tor-browser

cubeb_ringbuffer.h (15215B)

---

/*
* Copyright © 2016 Mozilla Foundation
*
* This program is made available under an ISC-style license.  See the
* accompanying file LICENSE for details.
*/

#ifndef CUBEB_RING_BUFFER_H
#define CUBEB_RING_BUFFER_H

#include "cubeb_utils.h"
#include <algorithm>
#include <atomic>
#include <cstdint>
#include <memory>
#include <thread>

/**
* Single producer single consumer lock-free and wait-free ring buffer.
*
* This data structure allows producing data from one thread, and consuming it
* on another thread, safely and without explicit synchronization. If used on
* two threads, this data structure uses atomics for thread safety. It is
* possible to disable the use of atomics at compile time and only use this data
* structure on one thread.
*
* The role for the producer and the consumer must be constant, i.e., the
* producer should always be on one thread and the consumer should always be on
* another thread.
*
* Some words about the inner workings of this class:
* - Capacity is fixed. Only one allocation is performed, in the constructor.
*   When reading and writing, the return value of the method allows checking if
*   the ring buffer is empty or full.
* - We always keep the read index at least one element ahead of the write
*   index, so we can distinguish between an empty and a full ring buffer: an
*   empty ring buffer is when the write index is at the same position as the
*   read index. A full buffer is when the write index is exactly one position
*   before the read index.
* - We synchronize updates to the read index after having read the data, and
*   the write index after having written the data. This means that the each
*   thread can only touch a portion of the buffer that is not touched by the
*   other thread.
* - Callers are expected to provide buffers. When writing to the queue,
*   elements are copied into the internal storage from the buffer passed in.
*   When reading from the queue, the user is expected to provide a buffer.
*   Because this is a ring buffer, data might not be contiguous in memory,
*   providing an external buffer to copy into is an easy way to have linear
*   data for further processing.
*/
template <typename T> class ring_buffer_base {
public:
 /**
  * Constructor for a ring buffer.
  *
  * This performs an allocation, but is the only allocation that will happen
  * for the life time of a `ring_buffer_base`.
  *
  * @param capacity The maximum number of element this ring buffer will hold.
  */
 ring_buffer_base(int capacity)
     /* One more element to distinguish from empty and full buffer. */
     : capacity_(capacity + 1)
 {
   assert(storage_capacity() < std::numeric_limits<int>::max() / 2 &&
          "buffer too large for the type of index used.");
   assert(capacity_ > 0);

   data_.reset(new T[storage_capacity()]);
   /* If this queue is using atomics, initializing those members as the last
    * action in the constructor acts as a full barrier, and allow capacity() to
    * be thread-safe. */
   write_index_ = 0;
   read_index_ = 0;
 }
 /**
  * Push `count` zero or default constructed elements in the array.
  *
  * Only safely called on the producer thread.
  *
  * @param count The number of elements to enqueue.
  * @return The number of element enqueued.
  */
 int enqueue_default(int count) { return enqueue(nullptr, count); }
 /**
  * @brief Put an element in the queue
  *
  * Only safely called on the producer thread.
  *
  * @param element The element to put in the queue.
  *
  * @return 1 if the element was inserted, 0 otherwise.
  */
 int enqueue(T & element) { return enqueue(&element, 1); }
 /**
  * Push `count` elements in the ring buffer.
  *
  * Only safely called on the producer thread.
  *
  * @param elements a pointer to a buffer containing at least `count` elements.
  * If `elements` is nullptr, zero or default constructed elements are
  * enqueued.
  * @param count The number of elements to read from `elements`
  * @return The number of elements successfully coped from `elements` and
  * inserted into the ring buffer.
  */
 int enqueue(T * elements, int count)
 {
#ifndef NDEBUG
   assert_correct_thread(producer_id);
#endif

   int wr_idx = write_index_.load(std::memory_order_relaxed);
   int rd_idx = read_index_.load(std::memory_order_acquire);

   if (full_internal(rd_idx, wr_idx)) {
     return 0;
   }

   int to_write = std::min(available_write_internal(rd_idx, wr_idx), count);

   /* First part, from the write index to the end of the array. */
   int first_part = std::min(storage_capacity() - wr_idx, to_write);
   /* Second part, from the beginning of the array */
   int second_part = to_write - first_part;

   if (elements) {
     Copy(data_.get() + wr_idx, elements, first_part);
     Copy(data_.get(), elements + first_part, second_part);
   } else {
     ConstructDefault(data_.get() + wr_idx, first_part);
     ConstructDefault(data_.get(), second_part);
   }

   write_index_.store(increment_index(wr_idx, to_write),
                      std::memory_order_release);

   return to_write;
 }
 /**
  * Retrieve at most `count` elements from the ring buffer, and copy them to
  * `elements`, if non-null.
  *
  * Only safely called on the consumer side.
  *
  * @param elements A pointer to a buffer with space for at least `count`
  * elements. If `elements` is `nullptr`, `count` element will be discarded.
  * @param count The maximum number of elements to dequeue.
  * @return The number of elements written to `elements`.
  */
 int dequeue(T * elements, int count)
 {
#ifndef NDEBUG
   assert_correct_thread(consumer_id);
#endif

   int rd_idx = read_index_.load(std::memory_order_relaxed);
   int wr_idx = write_index_.load(std::memory_order_acquire);

   if (empty_internal(rd_idx, wr_idx)) {
     return 0;
   }

   int to_read = std::min(available_read_internal(rd_idx, wr_idx), count);

   int first_part = std::min(storage_capacity() - rd_idx, to_read);
   int second_part = to_read - first_part;

   if (elements) {
     Copy(elements, data_.get() + rd_idx, first_part);
     Copy(elements + first_part, data_.get(), second_part);
   }

   read_index_.store(increment_index(rd_idx, to_read),
                     std::memory_order_release);

   return to_read;
 }
 /**
  * Get the number of available element for consuming.
  *
  * Only safely called on the consumer thread.
  *
  * @return The number of available elements for reading.
  */
 int available_read() const
 {
#ifndef NDEBUG
   assert_correct_thread(consumer_id);
#endif
   return available_read_internal(
       read_index_.load(std::memory_order_relaxed),
       write_index_.load(std::memory_order_acquire));
 }
 /**
  * Get the number of available elements for consuming.
  *
  * Only safely called on the producer thread.
  *
  * @return The number of empty slots in the buffer, available for writing.
  */
 int available_write() const
 {
#ifndef NDEBUG
   assert_correct_thread(producer_id);
#endif
   return available_write_internal(
       read_index_.load(std::memory_order_acquire),
       write_index_.load(std::memory_order_relaxed));
 }
 /**
  * Get the total capacity, for this ring buffer.
  *
  * Can be called safely on any thread.
  *
  * @return The maximum capacity of this ring buffer.
  */
 int capacity() const { return storage_capacity() - 1; }
 /**
  * Reset the consumer and producer thread identifier, in case the thread are
  * being changed. This has to be externally synchronized. This is no-op when
  * asserts are disabled.
  */
 void reset_thread_ids()
 {
#ifndef NDEBUG
   consumer_id = producer_id = std::thread::id();
#endif
 }

private:
 /** Return true if the ring buffer is empty.
  *
  * @param read_index the read index to consider
  * @param write_index the write index to consider
  * @return true if the ring buffer is empty, false otherwise.
  **/
 bool empty_internal(int read_index, int write_index) const
 {
   return write_index == read_index;
 }
 /** Return true if the ring buffer is full.
  *
  * This happens if the write index is exactly one element behind the read
  * index.
  *
  * @param read_index the read index to consider
  * @param write_index the write index to consider
  * @return true if the ring buffer is full, false otherwise.
  **/
 bool full_internal(int read_index, int write_index) const
 {
   return (write_index + 1) % storage_capacity() == read_index;
 }
 /**
  * Return the size of the storage. It is one more than the number of elements
  * that can be stored in the buffer.
  *
  * @return the number of elements that can be stored in the buffer.
  */
 int storage_capacity() const { return capacity_; }
 /**
  * Returns the number of elements available for reading.
  *
  * @return the number of available elements for reading.
  */
 int available_read_internal(int read_index, int write_index) const
 {
   if (write_index >= read_index) {
     return write_index - read_index;
   } else {
     return write_index + storage_capacity() - read_index;
   }
 }
 /**
  * Returns the number of empty elements, available for writing.
  *
  * @return the number of elements that can be written into the array.
  */
 int available_write_internal(int read_index, int write_index) const
 {
   /* We substract one element here to always keep at least one sample
    * free in the buffer, to distinguish between full and empty array. */
   int rv = read_index - write_index - 1;
   if (write_index >= read_index) {
     rv += storage_capacity();
   }
   return rv;
 }
 /**
  * Increments an index, wrapping it around the storage.
  *
  * @param index a reference to the index to increment.
  * @param increment the number by which `index` is incremented.
  * @return the new index.
  */
 int increment_index(int index, int increment) const
 {
   assert(increment >= 0);
   return (index + increment) % storage_capacity();
 }
 /**
  * @brief This allows checking that enqueue (resp. dequeue) are always called
  * by the right thread.
  *
  * @param id the id of the thread that has called the calling method first.
  */
#ifndef NDEBUG
 static void assert_correct_thread(std::thread::id & id)
 {
   if (id == std::thread::id()) {
     id = std::this_thread::get_id();
     return;
   }
   assert(id == std::this_thread::get_id());
 }
#endif
 /** Index at which the oldest element is at, in samples. */
 std::atomic<int> read_index_;
 /** Index at which to write new elements. `write_index` is always at
  * least one element ahead of `read_index_`. */
 std::atomic<int> write_index_;
 /** Maximum number of elements that can be stored in the ring buffer. */
 const int capacity_;
 /** Data storage */
 std::unique_ptr<T[]> data_;
#ifndef NDEBUG
 /** The id of the only thread that is allowed to read from the queue. */
 mutable std::thread::id consumer_id;
 /** The id of the only thread that is allowed to write from the queue. */
 mutable std::thread::id producer_id;
#endif
};

/**
* Adapter for `ring_buffer_base` that exposes an interface in frames.
*/
template <typename T> class audio_ring_buffer_base {
public:
 /**
  * @brief Constructor.
  *
  * @param channel_count       Number of channels.
  * @param capacity_in_frames  The capacity in frames.
  */
 audio_ring_buffer_base(int channel_count, int capacity_in_frames)
     : channel_count(channel_count),
       ring_buffer(frames_to_samples(capacity_in_frames))
 {
   assert(channel_count > 0);
 }
 /**
  * @brief Enqueue silence.
  *
  * Only safely called on the producer thread.
  *
  * @param frame_count The number of frames of silence to enqueue.
  * @return  The number of frames of silence actually written to the queue.
  */
 int enqueue_default(int frame_count)
 {
   return samples_to_frames(
       ring_buffer.enqueue(nullptr, frames_to_samples(frame_count)));
 }
 /**
  * @brief Enqueue `frames_count` frames of audio.
  *
  * Only safely called from the producer thread.
  *
  * @param [in] frames If non-null, the frames to enqueue.
  *                    Otherwise, silent frames are enqueued.
  * @param frame_count The number of frames to enqueue.
  *
  * @return The number of frames enqueued
  */

 int enqueue(T * frames, int frame_count)
 {
   return samples_to_frames(
       ring_buffer.enqueue(frames, frames_to_samples(frame_count)));
 }

 /**
  * @brief Removes `frame_count` frames from the buffer, and
  *        write them to `frames` if it is non-null.
  *
  * Only safely called on the consumer thread.
  *
  * @param frames      If non-null, the frames are copied to `frames`.
  *                    Otherwise, they are dropped.
  * @param frame_count The number of frames to remove.
  *
  * @return  The number of frames actually dequeud.
  */
 int dequeue(T * frames, int frame_count)
 {
   return samples_to_frames(
       ring_buffer.dequeue(frames, frames_to_samples(frame_count)));
 }
 /**
  * Get the number of available frames of audio for consuming.
  *
  * Only safely called on the consumer thread.
  *
  * @return The number of available frames of audio for reading.
  */
 int available_read() const
 {
   return samples_to_frames(ring_buffer.available_read());
 }
 /**
  * Get the number of available frames of audio for consuming.
  *
  * Only safely called on the producer thread.
  *
  * @return The number of empty slots in the buffer, available for writing.
  */
 int available_write() const
 {
   return samples_to_frames(ring_buffer.available_write());
 }
 /**
  * Get the total capacity, for this ring buffer.
  *
  * Can be called safely on any thread.
  *
  * @return The maximum capacity of this ring buffer.
  */
 int capacity() const { return samples_to_frames(ring_buffer.capacity()); }

private:
 /**
  * @brief Frames to samples conversion.
  *
  * @param frames The number of frames.
  *
  * @return  A number of samples.
  */
 int frames_to_samples(int frames) const { return frames * channel_count; }
 /**
  * @brief Samples to frames conversion.
  *
  * @param samples The number of samples.
  *
  * @return  A number of frames.
  */
 int samples_to_frames(int samples) const { return samples / channel_count; }
 /** Number of channels of audio that will stream through this ring buffer. */
 int channel_count;
 /** The underlying ring buffer that is used to store the data. */
 ring_buffer_base<T> ring_buffer;
};

/**
* Lock-free instantiation of the `ring_buffer_base` type. This is safe to use
* from two threads, one producer, one consumer (that never change role),
* without explicit synchronization.
*/
template <typename T> using lock_free_queue = ring_buffer_base<T>;
/**
* Lock-free instantiation of the `audio_ring_buffer` type. This is safe to use
* from two threads, one producer, one consumer (that never change role),
* without explicit synchronization.
*/
template <typename T>
using lock_free_audio_ring_buffer = audio_ring_buffer_base<T>;

#endif // CUBEB_RING_BUFFER_H