chris@63: // A callback which can be used to fulfill a promise. Only the first call to fulfill() or Chris@63: // reject() matters; subsequent calls are ignored. Chris@63: Chris@63: public: Chris@63: virtual void fulfill(T&& value) = 0; Chris@63: // Fulfill the promise with the given value. Chris@63: Chris@63: virtual void reject(Exception&& exception) = 0; Chris@63: // Reject the promise with an error. Chris@63: Chris@63: virtual bool isWaiting() = 0; Chris@63: // Returns true if the promise is still unfulfilled and someone is potentially waiting for it. Chris@63: // Returns false if fulfill()/reject() has already been called *or* if the promise to be Chris@63: // fulfilled has been discarded and therefore the result will never be used anyway. Chris@63: Chris@63: template Chris@63: bool rejectIfThrows(Func&& func); Chris@63: // Call the function (with no arguments) and return true. If an exception is thrown, call Chris@63: // `fulfiller.reject()` and then return false. When compiled with exceptions disabled, Chris@63: // non-fatal exceptions are still detected and handled correctly. Chris@63: }; Chris@63: Chris@63: template <> Chris@63: class PromiseFulfiller { Chris@63: // Specialization of PromiseFulfiller for void promises. See PromiseFulfiller. Chris@63: Chris@63: public: Chris@63: virtual void fulfill(_::Void&& value = _::Void()) = 0; Chris@63: // Call with zero parameters. The parameter is a dummy that only exists so that subclasses don't Chris@63: // have to specialize for . Chris@63: Chris@63: virtual void reject(Exception&& exception) = 0; Chris@63: virtual bool isWaiting() = 0; Chris@63: Chris@63: template Chris@63: bool rejectIfThrows(Func&& func); Chris@63: }; Chris@63: Chris@63: template Chris@63: Promise newAdaptedPromise(Params&&... adapterConstructorParams); Chris@63: // Creates a new promise which owns an instance of `Adapter` which encapsulates the operation Chris@63: // that will eventually fulfill the promise. This is primarily useful for adapting non-KJ Chris@63: // asynchronous APIs to use promises. Chris@63: // Chris@63: // An instance of `Adapter` will be allocated and owned by the returned `Promise`. A Chris@63: // `PromiseFulfiller&` will be passed as the first parameter to the adapter's constructor, Chris@63: // and `adapterConstructorParams` will be forwarded as the subsequent parameters. The adapter Chris@63: // is expected to perform some asynchronous operation and call the `PromiseFulfiller` once Chris@63: // it is finished. Chris@63: // Chris@63: // The adapter is destroyed when its owning Promise is destroyed. This may occur before the Chris@63: // Promise has been fulfilled. In this case, the adapter's destructor should cancel the Chris@63: // asynchronous operation. Once the adapter is destroyed, the fulfillment callback cannot be Chris@63: // called. Chris@63: // Chris@63: // An adapter implementation should be carefully written to ensure that it cannot accidentally Chris@63: // be left unfulfilled permanently because of an exception. Consider making liberal use of Chris@63: // `PromiseFulfiller::rejectIfThrows()`. Chris@63: Chris@63: template Chris@63: struct PromiseFulfillerPair { Chris@63: Promise<_::JoinPromises> promise; Chris@63: Own> fulfiller; Chris@63: }; Chris@63: Chris@63: template Chris@63: PromiseFulfillerPair newPromiseAndFulfiller(); Chris@63: // Construct a Promise and a separate PromiseFulfiller which can be used to fulfill the promise. Chris@63: // If the PromiseFulfiller is destroyed before either of its methods are called, the Promise is Chris@63: // implicitly rejected. Chris@63: // Chris@63: // Although this function is easier to use than `newAdaptedPromise()`, it has the serious drawback Chris@63: // that there is no way to handle cancellation (i.e. detect when the Promise is discarded). Chris@63: // Chris@63: // You can arrange to fulfill a promise with another promise by using a promise type for T. E.g. Chris@63: // `newPromiseAndFulfiller>()` will produce a promise of type `Promise` but the Chris@63: // fulfiller will be of type `PromiseFulfiller>`. Thus you pass a `Promise` to the Chris@63: // `fulfill()` callback, and the promises are chained. Chris@63: Chris@63: // ======================================================================================= Chris@63: // TaskSet Chris@63: Chris@63: class TaskSet { Chris@63: // Holds a collection of Promises and ensures that each executes to completion. Memory Chris@63: // associated with each promise is automatically freed when the promise completes. Destroying Chris@63: // the TaskSet itself automatically cancels all unfinished promises. Chris@63: // Chris@63: // This is useful for "daemon" objects that perform background tasks which aren't intended to Chris@63: // fulfill any particular external promise, but which may need to be canceled (and thus can't Chris@63: // use `Promise::detach()`). The daemon object holds a TaskSet to collect these tasks it is Chris@63: // working on. This way, if the daemon itself is destroyed, the TaskSet is detroyed as well, Chris@63: // and everything the daemon is doing is canceled. Chris@63: Chris@63: public: Chris@63: class ErrorHandler { Chris@63: public: Chris@63: virtual void taskFailed(kj::Exception&& exception) = 0; Chris@63: }; Chris@63: Chris@63: TaskSet(ErrorHandler& errorHandler); Chris@63: // `loop` will be used to wait on promises. `errorHandler` will be executed any time a task Chris@63: // throws an exception, and will execute within the given EventLoop. Chris@63: Chris@63: ~TaskSet() noexcept(false); Chris@63: Chris@63: void add(Promise&& promise); Chris@63: Chris@63: kj::String trace(); Chris@63: // Return debug info about all promises currently in the TaskSet. Chris@63: Chris@63: private: Chris@63: Own<_::TaskSetImpl> impl; Chris@63: }; Chris@63: Chris@63: // ======================================================================================= Chris@63: // The EventLoop class Chris@63: Chris@63: class EventPort { Chris@63: // Interfaces between an `EventLoop` and events originating from outside of the loop's thread. Chris@63: // All such events come in through the `EventPort` implementation. Chris@63: // Chris@63: // An `EventPort` implementation may interface with low-level operating system APIs and/or other Chris@63: // threads. You can also write an `EventPort` which wraps some other (non-KJ) event loop Chris@63: // framework, allowing the two to coexist in a single thread. Chris@63: Chris@63: public: Chris@63: virtual bool wait() = 0; Chris@63: // Wait for an external event to arrive, sleeping if necessary. Once at least one event has Chris@63: // arrived, queue it to the event loop (e.g. by fulfilling a promise) and return. Chris@63: // Chris@63: // This is called during `Promise::wait()` whenever the event queue becomes empty, in order to Chris@63: // wait for new events to populate the queue. Chris@63: // Chris@63: // It is safe to return even if nothing has actually been queued, so long as calling `wait()` in Chris@63: // a loop will eventually sleep. (That is to say, false positives are fine.) Chris@63: // Chris@63: // Returns true if wake() has been called from another thread. (Precisely, returns true if Chris@63: // no previous call to wait `wait()` nor `poll()` has returned true since `wake()` was last Chris@63: // called.) Chris@63: Chris@63: virtual bool poll() = 0; Chris@63: // Check if any external events have arrived, but do not sleep. If any events have arrived, Chris@63: // add them to the event queue (e.g. by fulfilling promises) before returning. Chris@63: // Chris@63: // This may be called during `Promise::wait()` when the EventLoop has been executing for a while Chris@63: // without a break but is still non-empty. Chris@63: // Chris@63: // Returns true if wake() has been called from another thread. (Precisely, returns true if Chris@63: // no previous call to wait `wait()` nor `poll()` has returned true since `wake()` was last Chris@63: // called.) Chris@63: Chris@63: virtual void setRunnable(bool runnable); Chris@63: // Called to notify the `EventPort` when the `EventLoop` has work to do; specifically when it Chris@63: // transitions from empty -> runnable or runnable -> empty. This is typically useful when Chris@63: // integrating with an external event loop; if the loop is currently runnable then you should Chris@63: // arrange to call run() on it soon. The default implementation does nothing. Chris@63: Chris@63: virtual void wake() const; Chris@63: // Wake up the EventPort's thread from another thread. Chris@63: // Chris@63: // Unlike all other methods on this interface, `wake()` may be called from another thread, hence Chris@63: // it is `const`. Chris@63: // Chris@63: // Technically speaking, `wake()` causes the target thread to cease sleeping and not to sleep Chris@63: // again until `wait()` or `poll()` has returned true at least once. Chris@63: // Chris@63: // The default implementation throws an UNIMPLEMENTED exception. Chris@63: }; Chris@63: Chris@63: class EventLoop { Chris@63: // Represents a queue of events being executed in a loop. Most code won't interact with Chris@63: // EventLoop directly, but instead use `Promise`s to interact with it indirectly. See the Chris@63: // documentation for `Promise`. Chris@63: // Chris@63: // Each thread can have at most one current EventLoop. To make an `EventLoop` current for Chris@63: // the thread, create a `WaitScope`. Async APIs require that the thread has a current EventLoop, Chris@63: // or they will throw exceptions. APIs that use `Promise::wait()` additionally must explicitly Chris@63: // be passed a reference to the `WaitScope` to make the caller aware that they might block. Chris@63: // Chris@63: // Generally, you will want to construct an `EventLoop` at the top level of your program, e.g. Chris@63: // in the main() function, or in the start function of a thread. You can then use it to Chris@63: // construct some promises and wait on the result. Example: Chris@63: // Chris@63: // int main() { Chris@63: // // `loop` becomes the official EventLoop for the thread. Chris@63: // MyEventPort eventPort; Chris@63: // EventLoop loop(eventPort); Chris@63: // Chris@63: // // Now we can call an async function. Chris@63: // Promise textPromise = getHttp("http://example.com"); Chris@63: // Chris@63: // // And we can wait for the promise to complete. Note that you can only use `wait()` Chris@63: // // from the top level, not from inside a promise callback. Chris@63: // String text = textPromise.wait(); Chris@63: // print(text); Chris@63: // return 0; Chris@63: // } Chris@63: // Chris@63: // Most applications that do I/O will prefer to use `setupAsyncIo()` from `async-io.h` rather Chris@63: // than allocate an `EventLoop` directly. Chris@63: Chris@63: public: Chris@63: EventLoop(); Chris@63: // Construct an `EventLoop` which does not receive external events at all. Chris@63: Chris@63: explicit EventLoop(EventPort& port); Chris@63: // Construct an `EventLoop` which receives external events through the given `EventPort`. Chris@63: Chris@63: ~EventLoop() noexcept(false); Chris@63: Chris@63: void run(uint maxTurnCount = maxValue); Chris@63: // Run the event loop for `maxTurnCount` turns or until there is nothing left to be done, Chris@63: // whichever comes first. This never calls the `EventPort`'s `sleep()` or `poll()`. It will Chris@63: // call the `EventPort`'s `setRunnable(false)` if the queue becomes empty. Chris@63: Chris@63: bool isRunnable(); Chris@63: // Returns true if run() would currently do anything, or false if the queue is empty. Chris@63: Chris@63: private: Chris@63: EventPort& port; Chris@63: Chris@63: bool running = false; Chris@63: // True while looping -- wait() is then not allowed. Chris@63: Chris@63: bool lastRunnableState = false; Chris@63: // What did we last pass to port.setRunnable()? Chris@63: Chris@63: _::Event* head = nullptr; Chris@63: _::Event** tail = &head; Chris@63: _::Event** depthFirstInsertPoint = &head; Chris@63: Chris@63: Own<_::TaskSetImpl> daemons; Chris@63: Chris@63: bool turn(); Chris@63: void setRunnable(bool runnable); Chris@63: void enterScope(); Chris@63: void leaveScope(); Chris@63: Chris@63: friend void _::detach(kj::Promise&& promise); Chris@63: friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, Chris@63: WaitScope& waitScope); Chris@63: friend class _::Event; Chris@63: friend class WaitScope; Chris@63: }; Chris@63: Chris@63: class WaitScope { Chris@63: // Represents a scope in which asynchronous programming can occur. A `WaitScope` should usually Chris@63: // be allocated on the stack and serves two purposes: Chris@63: // * While the `WaitScope` exists, its `EventLoop` is registered as the current loop for the Chris@63: // thread. Most operations dealing with `Promise` (including all of its methods) do not work Chris@63: // unless the thread has a current `EventLoop`. Chris@63: // * `WaitScope` may be passed to `Promise::wait()` to synchronously wait for a particular Chris@63: // promise to complete. See `Promise::wait()` for an extended discussion. Chris@63: Chris@63: public: Chris@63: inline explicit WaitScope(EventLoop& loop): loop(loop) { loop.enterScope(); } Chris@63: inline ~WaitScope() { loop.leaveScope(); } Chris@63: KJ_DISALLOW_COPY(WaitScope); Chris@63: Chris@63: private: Chris@63: EventLoop& loop; Chris@63: friend class EventLoop; Chris@63: friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, Chris@63: WaitScope& waitScope); Chris@63: }; Chris@63: Chris@63: } // namespace kj Chris@63: Chris@63: #include "async-inl.h" Chris@63: Chris@63: #endif // KJ_ASYNC_H_