mpd/src/input/BufferingInputStream.cxx

/*
 * Copyright 2003-2019 The Music Player Daemon Project
 * http://www.musicpd.org
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#include "BufferingInputStream.hxx"
#include "thread/Cond.hxx"
#include "thread/Name.hxx"

#include <string.h>

BufferingInputStream::BufferingInputStream(InputStreamPtr _input)
	:input(std::move(_input)),
	 mutex(input->mutex),
	 thread(BIND_THIS_METHOD(RunThread)),
	 buffer(input->GetSize())
{
	input->SetHandler(this);

	thread.Start();
}

BufferingInputStream::~BufferingInputStream() noexcept
{
	{
		const std::lock_guard<Mutex> lock(mutex);
		stop = true;
		wake_cond.notify_one();
	}

	thread.Join();
}

void
BufferingInputStream::Check()
{
	if (error)
		std::rethrow_exception(error);

	if (input)
		input->Check();
}

bool
BufferingInputStream::IsAvailable(size_t offset) noexcept
{
	if (offset >= size() || error)
		return true;

	if (buffer.Read(offset).HasData())
		return true;

	/* if no data is available now, make sure it will be soon */
	if (want_offset == INVALID_OFFSET)
		want_offset = offset;

	return false;
}

size_t
BufferingInputStream::Read(std::unique_lock<Mutex> &lock, size_t offset,
			   void *ptr, size_t s)
{
	if (offset >= size())
		return 0;

	while (true) {
		auto r = buffer.Read(offset);
		if (r.HasData()) {
			/* yay, we have some data */
			size_t nbytes = std::min(s, r.defined_buffer.size);
			memcpy(ptr, r.defined_buffer.data, nbytes);
			return nbytes;
		}

		if (error)
			std::rethrow_exception(error);

		if (want_offset == INVALID_OFFSET)
			want_offset = offset;

		client_cond.wait(lock);
	}
}

size_t
BufferingInputStream::FindFirstHole() const noexcept
{
	auto r = buffer.Read(0);
	if (r.undefined_size > 0)
		/* a hole at the beginning */
		return 0;

	if (r.defined_buffer.size < size())
		/* a hole in the middle */
		return r.defined_buffer.size;

	/* the file has been read completely */
	return INVALID_OFFSET;
}

inline void
BufferingInputStream::RunThreadLocked(std::unique_lock<Mutex> &lock)
{
	while (!stop) {
		if (want_offset != INVALID_OFFSET) {
			assert(want_offset < size());

			const size_t seek_offset = want_offset;
			want_offset = INVALID_OFFSET;
			if (!buffer.Read(seek_offset).HasData())
				input->Seek(lock, seek_offset);
		} else if (input->IsEOF()) {
			/* our input has reached its end: prepare
			   reading the first remaining hole */

			size_t new_offset = FindFirstHole();
			if (new_offset == INVALID_OFFSET) {
				/* the file has been read completely */
				break;
			}

			/* seek to the first hole */
			input->Seek(lock, new_offset);
		} else if (input->IsAvailable()) {
			const auto read_offset = input->GetOffset();
			auto w = buffer.Write(read_offset);

			if (w.empty()) {
				size_t new_offset = FindFirstHole();
				if (new_offset == INVALID_OFFSET)
					/* the file has been read
					   completely */
					break;

				input->Seek(lock, new_offset);

				continue;
			}

			size_t nbytes = input->Read(lock, w.data, w.size);
			buffer.Commit(read_offset, read_offset + nbytes);

			client_cond.notify_all();
			OnBufferAvailable();
		} else
			wake_cond.wait(lock);
	}
}

void
BufferingInputStream::RunThread() noexcept
{
	SetThreadName("buffering");

	std::unique_lock<Mutex> lock(mutex);

	try {
		RunThreadLocked(lock);
	} catch (...) {
		error = std::current_exception();
		client_cond.notify_all();
		OnBufferAvailable();
	}

	/* clear the "input" attribute while holding the mutex */
	auto _input = std::move(input);

	/* the mutex must be unlocked while an InputStream can be
	   destructed */
	lock.unlock();

	/* and now actually destruct the InputStream */
	_input.reset();
}