input/cache: first draft of the file cache

doc/user.rst

@@ -334,6 +334,39 @@ The following table lists the input options valid for all plugins:
 
 More information can be found in the :ref:`input_plugins` reference.
 
+Configuring the Input Cache
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The input cache prefetches queued song files before they are going to
+be played.  This has several advantages:
+
+- risk of buffer underruns during playback is reduced because this
+  decouples playback from disk (or network) I/O
+- bulk transfers may be faster and more energy efficient than loading
+  small chunks on-the-fly
+- by prefetching several songs at a time, the hard disk can spin down
+  for longer periods of time
+
+This comes at a cost:
+
+- memory usage
+- bulk transfers may reduce the performance of other applications
+  which also want to access the disk (if the kernel's I/O scheduler
+  isn't doing its job properly)
+
+To enable the input cache, add an ``input_cache`` block to the
+configuration file:
+
+.. code-block:: none
+
+    input_cache {
+        size "1 GB"
+    }
+
+This allocates a cache of 1 GB.  If the cache grows larger than that,
+older files will be evicted.
+
+
 Configuring decoder plugins
 ---------------------------
 

src/Instance.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -23,6 +23,7 @@
 #include "Idle.hxx"
 #include "Stats.hxx"
 #include "client/List.hxx"
+#include "input/cache/Manager.hxx"
 
 #ifdef ENABLE_CURL
 #include "RemoteTagCache.hxx"

src/Instance.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -54,6 +54,7 @@ struct Partition;
 class StateFile;
 class RemoteTagCache;
 class StickerDatabase;
+class InputCacheManager;
 
 /**
  * A utility class which, when used as the first base class, ensures
@@ -98,6 +99,8 @@ struct Instance final
 	Systemd::Watchdog systemd_watchdog;
 #endif
 
+	std::unique_ptr<InputCacheManager> input_cache;
+
 	MaskMonitor idle_monitor;
 
 #ifdef ENABLE_NEIGHBOR_PLUGINS

src/Main.cxx

@@ -38,6 +38,8 @@
 #include "Log.hxx"
 #include "LogInit.hxx"
 #include "input/Init.hxx"
+#include "input/cache/Config.hxx"
+#include "input/cache/Manager.hxx"
 #include "event/Loop.hxx"
 #include "fs/AllocatedPath.hxx"
 #include "fs/Config.hxx"
@@ -414,6 +416,12 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 		raw_config.GetPositive(ConfigOption::MAX_CONN, 10);
 	instance.client_list = std::make_unique<ClientList>(max_clients);
 
+	const auto *input_cache_config = raw_config.GetBlock(ConfigBlockOption::INPUT_CACHE);
+	if (input_cache_config != nullptr) {
+		const InputCacheConfig c(*input_cache_config);
+		instance.input_cache = std::make_unique<InputCacheManager>(c);
+	}
+
 	initialize_decoder_and_player(instance,
 				      raw_config, config.replay_gain);
 
@@ -461,6 +469,7 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 	client_manager_init(raw_config);
 	const ScopeInputPluginsInit input_plugins_init(raw_config,
 						       instance.io_thread.GetEventLoop());
+
 	const ScopePlaylistPluginsInit playlist_plugins_init(raw_config);
 
 #ifdef ENABLE_DAEMON

src/Partition.cxx

@@ -20,10 +20,15 @@
 #include "config.h"
 #include "Partition.hxx"
 #include "Instance.hxx"
+#include "Log.hxx"
 #include "song/DetachedSong.hxx"
 #include "mixer/Volume.hxx"
 #include "IdleFlags.hxx"
 #include "client/Listener.hxx"
+#include "input/cache/Manager.hxx"
+#include "util/Domain.hxx"
+
+static constexpr Domain cache_domain("cache");
 
 Partition::Partition(Instance &_instance,
 		     const char *_name,
@@ -37,7 +42,9 @@ Partition::Partition(Instance &_instance,
 	 global_events(instance.event_loop, BIND_THIS_METHOD(OnGlobalEvent)),
 	 playlist(max_length, *this),
 	 outputs(*this),
-	 pc(*this, outputs, buffer_chunks,
+	 pc(*this, outputs,
+	    instance.input_cache.get(),
+	    buffer_chunks,
 	    configured_audio_format, replay_gain_config)
 {
 	UpdateEffectiveReplayGainMode();
@@ -51,6 +58,43 @@ Partition::EmitIdle(unsigned mask) noexcept
 	instance.EmitIdle(mask);
 }
 
+static void
+PrefetchSong(InputCacheManager &cache, const char *uri) noexcept
+{
+	if (cache.Contains(uri))
+		return;
+
+	FormatDebug(cache_domain, "Prefetch '%s'", uri);
+
+	try {
+		cache.Prefetch(uri);
+	} catch (...) {
+		FormatError(std::current_exception(),
+			    "Prefetch '%s' failed", uri);
+	}
+}
+
+static void
+PrefetchSong(InputCacheManager &cache, const DetachedSong &song) noexcept
+{
+	PrefetchSong(cache, song.GetURI());
+}
+
+inline void
+Partition::PrefetchQueue() noexcept
+{
+	if (!instance.input_cache)
+		return;
+
+	auto &cache = *instance.input_cache;
+
+	int next = playlist.GetNextPosition();
+	if (next >= 0)
+		PrefetchSong(cache, playlist.queue.Get(next));
+
+	// TODO: prefetch more songs
+}
+
 void
 Partition::UpdateEffectiveReplayGainMode() noexcept
 {
@@ -106,6 +150,10 @@ void
 Partition::SyncWithPlayer() noexcept
 {
 	playlist.SyncWithPlayer(pc);
+
+	/* TODO: invoke this function in batches, to let the hard disk
+	   spin down in between */
+	PrefetchQueue();
 }
 
 void

src/Partition.hxx

@@ -81,6 +81,14 @@ struct Partition final : QueueListener, PlayerListener, MixerListener {
 
 	void EmitIdle(unsigned mask) noexcept;
 
+	/**
+	 * Populate the #InputCacheManager with soon-to-be-played song
+	 * files.
+	 *
+	 * Errors will be logged.
+	 */
+	void PrefetchQueue() noexcept;
+
 	void ClearQueue() noexcept {
 		playlist.Clear(pc);
 	}

src/config/Option.hxx

@@ -86,6 +86,7 @@ enum class ConfigBlockOption {
 	AUDIO_OUTPUT,
 	DECODER,
 	INPUT,
+	INPUT_CACHE,
 	PLAYLIST_PLUGIN,
 	RESAMPLER,
 	AUDIO_FILTER,

src/config/Templates.cxx

@@ -86,6 +86,7 @@ const ConfigTemplate config_block_templates[] = {
 	{ "audio_output", true },
 	{ "decoder", true },
 	{ "input", true },
+	{ "input_cache" },
 	{ "playlist_plugin", true },
 	{ "resampler" },
 	{ "filter", true },

src/decoder/Bridge.cxx

@@ -31,6 +31,8 @@
 #include "Log.hxx"
 #include "input/InputStream.hxx"
 #include "input/LocalOpen.hxx"
+#include "input/cache/Manager.hxx"
+#include "input/cache/Stream.hxx"
 #include "fs/Path.hxx"
 #include "util/ConstBuffer.hxx"
 #include "util/StringBuffer.hxx"
@@ -52,8 +54,18 @@ DecoderBridge::~DecoderBridge() noexcept
 }
 
 InputStreamPtr
-DecoderBridge::OpenLocal(Path path_fs)
+DecoderBridge::OpenLocal(Path path_fs, const char *uri_utf8)
 {
+	if (dc.input_cache != nullptr) {
+		auto lease = dc.input_cache->Get(uri_utf8, true);
+		if (lease) {
+			auto is = std::make_unique<CacheInputStream>(std::move(lease),
+								     dc.mutex);
+			is->SetHandler(&dc);
+			return is;
+		}
+	}
+
 	return OpenLocalInputStream(path_fs, dc.mutex);
 }
 

src/decoder/Bridge.hxx

@@ -157,7 +157,7 @@ public:
 	/**
 	 * Open a local file.
 	 */
-	InputStreamPtr OpenLocal(Path path_fs);
+	InputStreamPtr OpenLocal(Path path_fs, const char *uri_utf8);
 
 	/* virtual methods from DecoderClient */
 	void Ready(AudioFormat audio_format,

src/decoder/Control.cxx

@@ -26,9 +26,11 @@
 #include <assert.h>
 
 DecoderControl::DecoderControl(Mutex &_mutex, Cond &_client_cond,
+			       InputCacheManager *_input_cache,
 			       const AudioFormat _configured_audio_format,
 			       const ReplayGainConfig &_replay_gain_config) noexcept
 	:thread(BIND_THIS_METHOD(RunThread)),
+	 input_cache(_input_cache),
 	 mutex(_mutex), client_cond(_client_cond),
 	 configured_audio_format(_configured_audio_format),
 	 replay_gain_config(_replay_gain_config) {}

src/decoder/Control.hxx

@@ -46,6 +46,7 @@
 class DetachedSong;
 class MusicBuffer;
 class MusicPipe;
+class InputCacheManager;
 
 enum class DecoderState : uint8_t {
 	STOP = 0,
@@ -68,6 +69,8 @@ class DecoderControl final : public InputStreamHandler {
 	Thread thread;
 
 public:
+	InputCacheManager *const input_cache;
+
 	/**
 	 * This lock protects #state and #command.
 	 *
@@ -181,6 +184,7 @@ public:
 	 * @param _client_cond see #client_cond
 	 */
 	DecoderControl(Mutex &_mutex, Cond &_client_cond,
+		       InputCacheManager *_input_cache,
 		       const AudioFormat _configured_audio_format,
 		       const ReplayGainConfig &_replay_gain_config) noexcept;
 	~DecoderControl() noexcept;

src/decoder/Thread.cxx

@@ -346,7 +346,7 @@ decoder_run_file(DecoderBridge &bridge, const char *uri_utf8, Path path_fs)
 	InputStreamPtr input_stream;
 
 	try {
-		input_stream = bridge.OpenLocal(path_fs);
+		input_stream = bridge.OpenLocal(path_fs, uri_utf8);
 	} catch (const std::system_error &e) {
 		if (IsPathNotFound(e) &&
 		    /* ENOTDIR means this may be a path inside a

src/input/BufferingInputStream.cxx

@@ -152,6 +152,18 @@ BufferingInputStream::RunThreadLocked(std::unique_lock<Mutex> &lock)
 				continue;
 			}
 
+			/* enforce an upper limit for each
+			   InputStream::Read() call; this is necessary
+			   for plugins which are unable to do partial
+			   reads, e.g. when reading local files, the
+			   read() system call will not return until
+			   all requested bytes have been read from the
+			   hard disk, instead of returning when "some"
+			   data has been read */
+			constexpr size_t MAX_READ = 64 * 1024;
+			if (w.size > MAX_READ)
+				w.size = MAX_READ;
+
 			size_t nbytes = input->Read(lock, w.data, w.size);
 			buffer.Commit(read_offset, read_offset + nbytes);
 

src/input/cache/Config.cxx

@@ -0,0 +1,35 @@
+/*
+ * 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 "Config.hxx"
+#include "config/Block.hxx"
+#include "config/Parser.hxx"
+
+static constexpr size_t KILOBYTE = 1024;
+static constexpr size_t MEGABYTE = 1024 * KILOBYTE;
+
+InputCacheConfig::InputCacheConfig(const ConfigBlock &block)
+{
+	size = 256 * MEGABYTE;
+	const auto *size_param = block.GetBlockParam("size");
+	if (size_param != nullptr)
+		size = size_param->With([](const char *s){
+			return ParseSize(s);
+		});
+}

src/input/cache/Config.hxx

@@ -0,0 +1,33 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_INPUT_CACHE_CONFIG_HXX
+#define MPD_INPUT_CACHE_CONFIG_HXX
+
+#include <stddef.h>
+
+struct ConfigBlock;
+
+struct InputCacheConfig {
+	size_t size;
+
+	explicit InputCacheConfig(const ConfigBlock &block);
+};
+
+#endif

src/input/cache/Item.cxx

@@ -0,0 +1,62 @@
+/*
+ * 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 "Item.hxx"
+#include "Lease.hxx"
+
+#include <assert.h>
+
+InputCacheItem::InputCacheItem(InputStreamPtr _input) noexcept
+	:BufferingInputStream(std::move(_input)),
+	 uri(GetInput().GetURI())
+{
+}
+
+InputCacheItem::~InputCacheItem() noexcept
+{
+	assert(leases.empty());
+}
+
+void
+InputCacheItem::AddLease(InputCacheLease &lease) noexcept
+{
+	const std::lock_guard<Mutex> lock(mutex);
+	leases.push_back(lease);
+}
+
+void
+InputCacheItem::RemoveLease(InputCacheLease &lease) noexcept
+{
+	const std::lock_guard<Mutex> lock(mutex);
+	auto i = leases.iterator_to(lease);
+	if (i == next_lease)
+		++next_lease;
+	leases.erase(i);
+
+	// TODO: ensure that OnBufferAvailable() isn't currently running
+}
+
+void
+InputCacheItem::OnBufferAvailable() noexcept
+{
+	for (auto i = leases.begin(); i != leases.end(); i = next_lease) {
+		next_lease = std::next(i);
+		i->OnInputCacheAvailable();
+	}
+}

src/input/cache/Item.hxx

@@ -0,0 +1,80 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_INPUT_CACHE_ITEM_HXX
+#define MPD_INPUT_CACHE_ITEM_HXX
+
+#include "input/BufferingInputStream.hxx"
+#include "thread/Mutex.hxx"
+#include "util/SparseBuffer.hxx"
+
+#include <boost/intrusive/list.hpp>
+#include <boost/intrusive/set_hook.hpp>
+
+#include <memory>
+#include <string>
+
+class InputCacheLease;
+
+/**
+ * An item in the #InputCacheManager.  It caches the contents of a
+ * file, and reading and managing it through the base class
+ * #BufferingInputStream.
+ *
+ * Use the class #CacheInputStream to read from it.
+ */
+class InputCacheItem final
+	: public BufferingInputStream,
+	  public boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::auto_unlink>>,
+	  public boost::intrusive::set_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>>
+{
+	const std::string uri;
+
+	using LeaseList =
+		boost::intrusive::list<InputCacheLease,
+				       boost::intrusive::base_hook<boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>>>,
+				       boost::intrusive::constant_time_size<false>>;
+
+	LeaseList leases;
+	LeaseList::iterator next_lease = leases.end();
+
+public:
+	explicit InputCacheItem(InputStreamPtr _input) noexcept;
+	~InputCacheItem() noexcept;
+
+	const char *GetUri() const noexcept {
+		return uri.c_str();
+	}
+
+	using BufferingInputStream::size;
+
+	bool IsInUse() const noexcept {
+		const std::lock_guard<Mutex> lock(mutex);
+		return !leases.empty();
+	}
+
+	void AddLease(InputCacheLease &lease) noexcept;
+	void RemoveLease(InputCacheLease &lease) noexcept;
+
+private:
+	/* virtual methods from class BufferingInputStream */
+	void OnBufferAvailable() noexcept override;
+};
+
+#endif

src/input/cache/Lease.hxx

@@ -0,0 +1,94 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_INPUT_CACHE_LEASE_HXX
+#define MPD_INPUT_CACHE_LEASE_HXX
+
+#include "Item.hxx"
+
+#include <boost/intrusive/list_hook.hpp>
+
+#include <utility>
+
+/**
+ * A lease for an #InputCacheItem.
+ */
+class InputCacheLease
+	: public boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>>
+{
+	InputCacheItem *item = nullptr;
+
+public:
+	InputCacheLease() = default;
+
+	explicit InputCacheLease(InputCacheItem &_item) noexcept
+		:item(&_item)
+	{
+		item->AddLease(*this);
+	}
+
+	InputCacheLease(InputCacheLease &&src) noexcept
+		:item(std::exchange(src.item, nullptr))
+	{
+		if (item != nullptr) {
+			item->RemoveLease(src);
+			item->AddLease(*this);
+		}
+	}
+
+	~InputCacheLease() noexcept {
+		if (item != nullptr)
+			item->RemoveLease(*this);
+	}
+
+	InputCacheLease &operator=(InputCacheLease &&src) noexcept {
+		using std::swap;
+		swap(item, src.item);
+
+		if (item != nullptr) {
+			item->RemoveLease(src);
+			item->AddLease(*this);
+		}
+
+		return *this;
+	}
+
+	operator bool() const noexcept {
+		return item != nullptr;
+	}
+
+	auto &operator*() const noexcept {
+		return *item;
+	}
+
+	auto *operator->() const noexcept {
+		return item;
+	}
+
+	auto &GetCacheItem() const noexcept {
+		return *item;
+	}
+
+	/**
+	 * Caller locks #InputCacheItem::mutex.
+	 */
+	virtual void OnInputCacheAvailable() noexcept {}
+};
+
+#endif

src/input/cache/Manager.cxx

@@ -0,0 +1,163 @@
+/*
+ * 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 "Manager.hxx"
+#include "Config.hxx"
+#include "Item.hxx"
+#include "Lease.hxx"
+#include "input/InputStream.hxx"
+#include "fs/Traits.hxx"
+#include "util/DeleteDisposer.hxx"
+
+#include <string.h>
+
+inline bool
+InputCacheManager::ItemCompare::operator()(const InputCacheItem &a,
+					   const char *b) const noexcept
+{
+	return strcmp(a.GetUri(), b) < 0;
+}
+
+inline bool
+InputCacheManager::ItemCompare::operator()(const char *a,
+					   const InputCacheItem &b) const noexcept
+{
+	return strcmp(a, b.GetUri()) < 0;
+}
+
+inline bool
+InputCacheManager::ItemCompare::operator()(const InputCacheItem &a,
+					   const InputCacheItem &b) const noexcept
+{
+	return strcmp(a.GetUri(), b.GetUri()) < 0;
+}
+
+InputCacheManager::InputCacheManager(const InputCacheConfig &config) noexcept
+	:max_total_size(config.size)
+{
+}
+
+InputCacheManager::~InputCacheManager() noexcept
+{
+	items_by_time.clear_and_dispose(DeleteDisposer());
+}
+
+bool
+InputCacheManager::IsEligible(const InputStream &input) noexcept
+{
+	assert(input.IsReady());
+
+	return input.IsSeekable() && input.KnownSize() &&
+		input.GetSize() > 0 &&
+		input.GetSize() <= max_total_size / 2;
+}
+
+bool
+InputCacheManager::Contains(const char *uri) noexcept
+{
+	return Get(uri, false);
+}
+
+InputCacheLease
+InputCacheManager::Get(const char *uri, bool create)
+{
+	// TODO: allow caching remote files
+	if (!PathTraitsUTF8::IsAbsolute(uri))
+		return {};
+
+	UriMap::insert_commit_data hint;
+	auto result = items_by_uri.insert_check(uri, items_by_uri.key_comp(),
+						hint);
+	if (!result.second) {
+		auto &item = *result.first;
+
+		/* refresh */
+		items_by_time.erase(items_by_time.iterator_to(item));
+		items_by_time.push_back(item);
+
+		// TODO revalidate the cache item using the file's mtime?
+		// TODO if cache item contains error, retry now?
+
+		return InputCacheLease(item);
+	}
+
+	if (!create)
+		return {};
+
+	// TODO: wait for "ready" without blocking here
+	auto is = InputStream::OpenReady(uri, mutex);
+
+	if (!IsEligible(*is))
+		return {};
+
+	const size_t size = is->GetSize();
+	total_size += size;
+
+	while (total_size > max_total_size && EvictOldestUnused()) {}
+
+	auto *item = new InputCacheItem(std::move(is));
+	items_by_uri.insert_commit(*item, hint);
+	items_by_time.push_back(*item);
+
+	return InputCacheLease(*item);
+}
+
+void
+InputCacheManager::Prefetch(const char *uri)
+{
+	Get(uri, true);
+}
+
+void
+InputCacheManager::Remove(InputCacheItem &item) noexcept
+{
+	assert(total_size >= item.size());
+	total_size -= item.size();
+
+	items_by_time.erase(items_by_time.iterator_to(item));
+	items_by_uri.erase(items_by_uri.iterator_to(item));
+}
+
+void
+InputCacheManager::Delete(InputCacheItem *item) noexcept
+{
+	Remove(*item);
+	delete item;
+}
+
+InputCacheItem *
+InputCacheManager::FindOldestUnused() noexcept
+{
+	for (auto &i : items_by_time)
+		if (!i.IsInUse())
+			return &i;
+
+	return nullptr;
+}
+
+bool
+InputCacheManager::EvictOldestUnused() noexcept
+{
+	auto *item = FindOldestUnused();
+	if (item == nullptr)
+		return false;
+
+	Delete(item);
+	return true;
+}

src/input/cache/Manager.hxx

@@ -0,0 +1,114 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_INPUT_CACHE_MANAGER_HXX
+#define MPD_INPUT_CACHE_MANAGER_HXX
+
+#include "input/Offset.hxx"
+#include "thread/Mutex.hxx"
+#include "util/Compiler.h"
+
+#include <boost/intrusive/set.hpp>
+#include <boost/intrusive/list.hpp>
+
+class InputStream;
+class InputCacheItem;
+class InputCacheLease;
+struct InputCacheConfig;
+
+/**
+ * A class which caches files in RAM.  It is supposed to prefetch
+ * files before they are played.
+ */
+class InputCacheManager {
+	const size_t max_total_size;
+
+	mutable Mutex mutex;
+
+	size_t total_size = 0;
+
+	struct ItemCompare {
+		gcc_pure
+		bool operator()(const InputCacheItem &a,
+				const char *b) const noexcept;
+
+		gcc_pure
+		bool operator()(const char *a,
+				const InputCacheItem &b) const noexcept;
+
+		gcc_pure
+		bool operator()(const InputCacheItem &a,
+				const InputCacheItem &b) const noexcept;
+	};
+
+	boost::intrusive::list<InputCacheItem,
+			       boost::intrusive::base_hook<boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::auto_unlink>>>,
+			       boost::intrusive::constant_time_size<false>> items_by_time;
+
+	using UriMap =
+		boost::intrusive::set<InputCacheItem,
+				      boost::intrusive::base_hook<boost::intrusive::set_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>>>,
+				      boost::intrusive::compare<ItemCompare>,
+				      boost::intrusive::constant_time_size<false>>;
+
+	UriMap items_by_uri;
+
+public:
+	explicit InputCacheManager(const InputCacheConfig &config) noexcept;
+	~InputCacheManager() noexcept;
+
+	gcc_pure
+	bool Contains(const char *uri) noexcept;
+
+	/**
+	 * Throws if opening the #InputStream fails.
+	 *
+	 * @param create if true, then the cache item will be created
+	 * if it did not exist
+	 * @return a lease of the new item or nullptr if the file is
+	 * not eligible for caching
+	 */
+	InputCacheLease Get(const char *uri, bool create);
+
+	/**
+	 * Shortcut for "Get(uri,true)", discarding the returned
+	 * lease.
+	 */
+	void Prefetch(const char *uri);
+
+private:
+	/**
+	 * Check whether the given #InputStream can be stored in this
+	 * cache.
+	 */
+	bool IsEligible(const InputStream &input) noexcept;
+
+	void Remove(InputCacheItem &item) noexcept;
+	void Delete(InputCacheItem *item) noexcept;
+
+	InputCacheItem *FindOldestUnused() noexcept;
+
+	/**
+	 * @return true if one item has been evicted, false if no
+	 * unused item was found
+	 */
+	bool EvictOldestUnused() noexcept;
+};
+
+#endif

src/input/cache/Stream.cxx

@@ -0,0 +1,96 @@
+/*
+ * 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 "Stream.hxx"
+
+CacheInputStream::CacheInputStream(InputCacheLease _lease,
+				   Mutex &_mutex) noexcept
+	:InputStream(_lease->GetUri(), _mutex),
+	 InputCacheLease(std::move(_lease))
+{
+	auto &i = GetCacheItem();
+	size = i.size();
+	seekable = true;
+	SetReady();
+}
+
+void
+CacheInputStream::Check()
+{
+	const ScopeUnlock unlock(mutex);
+
+	auto &i = GetCacheItem();
+	const std::lock_guard<Mutex> protect(i.mutex);
+
+	i.Check();
+}
+
+void
+CacheInputStream::Seek(std::unique_lock<Mutex> &, offset_type new_offset)
+{
+	offset = new_offset;
+}
+
+bool
+CacheInputStream::IsEOF() const noexcept
+{
+	return offset == size;
+}
+
+bool
+CacheInputStream::IsAvailable() const noexcept
+{
+	const auto _offset = offset;
+	const ScopeUnlock unlock(mutex);
+
+	auto &i = GetCacheItem();
+	const std::lock_guard<Mutex> protect(i.mutex);
+
+	return i.IsAvailable(_offset);
+}
+
+size_t
+CacheInputStream::Read(std::unique_lock<Mutex> &lock,
+		       void *ptr, size_t read_size)
+{
+	const auto _offset = offset;
+	auto &i = GetCacheItem();
+
+	size_t nbytes;
+
+	{
+		const ScopeUnlock unlock(mutex);
+		const std::lock_guard<Mutex> protect(i.mutex);
+
+		nbytes = i.Read(lock, _offset, ptr, read_size);
+	}
+
+	offset += nbytes;
+	return nbytes;
+}
+
+void
+CacheInputStream::OnInputCacheAvailable() noexcept
+{
+	auto &i = GetCacheItem();
+	const ScopeUnlock unlock(i.mutex);
+
+	const std::lock_guard<Mutex> protect(mutex);
+	InvokeOnAvailable();
+}

src/input/cache/Stream.hxx

@@ -0,0 +1,52 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_CACHE_INPUT_STREAM_HXX
+#define MPD_CACHE_INPUT_STREAM_HXX
+
+#include "Lease.hxx"
+#include "input/InputStream.hxx"
+
+/**
+ * An #InputStream implementation which reads data from an
+ * #InputCacheItem.
+ */
+class CacheInputStream final : public InputStream, InputCacheLease {
+public:
+	CacheInputStream(InputCacheLease _lease, Mutex &_mutex) noexcept;
+
+	/* virtual methods from class InputStream */
+	void Check() override;
+	/* we don't need to implement Update() because all attributes
+	   have been copied already in our constructor */
+	//void Update() noexcept;
+	void Seek(std::unique_lock<Mutex> &lock, offset_type offset) override;
+	bool IsEOF() const noexcept override;
+	/* we don't support tags */
+	// std::unique_ptr<Tag> ReadTag() override;
+	bool IsAvailable() const noexcept override;
+	size_t Read(std::unique_lock<Mutex> &lock,
+		    void *ptr, size_t size) override;
+
+private:
+	/* virtual methods from class InputCacheLease */
+	void OnInputCacheAvailable() noexcept override;
+};
+
+#endif

src/input/meson.build

@@ -35,6 +35,10 @@ input_glue = static_library(
   'BufferingInputStream.cxx',
   'BufferedInputStream.cxx',
   'MaybeBufferedInputStream.cxx',
+  'cache/Config.cxx',
+  'cache/Manager.cxx',
+  'cache/Item.cxx',
+  'cache/Stream.cxx',
   include_directories: inc,
 )
 

src/player/Control.cxx

@@ -28,10 +28,12 @@
 
 PlayerControl::PlayerControl(PlayerListener &_listener,
 			     PlayerOutputs &_outputs,
+			     InputCacheManager *_input_cache,
 			     unsigned _buffer_chunks,
 			     AudioFormat _configured_audio_format,
 			     const ReplayGainConfig &_replay_gain_config) noexcept
 	:listener(_listener), outputs(_outputs),
+	 input_cache(_input_cache),
 	 buffer_chunks(_buffer_chunks),
 	 configured_audio_format(_configured_audio_format),
 	 thread(BIND_THIS_METHOD(RunThread)),

src/player/Control.hxx

@@ -39,6 +39,7 @@
 struct Tag;
 class PlayerListener;
 class PlayerOutputs;
+class InputCacheManager;
 class DetachedSong;
 
 enum class PlayerState : uint8_t {
@@ -116,6 +117,8 @@ class PlayerControl final : public AudioOutputClient {
 
 	PlayerOutputs &outputs;
 
+	InputCacheManager *const input_cache;
+
 	const unsigned buffer_chunks;
 
 	/**
@@ -234,6 +237,7 @@ class PlayerControl final : public AudioOutputClient {
 public:
 	PlayerControl(PlayerListener &_listener,
 		      PlayerOutputs &_outputs,
+		      InputCacheManager *_input_cache,
 		      unsigned buffer_chunks,
 		      AudioFormat _configured_audio_format,
 		      const ReplayGainConfig &_replay_gain_config) noexcept;

src/player/Thread.cxx

@@ -1132,6 +1132,7 @@ try {
 	SetThreadName("player");
 
 	DecoderControl dc(mutex, cond,
+			  input_cache,
 			  configured_audio_format,
 			  replay_gain_config);
 	dc.StartThread();