--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/CSP2/CSP2_env/env-d9b9114564458d9d-741b3de822f2aaca6c6caa4325c4afce/include/kj/async.h	Tue Mar 18 17:55:14 2025 -0400
@@ -0,0 +1,1384 @@
+// 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 <kj/exception.h>
+#include <kj/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>>.
+
+// =======================================================================================
+
+class AsyncObject {
+  // You may optionally inherit privately from this to indicate that the type is a KJ async object,
+  // meaning it deals with KJ async I/O making it tied to a specific thread and event loop. This
+  // enables some additional debug checks, but does not otherwise have any effect on behavior as
+  // long as there are no bugs.
+  //
+  // (We prefer inheritance rather than composition here because inheriting an empty type adds zero
+  // size to the derived class.)
+
+public:
+  ~AsyncObject();
+
+private:
+  KJ_NORETURN(static void failed() noexcept);
+};
+
+class DisallowAsyncDestructorsScope {
+  // Create this type on the stack in order to specify that during its scope, no KJ async objects
+  // should be destroyed. If AsyncObject's destructor is called in this scope, the process will
+  // crash with std::terminate().
+  //
+  // This is useful as a sort of "sanitizer" to catch bugs. When tearing down an object that is
+  // intended to be passed between threads, you can set up one of these scopes to catch whether
+  // the object contains any async objects, which are not legal to pass across threads.
+
+public:
+  explicit DisallowAsyncDestructorsScope(kj::StringPtr reason);
+  ~DisallowAsyncDestructorsScope();
+  KJ_DISALLOW_COPY_AND_MOVE(DisallowAsyncDestructorsScope);
+
+private:
+  kj::StringPtr reason;
+  DisallowAsyncDestructorsScope* previousValue;
+
+  friend class AsyncObject;
+};
+
+class AllowAsyncDestructorsScope {
+  // Negates the effect of DisallowAsyncDestructorsScope.
+
+public:
+  AllowAsyncDestructorsScope();
+  ~AllowAsyncDestructorsScope();
+  KJ_DISALLOW_COPY_AND_MOVE(AllowAsyncDestructorsScope);
+
+private:
+  DisallowAsyncDestructorsScope* previousValue;
+};
+
+// =======================================================================================
+// 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(),
+                                 SourceLocation location = {}) 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, SourceLocation location = {}) KJ_WARN_UNUSED_RESULT;
+  // Equivalent to `.then(identityFunc, errorHandler)`, where `identifyFunc` is a function that
+  // just returns its input.
+
+  T wait(WaitScope& waitScope, SourceLocation location = {});
+  // 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, SourceLocation location = {});
+  // 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(SourceLocation location = {}) 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(SourceLocation location = {});
+  // 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, SourceLocation location = {}) 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, SourceLocation location = {})
+      KJ_WARN_UNUSED_RESULT;
+  Promise<T> eagerlyEvaluate(decltype(nullptr), SourceLocation location = {}) 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, _::OwnPromiseNode&& 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 _::ReadyNow READY_NOW = _::ReadyNow();
+// 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 T, T value>
+Promise<T> constPromise();
+// Construct a Promise which resolves to the given constant value. This function is equivalent to
+// `Promise<T>(value)` except that it avoids an allocation.
+
+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, SourceLocation location = {}) 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_AND_MOVE(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, SourceLocation location = {}) 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, SourceLocation location = {});
+// Join an array of promises into a promise for an array. Trailing continuations on promises are not
+// evaluated until all promises have settled. Exceptions are propagated only after the last promise
+// has settled.
+//
+// TODO(cleanup): It is likely that `joinPromisesFailFast()` is what everyone should be using.
+//   Deprecate this function.
+
+template <typename T>
+Promise<Array<T>> joinPromisesFailFast(Array<Promise<T>>&& promises, SourceLocation location = {});
+// Join an array of promises into a promise for an array. Trailing continuations on promises are
+// evaluated eagerly. If any promise results in an exception, the exception is immediately
+// propagated to the returned join promise.
+
+// =======================================================================================
+// 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)
+    KJ_DEPRECATED("Use C++14 generalized captures instead.");
+
+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));
+}
+
+// =======================================================================================
+// Hack for safely using a lambda as a coroutine.
+
+#if KJ_HAS_COROUTINE
+
+namespace _ {
+
+void throwMultipleCoCaptureInvocations();
+
+template<typename Functor>
+struct CaptureForCoroutine {
+  kj::Maybe<Functor> maybeFunctor;
+
+  explicit CaptureForCoroutine(Functor&& f) : maybeFunctor(kj::mv(f)) {}
+
+  template<typename ...Args>
+  static auto coInvoke(Functor functor, Args&&... args)
+      -> decltype(functor(kj::fwd<Args>(args)...)) {
+    // Since the functor is now in the local scope and no longer a member variable, it will be
+    // persisted in the coroutine state.
+
+    // Note that `co_await functor(...)` can still return `void`. It just happens that
+    // `co_return voidReturn();` is explicitly allowed.
+    co_return co_await functor(kj::fwd<Args>(args)...);
+  }
+
+  template<typename ...Args>
+  auto operator()(Args&&... args) {
+    if (maybeFunctor == nullptr) {
+      throwMultipleCoCaptureInvocations();
+    }
+    auto localFunctor = kj::mv(*kj::_::readMaybe(maybeFunctor));
+    maybeFunctor = nullptr;
+    return coInvoke(kj::mv(localFunctor), kj::fwd<Args>(args)...);
+  }
+};
+
+}  // namespace _
+
+template <typename Functor>
+auto coCapture(Functor&& f) {
+  // Assuming `f()` returns a Promise<T> `p`, wrap `f` in such a way that it will outlive its
+  // returned Promise. Note that the returned object may only be invoked once.
+  //
+  // This function is meant to help address this pain point with functors that return a coroutine:
+  // https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rcoro-capture
+  //
+  // The two most common patterns where this may be useful look like so:
+  // ```
+  // void addTask(Value myValue) {
+  //   auto myFun = [myValue]() -> kj::Promise<void> {
+  //     ...
+  //     co_return;
+  //   };
+  //   tasks.add(myFun());
+  // }
+  // ```
+  // and
+  // ```
+  // kj::Promise<void> afterPromise(kj::Promise<void> promise, Value myValue) {
+  //   auto myFun = [myValue]() -> kj::Promise<void> {
+  //     ...
+  //     co_return;
+  //   };
+  //   return promise.then(kj::mv(myFun));
+  // }
+  // ```
+  //
+  // Note that there are potentially more optimal alternatives to both of these patterns:
+  // ```
+  // void addTask(Value myValue) {
+  //   auto myFun = [](auto myValue) -> kj::Promise<void> {
+  //     ...
+  //     co_return;
+  //   };
+  //   tasks.add(myFun(myValue));
+  // }
+  // ```
+  // and
+  // ```
+  // kj::Promise<void> afterPromise(kj::Promise<void> promise, Value myValue) {
+  //   auto myFun = [&]() -> kj::Promise<void> {
+  //     ...
+  //     co_return;
+  //   };
+  //   co_await promise;
+  //   co_await myFun();
+  //   co_return;
+  // }
+  // ```
+  //
+  // For situations where you are trying to capture a specific local variable, kj::mvCapture() can
+  // also be useful:
+  // ```
+  // kj::Promise<void> reactToPromise(kj::Promise<MyType> promise) {
+  //   BigA a;
+  //   TinyB b;
+  //
+  //   doSomething(a, b);
+  //   return promise.then(kj::mvCapture(b, [](TinyB b, MyType type) -> kj::Promise<void> {
+  //     ...
+  //     co_return;
+  //   });
+  // }
+  // ```
+
+  return _::CaptureForCoroutine(kj::mv(f));
+}
+
+#endif  // KJ_HAS_COROUTINE
+
+// =======================================================================================
+// Advanced promise construction
+
+class PromiseRejector: private AsyncObject {
+  // 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(SourceLocation location = {});
+// 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: private AsyncObject {
+  // 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 cancellation 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_AND_MOVE(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: private AsyncObject {
+  // 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 destroyed as well,
+  // and everything the daemon is doing is canceled.
+
+public:
+  class ErrorHandler {
+  public:
+    virtual void taskFailed(kj::Exception&& exception) = 0;
+  };
+
+  TaskSet(ErrorHandler& errorHandler, SourceLocation location = {});
+  // `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.
+
+  void clear();
+  // Cancel all tasks.
+  //
+  // As always, it is not safe to cancel the task that is currently running, so you could not call
+  // this from inside a task in the TaskSet. However, it IS safe to call this from the
+  // `taskFailed()` callback.
+  //
+  // Calling this will always trigger onEmpty(), if anyone is listening.
+
+private:
+  class Task;
+  using OwnTask = Own<Task, _::PromiseDisposer>;
+
+  TaskSet::ErrorHandler& errorHandler;
+  Maybe<OwnTask> tasks;
+  Maybe<Own<PromiseFulfiller<void>>> emptyFulfiller;
+  SourceLocation location;
+};
+
+// =======================================================================================
+// 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, SourceLocation location = {}) 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, SourceLocation location = {}) 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(_::OwnPromiseNode&& node, _::ExceptionOrValue& result,
+                          WaitScope& waitScope, SourceLocation location);
+  friend bool _::pollImpl(_::PromiseNode& node, WaitScope& waitScope, SourceLocation location);
+  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_AND_MOVE(WaitScope);
+
+  uint poll(uint maxTurnCount = maxValue);
+  // Pumps the event queue and polls for I/O until there's nothing left to do (without blocking) or
+  // the maximum turn count has been reached. Returns the number of events popped off the event
+  // queue.
+  //
+  // 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(_::OwnPromiseNode&& node, _::ExceptionOrValue& result,
+                          WaitScope& waitScope, SourceLocation location);
+  friend bool _::pollImpl(_::PromiseNode& node, WaitScope& waitScope, SourceLocation location);
+};
+
+}  // namespace kj
+
+#define KJ_ASYNC_H_INCLUDED
+#include "async-inl.h"
+
+KJ_END_HEADER