capnproto

async.h (57525B)

---

 // 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.
 
 #pragma once
 
 #include "async-prelude.h"
 #include "exception.h"
 #include "refcount.h"
 
 KJ_BEGIN_HEADER
 
 #ifndef KJ_USE_FIBERS
   #if __BIONIC__ || __FreeBSD__ || __OpenBSD__ || KJ_NO_EXCEPTIONS
     // These platforms don't support fibers.
     #define KJ_USE_FIBERS 0
   #else
     #define KJ_USE_FIBERS 1
   #endif
 #else
   #if KJ_NO_EXCEPTIONS && KJ_USE_FIBERS
     #error "Fibers cannot be enabled when exceptions are disabled."
   #endif
 #endif
 
 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>
 class FunctionParam;
 
 template <typename Func, typename T>
 using PromiseForResult = _::ReducePromises<_::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.
 
   Promise<void> ignoreResult() KJ_WARN_UNUSED_RESULT { return then([](T&&) {}); }
   // Convenience method to convert the promise to a void promise by ignoring the return value.
   //
   // You must still wait on the returned promise if you want the task to execute.
 
   template <typename ErrorFunc>
   Promise<T> catch_(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT;
   // Equivalent to `.then(identityFunc, errorHandler)`, where `identifyFunc` is a function that
   // just returns its input.
 
   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.
   //
   // Usually, there is only one `WaitScope` for each `EventLoop`, and it can only be used at the
   // top level of the thread owning the loop. Calling `wait()` with this `WaitScope` is what
   // actually causes the event loop to run at all. This top-level `WaitScope` cannot be used
   // recursively, so cannot be used within an event callback.
   //
   // However, it is possible to obtain a `WaitScope` in lower-level code by using fibers. Use
   // kj::startFiber() to start some code executing on an alternate call stack. That code will get
   // its own `WaitScope` allowing it to operate in a synchronous style. In this case, `wait()`
   // switches back to the main stack in order to run the event loop, returning to the fiber's stack
   // once the awaited promise resolves.
 
   bool poll(WaitScope& waitScope);
   // Returns true if a call to wait() would complete without blocking, false if it would block.
   //
   // If the promise is not yet resolved, poll() will pump the event loop and poll for I/O in an
   // attempt to resolve it. Only when there is nothing left to do will it return false.
   //
   // Generally, poll() is most useful in tests. Often, you may want to verify that a promise does
   // not resolve until some specific event occurs. To do so, poll() the promise before the event to
   // verify it isn't resolved, then trigger the event, then poll() again to verify that it resolves.
   // The first poll() verifies that the promise doesn't resolve early, which would otherwise be
   // hard to do deterministically. The second poll() allows you to check that the promise has
   // resolved and avoid a wait() that might deadlock in the case that it hasn't.
   //
   // poll() is not supported in fibers; it will throw an exception.
 
   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).
 
   _::SplitTuplePromise<T> split();
   // Split a promise for a tuple into a tuple of promises.
   //
   // E.g. if you have `Promise<kj::Tuple<T, U>>`, `split()` returns
   // `kj::Tuple<Promise<T>, Promise<U>>`.
 
   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()`, or the parameter to `catch_()`.  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.
 
   friend class _::PromiseNode;
 };
 
 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.
 
   bool hasBranches();
   // Returns true if there are any branches that haven't been canceled.
 
 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) KJ_WARN_UNUSED_RESULT;
 // 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 Func>
 PromiseForResult<Func, void> evalNow(Func&& func) KJ_WARN_UNUSED_RESULT;
 // Run `func()` and return a promise for its result. `func()` executes before `evalNow()` returns.
 // If `func()` throws an exception, the exception is caught and wrapped in a promise -- this is the
 // main reason why `evalNow()` is useful.
 
 template <typename Func>
 PromiseForResult<Func, void> evalLast(Func&& func) KJ_WARN_UNUSED_RESULT;
 // Like `evalLater()`, except that the function doesn't run until the event queue is otherwise
 // completely empty and the thread is about to suspend waiting for I/O.
 //
 // This is useful when you need to perform some disruptive action and you want to make sure that
 // you don't interrupt some other task between two .then() continuations. For example, say you want
 // to cancel a read() operation on a socket and know for sure that if any bytes were read, you saw
 // them. It could be that a read() has completed and bytes have been transferred to the target
 // buffer, but the .then() callback that handles the read result hasn't executed yet. If you
 // cancel the promise at this inopportune moment, the bytes in the buffer are lost. If you do
 // evalLast(), then you can be sure that any pending .then() callbacks had a chance to finish out
 // and if you didn't receive the read result yet, then you know nothing has been read, and you can
 // simply drop the promise.
 //
 // If evalLast() is called multiple times, functions are executed in LIFO order. If the first
 // callback enqueues new events, then latter callbacks will not execute until those events are
 // drained.
 
 ArrayPtr<void* const> getAsyncTrace(ArrayPtr<void*> space);
 kj::String getAsyncTrace();
 // If the event loop is currently running in this thread, get a trace back through the promise
 // chain leading to the currently-executing event. The format is the same as kj::getStackTrace()
 // from exception.c++.
 
 template <typename Func>
 PromiseForResult<Func, void> retryOnDisconnect(Func&& func) KJ_WARN_UNUSED_RESULT;
 // Promises to run `func()` asynchronously, retrying once if it fails with a DISCONNECTED exception.
 // If the retry also fails, the exception is passed through.
 //
 // `func()` should return a `Promise`. `retryOnDisconnect(func)` returns the same promise, except
 // with the retry logic added.
 
 template <typename Func>
 PromiseForResult<Func, WaitScope&> startFiber(size_t stackSize, Func&& func) KJ_WARN_UNUSED_RESULT;
 // Executes `func()` in a fiber, returning a promise for the eventual reseult. `func()` will be
 // passed a `WaitScope&` as its parameter, allowing it to call `.wait()` on promises. Thus, `func()`
 // can be written in a synchronous, blocking style, instead of using `.then()`. This is often much
 // easier to write and read, and may even be significantly faster if it allows the use of stack
 // allocation rather than heap allocation.
 //
 // However, fibers have a major disadvantage: memory must be allocated for the fiber's call stack.
 // The entire stack must be allocated at once, making it necessary to choose a stack size upfront
 // that is big enough for whatever the fiber needs to do. Estimating this is often difficult. That
 // said, over-estimating is not too terrible since pages of the stack will actually be allocated
 // lazily when first accessed; actual memory usage will correspond to the "high watermark" of the
 // actual stack usage. That said, this lazy allocation forces page faults, which can be quite slow.
 // Worse, freeing a stack forces a TLB flush and shootdown -- all currently-executing threads will
 // have to be interrupted to flush their CPU cores' TLB caches.
 //
 // In short, when performance matters, you should try to avoid creating fibers very frequently.
 
 class FiberPool final {
   // A freelist pool of fibers with a set stack size. This improves CPU usage with fibers at
   // the expense of memory usage. Fibers in this pool will always use the max amount of memory
   // used until the pool is destroyed.
 
 public:
   explicit FiberPool(size_t stackSize);
   ~FiberPool() noexcept(false);
   KJ_DISALLOW_COPY(FiberPool);
 
   void setMaxFreelist(size_t count);
   // Set the maximum number of stacks to add to the freelist. If the freelist is full, stacks will
   // be deleted rather than returned to the freelist.
 
   void useCoreLocalFreelists();
   // EXPERIMENTAL: Call to tell FiberPool to try to use core-local stack freelists, which
   //   in theory should increase L1/L2 cache efficacy for freelisted stacks. In practice, as of
   //   this writing, no performance advantage has yet been demonstrated. Note that currently this
   //   feature is only supported on Linux (the flag has no effect on other operating systems).
 
   template <typename Func>
   PromiseForResult<Func, WaitScope&> startFiber(Func&& func) const KJ_WARN_UNUSED_RESULT;
   // Executes `func()` in a fiber from this pool, returning a promise for the eventual result.
   // `func()` will be passed a `WaitScope&` as its parameter, allowing it to call `.wait()` on
   // promises. Thus, `func()` can be written in a synchronous, blocking style, instead of
   // using `.then()`. This is often much easier to write and read, and may even be significantly
   // faster if it allows the use of stack allocation rather than heap allocation.
 
   void runSynchronously(kj::FunctionParam<void()> func) const;
   // Use one of the stacks in the pool to synchronously execute func(), returning the result that
   // func() returns. This is not the usual use case for fibers, but can be a nice optimization
   // in programs that have many threads that mostly only need small stacks, but occasionally need
   // a much bigger stack to run some deeply recursive algorithm. If the algorithm is run on each
   // thread's normal call stack, then every thread's stack will tend to grow to be very big
   // (usually, stacks automatically grow as needed, but do not shrink until the thread exits
   // completely). If the thread can share a small set of big stacks that they use only when calling
   // the deeply recursive algorithm, and use small stacks for everything else, overall memory usage
   // is reduced.
   //
   // TODO(someday): If func() returns a value, return it from runSynchronously? Current use case
   //   doesn't need it.
 
   size_t getFreelistSize() const;
   // Get the number of stacks currently in the freelist. Does not count stacks that are active.
 
 private:
   class Impl;
   Own<Impl> impl;
 
   friend class _::FiberStack;
   friend class _::FiberBase;
 };
 
 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
 
 class PromiseRejector {
   // Superclass of PromiseFulfiller containing the non-typed methods. Useful when you only really
   // need to be able to reject a promise, and you need to operate on fulfillers of different types.
 public:
   virtual void reject(Exception&& exception) = 0;
   virtual bool isWaiting() = 0;
 };
 
 template <typename T>
 class PromiseFulfiller: public PromiseRejector {
   // 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>: public PromiseRejector {
   // 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>
 _::ReducePromises<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 {
   _::ReducePromises<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.
 
 template <typename T>
 class CrossThreadPromiseFulfiller: public kj::PromiseFulfiller<T> {
   // Like PromiseFulfiller<T> but the methods are `const`, indicating they can safely be called
   // from another thread.
 
 public:
   virtual void fulfill(T&& value) const = 0;
   virtual void reject(Exception&& exception) const = 0;
   virtual bool isWaiting() const = 0;
 
   void fulfill(T&& value) override { return constThis()->fulfill(kj::fwd<T>(value)); }
   void reject(Exception&& exception) override { return constThis()->reject(kj::mv(exception)); }
   bool isWaiting() override { return constThis()->isWaiting(); }
 
 private:
   const CrossThreadPromiseFulfiller* constThis() { return this; }
 };
 
 template <>
 class CrossThreadPromiseFulfiller<void>: public kj::PromiseFulfiller<void> {
   // Specialization of CrossThreadPromiseFulfiller for void promises.  See
   // CrossThreadPromiseFulfiller<T>.
 
 public:
   virtual void fulfill(_::Void&& value = _::Void()) const = 0;
   virtual void reject(Exception&& exception) const = 0;
   virtual bool isWaiting() const = 0;
 
   void fulfill(_::Void&& value) override { return constThis()->fulfill(kj::mv(value)); }
   void reject(Exception&& exception) override { return constThis()->reject(kj::mv(exception)); }
   bool isWaiting() override { return constThis()->isWaiting(); }
 
 private:
   const CrossThreadPromiseFulfiller* constThis() { return this; }
 };
 
 template <typename T>
 struct PromiseCrossThreadFulfillerPair {
   _::ReducePromises<T> promise;
   Own<CrossThreadPromiseFulfiller<T>> fulfiller;
 };
 
 template <typename T>
 PromiseCrossThreadFulfillerPair<T> newPromiseAndCrossThreadFulfiller();
 // Like `newPromiseAndFulfiller()`, but the fulfiller is allowed to be invoked from any thread,
 // not just the one that called this method. Note that the Promise is still tied to the calling
 // thread's event loop and *cannot* be used from another thread -- only the PromiseFulfiller is
 // cross-thread.
 
 // =======================================================================================
 // Canceler
 
 class Canceler {
   // A Canceler can wrap some set of Promises and then forcefully cancel them on-demand, or
   // implicitly when the Canceler is destroyed.
   //
   // The cancellation is done in such a way that once cancel() (or the Canceler's destructor)
   // returns, it's guaranteed that the promise has already been canceled and destroyed. This
   // guarantee is important for enforcing ownership constraints. For example, imagine that Alice
   // calls a method on Bob that returns a Promise. That Promise encapsulates a task that uses Bob's
   // internal state. But, imagine that Alice does not own Bob, and indeed Bob might be destroyed
   // at random without Alice having canceled the promise. In this case, it is necessary for Bob to
   // ensure that the promise will be forcefully canceled. Bob can do this by constructing a
   // Canceler and using it to wrap promises before returning them to callers. When Bob is
   // destroyed, the Canceler is destroyed too, and all promises Bob wrapped with it throw errors.
   //
   // Note that another common strategy for cancelation is to use exclusiveJoin() to join a promise
   // with some "cancellation promise" which only resolves if the operation should be canceled. The
   // cancellation promise could itself be created by newPromiseAndFulfiller<void>(), and thus
   // calling the PromiseFulfiller cancels the operation. There is a major problem with this
   // approach: upon invoking the fulfiller, an arbitrary amount of time may pass before the
   // exclusive-joined promise actually resolves and cancels its other fork. During that time, the
   // task might continue to execute. If it holds pointers to objects that have been destroyed, this
   // might cause segfaults. Thus, it is safer to use a Canceler.
 
 public:
   inline Canceler() {}
   ~Canceler() noexcept(false);
   KJ_DISALLOW_COPY(Canceler);
 
   template <typename T>
   Promise<T> wrap(Promise<T> promise) {
     return newAdaptedPromise<T, AdapterImpl<T>>(*this, kj::mv(promise));
   }
 
   void cancel(StringPtr cancelReason);
   void cancel(const Exception& exception);
   // Cancel all previously-wrapped promises that have not already completed, causing them to throw
   // the given exception. If you provide just a description message instead of an exception, then
   // an exception object will be constructed from it -- but only if there are requests to cancel.
 
   void release();
   // Releases previously-wrapped promises, so that they will not be canceled regardless of what
   // happens to this Canceler.
 
   bool isEmpty() const { return list == nullptr; }
   // Indicates if any previously-wrapped promises are still executing. (If this returns true, then
   // cancel() would be a no-op.)
 
 private:
   class AdapterBase {
   public:
     AdapterBase(Canceler& canceler);
     ~AdapterBase() noexcept(false);
 
     virtual void cancel(Exception&& e) = 0;
 
     void unlink();
 
   private:
     Maybe<Maybe<AdapterBase&>&> prev;
     Maybe<AdapterBase&> next;
     friend class Canceler;
   };
 
   template <typename T>
   class AdapterImpl: public AdapterBase {
   public:
     AdapterImpl(PromiseFulfiller<T>& fulfiller,
                 Canceler& canceler, Promise<T> inner)
         : AdapterBase(canceler),
           fulfiller(fulfiller),
           inner(inner.then(
               [&fulfiller](T&& value) { fulfiller.fulfill(kj::mv(value)); },
               [&fulfiller](Exception&& e) { fulfiller.reject(kj::mv(e)); })
               .eagerlyEvaluate(nullptr)) {}
 
     void cancel(Exception&& e) override {
       fulfiller.reject(kj::mv(e));
       inner = nullptr;
     }
 
   private:
     PromiseFulfiller<T>& fulfiller;
     Promise<void> inner;
   };
 
   Maybe<AdapterBase&> list;
 };
 
 template <>
 class Canceler::AdapterImpl<void>: public AdapterBase {
 public:
   AdapterImpl(kj::PromiseFulfiller<void>& fulfiller,
               Canceler& canceler, kj::Promise<void> inner);
   void cancel(kj::Exception&& e) override;
   // These must be defined in async.c++ to prevent translation units compiled by MSVC from trying to
   // link with symbols defined in async.c++ merely because they included async.h.
 
 private:
   kj::PromiseFulfiller<void>& fulfiller;
   kj::Promise<void> inner;
 };
 
 // =======================================================================================
 // 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);
   // `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.
 
   bool isEmpty() { return tasks == nullptr; }
   // Check if any tasks are running.
 
   Promise<void> onEmpty();
   // Returns a promise that fulfills the next time the TaskSet is empty. Only one such promise can
   // exist at a time.
 
 private:
   class Task;
 
   TaskSet::ErrorHandler& errorHandler;
   Maybe<Own<Task>> tasks;
   Maybe<Own<PromiseFulfiller<void>>> emptyFulfiller;
 };
 
 // =======================================================================================
 // Cross-thread execution.
 
 class Executor {
   // Executes code on another thread's event loop.
   //
   // Use `kj::getCurrentThreadExecutor()` to get an executor that schedules calls on the current
   // thread's event loop. You may then pass the reference to other threads to enable them to call
   // back to this one.
 
 public:
   Executor(EventLoop& loop, Badge<EventLoop>);
   ~Executor() noexcept(false);
 
   virtual kj::Own<const Executor> addRef() const = 0;
   // Add a reference to this Executor. The Executor will not be destroyed until all references are
   // dropped. This uses atomic refcounting for thread-safety.
   //
   // Use this when you can't guarantee that the target thread's event loop won't concurrently exit
   // (including due to an uncaught exception!) while another thread is still using the Executor.
   // Otherwise, the Executor object is destroyed when the owning event loop exits.
   //
   // If the target event loop has exited, then `execute{Async,Sync}` will throw DISCONNECTED
   // exceptions.
 
   bool isLive() const;
   // Returns true if the remote event loop still exists, false if it has been destroyed. In the
   // latter case, `execute{Async,Sync}()` will definitely throw. Of course, if this returns true,
   // it could still change to false at any moment, and `execute{Async,Sync}()` could still throw as
   // a result.
   //
   // TODO(cleanup): Should we have tryExecute{Async,Sync}() that return Maybes that are null if
   //   the remote event loop exited? Currently there are multiple known use cases that check
   //   isLive() after catching a DISCONNECTED exception to decide whether it is due to the executor
   //   exiting, and then handling that case. This is borderline in violation of KJ exception
   //   philosophy, but right now I'm not excited about the extra template metaprogramming needed
   //   for "try" versions...
 
   template <typename Func>
   PromiseForResult<Func, void> executeAsync(Func&& func) const;
   // Call from any thread to request that the given function be executed on the executor's thread,
   // returning a promise for the result.
   //
   // The Promise returned by executeAsync() belongs to the requesting thread, not the executor
   // thread. Hence, for example, continuations added to this promise with .then() will execute in
   // the requesting thread.
   //
   // If func() itself returns a Promise, that Promise is *not* returned verbatim to the requesting
   // thread -- after all, Promise objects cannot be used cross-thread. Instead, the executor thread
   // awaits the promise. Once it resolves to a final result, that result is transferred to the
   // requesting thread, resolving the promise that executeAsync() returned earlier.
   //
   // `func` will be destroyed in the requesting thread, after the final result has been returned
   // from the executor thread. This means that it is safe for `func` to capture objects that cannot
   // safely be destroyed from another thread. It is also safe for `func` to be an lvalue reference,
   // so long as the functor remains live until the promise completes or is canceled, and the
   // function is thread-safe.
   //
   // Of course, the body of `func` must be careful that any access it makes on these objects is
   // safe cross-thread. For example, it must not attempt to access Promise-related objects
   // cross-thread; you cannot create a `PromiseFulfiller` in one thread and then `fulfill()` it
   // from another. Unfortunately, the usual convention of using const-correctness to enforce
   // thread-safety does not work here, because applications can often ensure that `func` has
   // exclusive access to captured objects, and thus can safely mutate them even in non-thread-safe
   // ways; the const qualifier is not sufficient to express this.
   //
   // The final return value of `func` is transferred between threads, and hence is constructed and
   // destroyed in separate threads. It is the app's responsibility to make sure this is OK.
   // Alternatively, the app can perhaps arrange to send the return value back to the original
   // thread for destruction, if needed.
   //
   // If the requesting thread destroys the returned Promise, the destructor will block waiting for
   // the executor thread to acknowledge cancellation. This ensures that `func` can be destroyed
   // before the Promise's destructor returns.
   //
   // Multiple calls to executeAsync() from the same requesting thread to the same target thread
   // will be delivered in the same order in which they were requested. (However, if func() returns
   // a promise, delivery of subsequent calls is not blocked on that promise. In other words, this
   // call provides E-Order in the same way as Cap'n Proto.)
 
   template <typename Func>
   _::UnwrapPromise<PromiseForResult<Func, void>> executeSync(Func&& func) const;
   // Schedules `func()` to execute on the executor thread, and then blocks the requesting thread
   // until `func()` completes. If `func()` returns a Promise, then the wait will continue until
   // that promise resolves, and the final result will be returned to the requesting thread.
   //
   // The requesting thread does not need to have an EventLoop. If it does have an EventLoop, that
   // loop will *not* execute while the thread is blocked. This method is particularly useful to
   // allow non-event-loop threads to perform I/O via a separate event-loop thread.
   //
   // As with `executeAsync()`, `func` is always destroyed on the requesting thread, after the
   // executor thread has signaled completion. The return value is transferred between threads.
 
 private:
   struct Impl;
   Own<Impl> impl;
   // To avoid including mutex.h...
 
   friend class EventLoop;
   friend class _::XThreadEvent;
   friend class _::XThreadPaf;
 
   void send(_::XThreadEvent& event, bool sync) const;
   void wait();
   bool poll();
 
   EventLoop& getLoop() const;
 };
 
 const Executor& getCurrentThreadExecutor();
 // Get the executor for the current thread's event loop. This reference can then be passed to other
 // threads.
 
 // =======================================================================================
 // 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 `setupAsyncIo()` 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.
 
   const Executor& getExecutor();
   // Returns an Executor that can be used to schedule events on this EventLoop from another thread.
   //
   // Use the global function kj::getCurrentThreadExecutor() to get the current thread's EventLoop's
   // Executor.
   //
   // Note that this is only needed for cross-thread scheduling. To schedule code to run later in
   // the current thread, use `kj::evalLater()`, which will be more efficient.
 
 private:
   kj::Maybe<EventPort&> port;
   // If null, this thread doesn't receive I/O events from the OS. It can potentially receive
   // events from other threads via the Executor.
 
   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;
   _::Event** breadthFirstInsertPoint = &head;
 
   kj::Maybe<Own<Executor>> executor;
   // Allocated the first time getExecutor() is requested, making cross-thread request possible.
 
   Own<TaskSet> daemons;
 
   _::Event* currentlyFiring = nullptr;
 
   bool turn();
   void setRunnable(bool runnable);
   void enterScope();
   void leaveScope();
 
   void wait();
   void poll();
 
   friend void _::detach(kj::Promise<void>&& promise);
   friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result,
                           WaitScope& waitScope);
   friend bool _::pollImpl(_::PromiseNode& node, WaitScope& waitScope);
   friend class _::Event;
   friend class WaitScope;
   friend class Executor;
   friend class _::XThreadEvent;
   friend class _::XThreadPaf;
   friend class _::FiberBase;
   friend class _::FiberStack;
   friend ArrayPtr<void* const> getAsyncTrace(ArrayPtr<void*> space);
 };
 
 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() { if (fiber == nullptr) loop.leaveScope(); }
   KJ_DISALLOW_COPY(WaitScope);
 
   void poll();
   // Pumps the event queue and polls for I/O until there's nothing left to do (without blocking).
   //
   // Not supported in fibers.
 
   void setBusyPollInterval(uint count) { busyPollInterval = count; }
   // Set the maximum number of events to run in a row before calling poll() on the EventPort to
   // check for new I/O.
   //
   // This has no effect when used in a fiber.
 
   void runEventCallbacksOnStackPool(kj::Maybe<const FiberPool&> pool) { runningStacksPool = pool; }
   // Arranges to switch stacks while event callbacks are executing. This is an optimization that
   // is useful for programs that use extremely high thread counts, where each thread has its own
   // event loop, but each thread has relatively low event throughput, i.e. each thread spends
   // most of its time waiting for I/O. Normally, the biggest problem with having lots of threads
   // is that each thread must allocate a stack, and stacks can take a lot of memory if the
   // application commonly makes deep calls. But, most of that stack space is only needed while
   // the thread is executing, not while it's sleeping. So, if threads only switch to a big stack
   // during execution, switching back when it's time to sleep, and if those stacks are freelisted
   // so that they can be shared among threads, then a lot of memory is saved.
   //
   // We use the `FiberPool` type here because it implements a freelist of stacks, which is exactly
   // what we happen to want! In our case, though, we don't use those stacks to implement fibers;
   // we use them as the main thread stack.
   //
   // This has no effect if this WaitScope itself is for a fiber.
   //
   // Pass `nullptr` as the parameter to go back to running events on the main stack.
 
   void cancelAllDetached();
   // HACK: Immediately cancel all detached promises.
   //
   // New code should not use detached promises, and therefore should not need this.
   //
   // This method exists to help existing code deal with the problems of detached promises,
   // especially at teardown time.
   //
   // This method may be removed in the future.
 
 private:
   EventLoop& loop;
   uint busyPollInterval = kj::maxValue;
 
   kj::Maybe<_::FiberBase&> fiber;
   kj::Maybe<const FiberPool&> runningStacksPool;
 
   explicit WaitScope(EventLoop& loop, _::FiberBase& fiber)
       : loop(loop), fiber(fiber) {}
 
   template <typename Func>
   inline void runOnStackPool(Func&& func) {
     KJ_IF_MAYBE(pool, runningStacksPool) {
       pool->runSynchronously(kj::fwd<Func>(func));
     } else {
       func();
     }
   }
 
   friend class EventLoop;
   friend class _::FiberBase;
   friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result,
                           WaitScope& waitScope);
   friend bool _::pollImpl(_::PromiseNode& node, WaitScope& waitScope);
 };
 
 }  // namespace kj
 
 #define KJ_ASYNC_H_INCLUDED
 #include "async-inl.h"
 
 KJ_END_HEADER