channel.h

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// uvco (c) 2024 Lewin Bormann. See LICENSE for specific terms.
 
#pragma once
 
#include <uv.h>
 
#include "uvco/bounded_queue.h"
#include "uvco/exception.h"
#include "uvco/promise/multipromise.h"
#include "uvco/promise/promise.h"
#include "uvco/run.h"
 
#include <boost/assert.hpp>
#include <coroutine>
 
namespace uvco {
 
 
template <typename T> class Channel {
public:
  explicit Channel(unsigned capacity, unsigned max_waiters = 16)
      : queue_{capacity}, read_waiting_{max_waiters},
        write_waiting_{max_waiters} {}
 
  template <typename U> Promise<void> put(U &&value) {
    if (!queue_.hasSpace()) {
      // Block until a reader has popped an item.
      ChannelAwaiter_ awaiter{queue_, write_waiting_};
      // Return value indicates if queue is filled with >= 1 item (true) or
      // empty (false).
      co_await awaiter;
      BOOST_VERIFY(queue_.hasSpace());
    }
    queue_.put(std::forward<U>(value));
 
    // NOTE: this will switch control to the reader until it suspends; keep this
    // in mind.
    //
    // For a filled queue, this will result in a nice lock-step switching back
    // and forth.
    awake_reader();
    co_return;
  }
 
  Promise<T> get() {
    if (queue_.empty()) {
      ChannelAwaiter_ awaiter{queue_, read_waiting_};
      BOOST_VERIFY(co_await awaiter);
    }
    T item{queue_.get()};
    // NOTE: this will switch control to the writer until it suspends; keep this
    // in mind.
    awake_writer();
    co_return item;
  }
 
  MultiPromise<T> getAll() {
    while (true) {
      if (queue_.empty()) {
        ChannelAwaiter_ awaiter{queue_, read_waiting_};
        BOOST_VERIFY(co_await awaiter);
      }
      T item = queue_.get();
      awake_writer();
      // Suspends until consumer asks for next item.
      co_yield item;
    }
  }
 
private:
  BoundedQueue<T> queue_;
  // TODO: a multi-reader/writer queue is easily achieved by converting the
  // optionals into queues. This may be interesting in future.
  BoundedQueue<std::coroutine_handle<>> read_waiting_;
  BoundedQueue<std::coroutine_handle<>> write_waiting_;
 
  void awake_reader() {
    if (!read_waiting_.empty()) {
      auto handle = read_waiting_.get();
      // Slower than direct resume but interacts more nicely with other
      // coroutines.
      Loop::enqueue(handle);
    }
  }
  void awake_writer() {
    if (!write_waiting_.empty()) {
      auto handle = write_waiting_.get();
      Loop::enqueue(handle);
    }
  }
 
  struct ChannelAwaiter_ {
    explicit ChannelAwaiter_(BoundedQueue<T> &queue,
                             BoundedQueue<std::coroutine_handle<>> &slot)
        : queue_{queue}, waiters_{slot} {}
 
    bool await_ready() { return false; }
 
    bool await_suspend(std::coroutine_handle<> handle) {
      if (!waiters_.hasSpace()) {
        throw UvcoException(
            UV_EBUSY,
            "too many coroutines waiting for reading/writing a channel");
      }
      waiters_.put(handle);
      return true;
    }
 
    bool await_resume() { return !queue_.empty(); }
 
    // References Channel<T>::queue_
    BoundedQueue<T> &queue_;
    BoundedQueue<std::coroutine_handle<>> &waiters_;
  };
};
 
 
} // namespace uvco