protocol: add command "binarylimit"

Increasing the protocol version to 0.22.4 to allow clients to detect
this feature.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1038

NEWS

@@ -1,5 +1,6 @@
 ver 0.22.4 (not yet released)
 * protocol
+  - add command "binarylimit" to allow larger chunk sizes
   - fix "readpicture" on 32 bit machines
   - show duration and tags of songs in virtual playlist (CUE) folders
 * storage

doc/protocol.rst

@@ -69,11 +69,14 @@ that, the specified number of bytes of binary data follows, then a
 newline, and finally the ``OK`` line.
 
 If the object to be transmitted is large, the server may choose a
-reasonable chunk size and transmit only a portion.  Usually, the
-response also contains a ``size`` line which specifies the total
-(uncropped) size, and the command usually has a way to specify an
-offset into the object; this way, the client can copy the whole file
-without blocking the connection for too long.
+reasonable chunk size and transmit only a portion.  The maximum chunk
+size can be changed by clients with the :ref:`binarylimit
+<command_binarylimit>` command.
+
+Usually, the response also contains a ``size`` line which specifies
+the total (uncropped) size, and the command usually has a way to
+specify an offset into the object; this way, the client can copy the
+whole file without blocking the connection for too long.
 
 Example::
 
@@ -1355,6 +1358,17 @@ Connection settings
 :command:`ping`
     Does nothing but return "OK".
 
+.. _command_binarylimit:
+
+:command:`binarylimit SIZE` [#since_0_22_4]_
+
+    Set the maximum :ref:`binary response <binary>` size for the
+    current connection to the specified number of bytes.
+
+    A bigger value means less overhead for transmitting large
+    entities, but it also means that the connection is blocked for a
+    longer time.
+
 .. _command_tagtypes:
 
 :command:`tagtypes`
@@ -1583,3 +1597,4 @@ client-to-client messages are local to the current partition.
 .. [#since_0_19] Since :program:`MPD` 0.19
 .. [#since_0_20] Since :program:`MPD` 0.20
 .. [#since_0_21] Since :program:`MPD` 0.21
+.. [#since_0_22_4] Since :program:`MPD` 0.22.4

meson.build

@@ -32,7 +32,7 @@ version_conf = configuration_data()
 version_conf.set_quoted('PACKAGE', meson.project_name())
 version_conf.set_quoted('PACKAGE_NAME', meson.project_name())
 version_conf.set_quoted('VERSION', meson.project_version())
-version_conf.set_quoted('PROTOCOL_VERSION', '0.22.0')
+version_conf.set_quoted('PROTOCOL_VERSION', '0.22.4')
 configure_file(output: 'Version.h', configuration: version_conf)
 
 conf = configuration_data()

src/client/Client.hxx

@@ -84,6 +84,12 @@ public:
 	 */
 	TagMask tag_mask = TagMask::All();
 
+	/**
+	 * The maximum number of bytes transmitted in a binary
+	 * response.  Can be changed with the "binarylimit" command.
+	 */
+	size_t binary_limit = 8192;
+
 private:
 	static constexpr size_t MAX_SUBSCRIPTIONS = 16;
 
@@ -122,6 +128,7 @@ public:
 	~Client() noexcept;
 
 	using FullyBufferedSocket::GetEventLoop;
+	using FullyBufferedSocket::GetOutputMaxSize;
 
 	gcc_pure
 	bool IsExpired() const noexcept {

src/client/Response.cxx

@@ -59,7 +59,7 @@ Response::Format(const char *fmt, ...) noexcept
 bool
 Response::WriteBinary(ConstBuffer<void> payload) noexcept
 {
-	assert(payload.size <= MAX_BINARY_SIZE);
+	assert(payload.size <= client.binary_limit);
 
 	return Format("binary: %zu\n", payload.size) &&
 		Write(payload.data, payload.size) &&

src/client/Response.hxx

@@ -79,8 +79,6 @@ public:
 	gcc_printf(2,3)
 	bool Format(const char *fmt, ...) noexcept;
 
-	static constexpr size_t MAX_BINARY_SIZE = 8192;
-
 	/**
 	 * Write a binary chunk; this writes the "binary" line, the
 	 * given chunk and the trailing newline.

src/command/AllCommands.cxx

@@ -87,6 +87,7 @@ static constexpr struct command commands[] = {
 	{ "addid", PERMISSION_ADD, 1, 2, handle_addid },
 	{ "addtagid", PERMISSION_ADD, 3, 3, handle_addtagid },
 	{ "albumart", PERMISSION_READ, 2, 2, handle_album_art },
+	{ "binarylimit", PERMISSION_NONE, 1, 1, handle_binary_limit },
 	{ "channels", PERMISSION_READ, 0, 0, handle_channels },
 	{ "clear", PERMISSION_CONTROL, 0, 0, handle_clear },
 	{ "clearerror", PERMISSION_CONTROL, 0, 0, handle_clearerror },

src/command/ClientCommands.cxx

@@ -40,6 +40,21 @@ handle_ping([[maybe_unused]] Client &client, [[maybe_unused]] Request args,
 	return CommandResult::OK;
 }
 
+CommandResult
+handle_binary_limit(Client &client, Request args,
+		    [[maybe_unused]] Response &r)
+{
+	size_t value = args.ParseUnsigned(0, client.GetOutputMaxSize() - 4096);
+	if (value < 64) {
+		r.Error(ACK_ERROR_ARG, "Value too small");
+		return CommandResult::ERROR;
+	}
+
+	client.binary_limit = value;
+
+	return CommandResult::OK;
+}
+
 CommandResult
 handle_password(Client &client, Request args, Response &r)
 {

src/command/ClientCommands.hxx

@@ -32,6 +32,9 @@ handle_close(Client &client, Request request, Response &response);
 CommandResult
 handle_ping(Client &client, Request request, Response &response);
 
+CommandResult
+handle_binary_limit(Client &client, Request request, Response &response);
+
 CommandResult
 handle_password(Client &client, Request request, Response &response);
 

src/command/FileCommands.cxx

@@ -43,6 +43,7 @@
 #include "thread/Mutex.hxx"
 #include "Log.hxx"
 
+#include <algorithm>
 #include <cassert>
 #include <cinttypes> /* for PRIu64 */
 
@@ -205,28 +206,26 @@ read_stream_art(Response &r, const char *uri, size_t offset)
 
 	const offset_type art_file_size = is->GetSize();
 
-	if (offset >= art_file_size) {
-		if (offset > art_file_size) {
-			r.Error(ACK_ERROR_ARG, "Offset too large");
-			return CommandResult::ERROR;
-		} else {
-			r.Format("size: %" PRIoffset "\n", art_file_size);
-			r.WriteBinary(nullptr);
-			return CommandResult::OK;
-		}
+	if (offset > art_file_size) {
+		r.Error(ACK_ERROR_ARG, "Offset too large");
+		return CommandResult::ERROR;
 	}
 
-	uint8_t buffer[Response::MAX_BINARY_SIZE];
-	size_t read_size;
+	std::size_t buffer_size =
+		std::min<offset_type>(art_file_size - offset,
+				      r.GetClient().binary_limit);
 
-	{
+	std::unique_ptr<std::byte[]> buffer(new std::byte[buffer_size]);
+
+	std::size_t read_size = 0;
+	if (buffer_size > 0) {
 		std::unique_lock<Mutex> lock(mutex);
 		is->Seek(lock, offset);
-		read_size = is->Read(lock, &buffer, sizeof(buffer));
+		read_size = is->Read(lock, buffer.get(), buffer_size);
 	}
 
 	r.Format("size: %" PRIoffset "\n", art_file_size);
-	r.WriteBinary({buffer, read_size});
+	r.WriteBinary({buffer.get(), read_size});
 
 	return CommandResult::OK;
 }
@@ -313,8 +312,10 @@ public:
 			response.Format("type: %s\n", mime_type);
 
 		buffer.size -= offset;
-		if (buffer.size > Response::MAX_BINARY_SIZE)
-			buffer.size = Response::MAX_BINARY_SIZE;
+
+		const std::size_t binary_limit = response.GetClient().binary_limit;
+		if (buffer.size > binary_limit)
+			buffer.size = binary_limit;
 		buffer.data = OffsetPointer(buffer.data, offset);
 
 		response.WriteBinary(buffer);