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/// @file WASM shim for kj/async.h
/// kj::Promise<T> implemented as a C++20 coroutine return type.
/// All promises are immediately resolved — the KJ event loop is never entered.
#pragma once
#include "common.h"
#include <coroutine>
#include <exception>
#include <optional>
#include <stdexcept>
#include <type_traits>
namespace kj {
// ---- WaitScope ----
struct WaitScope
{};
// ========================================================================
// Promise<T> — forward-declare both primary and void specialization
// so no implicit instantiation of the primary happens for void.
// ========================================================================
template<typename T>
struct Promise;
template<>
struct Promise<void>;
// Define ForkedPromise early so Promise::fork() can use it.
template<typename T>
struct ForkedPromise
{
ForkedPromise() = default;
ForkedPromise(std::nullptr_t) {} // allow {nullptr} initialization
Promise<T> addBranch()
{
return {};
}
};
// ---- Internal storage helpers ----
namespace _p {
template<typename T>
struct Storage
{
std::exception_ptr exc;
std::optional<T> val;
void set_value(T t)
{
val.emplace(std::move(t));
}
void set_exception(std::exception_ptr e)
{
exc = e;
}
T take()
{
if (exc) {
std::rethrow_exception(exc);
}
if (!val) {
throw std::logic_error("Promise: no value");
}
return std::move(*val);
}
};
template<>
struct Storage<void>
{
std::exception_ptr exc;
bool ok = false;
void set_value()
{
ok = true;
}
void set_exception(std::exception_ptr e)
{
exc = e;
}
void take()
{
if (exc) {
std::rethrow_exception(exc);
}
if (!ok) {
throw std::logic_error("Promise<void>: no value");
}
}
};
// promise_type for T != void
template<typename T>
struct PP
{
Storage<T> * storage = nullptr;
Promise<T> get_return_object();
std::suspend_never initial_suspend() noexcept
{
return {};
}
std::suspend_never final_suspend() noexcept
{
return {};
}
void return_value(T val)
{
if (storage) {
storage->set_value(std::move(val));
}
}
void unhandled_exception()
{
if (storage) {
storage->set_exception(std::current_exception());
}
}
};
// promise_type for void
template<>
struct PP<void>
{
Storage<void> * storage = nullptr;
Promise<void> get_return_object();
std::suspend_never initial_suspend() noexcept
{
return {};
}
std::suspend_never final_suspend() noexcept
{
return {};
}
void return_void()
{
if (storage) {
storage->set_value();
}
}
void unhandled_exception()
{
if (storage) {
storage->set_exception(std::current_exception());
}
}
};
} // namespace _p
// ========================================================================
// Promise<T> primary (T != void)
// ========================================================================
template<typename T>
struct Promise
{
using promise_type = _p::PP<T>;
// Default-constructed promise has no value (used by coroutine machinery).
Promise() = default;
// nullptr means "unresolved placeholder" (used in lix's async retry loops).
Promise(std::nullptr_t) {} // NOLINT
// Immediately-resolved promise — non-explicit to allow brace-init return statements.
Promise(T val)
{
st_.set_value(std::move(val));
} // NOLINT
Promise(std::exception_ptr e)
{
st_.set_exception(e);
} // NOLINT
// co_await
bool await_ready() const noexcept
{
return true;
}
void await_suspend(std::coroutine_handle<>) noexcept {}
T await_resume()
{
return st_.take();
}
T wait(WaitScope &)
{
return st_.take();
}
// then() is defined out-of-line after _p::UnwrapT and Promise<void> are complete.
template<typename F>
auto then(F && f); // return type deduced at definition site
Promise<T> exclusiveJoin(Promise<T>)
{
return std::move(*this);
}
template<typename... Args>
Promise<T> attach(Args &&...)
{
return std::move(*this);
}
ForkedPromise<T> fork()
{
return {};
}
_p::Storage<T> st_;
};
// ========================================================================
// Promise<void> specialization
// ========================================================================
template<>
struct Promise<void>
{
using promise_type = _p::PP<void>;
Promise()
{
st_.set_value();
}
Promise(std::nullptr_t) {} // NOLINT — unresolved placeholder
Promise(std::exception_ptr e)
{
st_.set_exception(e);
} // NOLINT
bool await_ready() const noexcept
{
return true;
}
void await_suspend(std::coroutine_handle<>) noexcept {}
void await_resume()
{
st_.take();
}
void wait(WaitScope &)
{
st_.take();
}
// Defined out-of-line after _p::UnwrapT is available.
template<typename F>
auto then(F && f);
Promise<void> exclusiveJoin(Promise<void>)
{
return {};
}
template<typename... Args>
Promise<void> attach(Args &&...)
{
return {};
}
ForkedPromise<void> fork()
{
return {};
}
_p::Storage<void> st_;
};
// Connect PP::get_return_object and define flatten helpers now that both
// Promise specializations are complete.
namespace _p {
template<typename T>
Promise<T> PP<T>::get_return_object()
{
Promise<T> p;
storage = &p.st_;
return p;
}
inline Promise<void> PP<void>::get_return_object()
{
Promise<void> p;
storage = &p.st_;
return p;
}
// Promise flattening helpers (monadic bind).
template<typename T>
struct Unwrap
{
using type = Promise<T>;
};
template<typename T>
struct Unwrap<Promise<T>>
{
using type = Promise<T>;
};
template<typename T>
using UnwrapT = typename Unwrap<T>::type;
template<typename T>
Promise<T> flatten(Promise<T> p)
{
return p;
}
template<typename T>
Promise<T> flatten(Promise<Promise<T>> pp)
{
try {
return pp.st_.take();
} catch (...) {
return Promise<T>(std::current_exception());
}
}
} // namespace _p
// Define Promise<T>::then() and Promise<void>::then() out-of-line
// now that _p::UnwrapT and _p::flatten are available.
template<typename T>
template<typename F>
auto Promise<T>::then(F && f)
{
using R = std::invoke_result_t<F, T>;
using Flat = _p::UnwrapT<R>;
if constexpr (std::is_void_v<R>) {
try {
f(st_.take());
} catch (...) {
}
return Flat{};
} else {
try {
R r = f(st_.take());
return _p::flatten(Promise<R>(std::move(r)));
} catch (...) {
return Flat(std::current_exception());
}
}
}
template<typename F>
auto Promise<void>::then(F && f)
{
using R = std::invoke_result_t<F>;
using Flat = _p::UnwrapT<R>;
if constexpr (std::is_void_v<R>) {
try {
st_.take();
f();
} catch (...) {
}
return Flat{};
} else {
try {
st_.take();
return _p::flatten(Promise<R>(f()));
} catch (...) {
return Flat(std::current_exception());
}
}
}
// ========================================================================
// Supporting types
// ========================================================================
template<typename T>
struct PromiseFulfiller
{
virtual void fulfill(T &&) = 0;
virtual void reject(Exception &&) = 0;
virtual bool isWaiting()
{
return false;
}
virtual ~PromiseFulfiller() = default;
};
template<>
struct PromiseFulfiller<void>
{
virtual void fulfill() = 0;
virtual void reject(Exception &&) = 0;
virtual bool isWaiting()
{
return false;
}
virtual ~PromiseFulfiller() = default;
};
template<typename T>
using CrossThreadPromiseFulfiller = PromiseFulfiller<T>;
template<typename T>
struct PromiseFulfillerPair
{
Promise<T> promise;
Own<PromiseFulfiller<T>> fulfiller;
};
template<typename T>
PromiseFulfillerPair<T> newPromiseAndFulfiller()
{
return {};
}
template<typename T>
PromiseFulfillerPair<T> newPromiseAndCrossThreadFulfiller()
{
return {};
}
template<typename T, typename Adapter, typename... Args>
Promise<T> newAdaptedPromise(Args &&...)
{
return {};
}
struct TaskSet
{
struct ErrorHandler
{
virtual void taskFailed(std::exception_ptr) {}
virtual ~ErrorHandler() = default;
};
explicit TaskSet(ErrorHandler &) {}
template<typename T>
void add(Promise<T>)
{
}
bool isEmpty() const
{
return true;
}
Promise<void> onEmpty()
{
return {};
}
};
template<typename T>
Promise<Array<T>> joinPromisesFailFast(Array<Promise<T>>)
{
return {};
}
inline Promise<void> joinPromisesFailFast(Array<Promise<void>>)
{
return {};
}
struct CanceledException : std::exception
{
const char * what() const noexcept override
{
return "kj::CanceledException";
}
};
struct FiberPool
{
explicit FiberPool(size_t) {}
template<typename F>
auto startFiber(WaitScope &, F && f) -> decltype(f(std::declval<WaitScope &>()))
{
// In WASM we have no fiber scheduler; just call synchronously.
WaitScope ws;
return f(ws);
}
};
struct Executor
{
template<typename F>
void executeSync(F &&)
{
}
};
static inline const Executor & getCurrentThreadExecutor()
{
static Executor e;
return e;
}
} // namespace kj
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