This commit adds a new protocol command to toggle protocol features

for a client connection. It works like the tag_mask and the associated
tagtypes command.

New commands:

- protocol
  Shows enabled protocol features.

- protocol available
  Show all available protocol features.

- protocol enable {feature...}
  Enables protocol features.

- protocol disable {feature...}
  Disables protocol features.

- protocol all
  Enables all available protocol features.

- protocol clear
  Disables all protocol features.

This commit adds also the first protocol feature.

hide_playlists_in_root
  Disables the listing of playlists in the root folder
  for the lsinfo command.

doc/protocol.rst

@@ -1678,6 +1678,44 @@ Connection settings
     Announce that this client is interested in all tag
     types.  This is the default setting for new clients.
 
+.. _command_protocol:
+
+:command:`protocol`
+    Shows a list of enabled protocol features.
+
+    Available features:
+
+    - ``hide_playlists_in_root``: disables the listing of
+      stored playlists for the :ref:`lsinfo <command_lsinfo>`.
+
+    The following ``protocol`` sub commands configure the
+    protocol features.
+
+.. _command_protocol_disable:
+
+:command:`protocol disable {FEATURE...}`
+    Disables one or more features.
+
+.. _command_protocol_enable:
+
+:command:`protocol enable {FEATURE...}`
+    Enables one or more features.
+
+.. _command_protocol_clear:
+
+:command:`protocol clear`
+    Disables all protocol features.
+
+.. _command_protocol_all:
+
+:command:`protocol all`
+    Enables all protocol features.
+
+.. _command_protocol_available:
+
+:command:`protocol available`
+    Lists all available protocol features.
+
 .. _partition_commands:
 
 Partition commands

meson.build

@@ -379,6 +379,7 @@ sources = [
   'src/client/File.cxx',
   'src/client/Response.cxx',
   'src/client/ThreadBackgroundCommand.cxx',
+  'src/client/ProtocolFeature.cxx',
   'src/Listen.cxx',
   'src/LogInit.cxx',
   'src/ls.cxx',

src/client/Client.hxx

@@ -5,6 +5,7 @@
 
 #include "IClient.hxx"
 #include "Message.hxx"
+#include "ProtocolFeature.hxx"
 #include "command/CommandResult.hxx"
 #include "command/CommandListBuilder.hxx"
 #include "input/LastInputStream.hxx"
@@ -111,6 +112,11 @@ private:
 	 */
 	std::unique_ptr<BackgroundCommand> background_command;
 
+	/**
+	 * Bitmask of protocol features.
+	 */
+	ProtocolFeature protocol_feature = ProtocolFeature::None();
+
 public:
 	Client(EventLoop &loop, Partition &partition,
 	       UniqueSocketDescriptor fd, int uid,
@@ -167,6 +173,29 @@ public:
 		permission = _permission;
 	}
 
+	ProtocolFeature GetProtocolFeatures() const noexcept {
+		return protocol_feature;
+	}
+
+	void SetProtocolFeatures(ProtocolFeature features, bool enable) noexcept {
+		if (enable)
+			protocol_feature.Set(features);
+		else
+			protocol_feature.Unset(features);
+	}
+
+	void AllProtocolFeatures() noexcept {
+		protocol_feature.SetAll();
+	}
+
+	void ClearProtocolFeatures() noexcept {
+		protocol_feature.Clear();
+	}
+
+	bool ProtocolFeatureEnabled(enum ProtocolFeatureType value) noexcept {
+		return protocol_feature.Test(value);
+	}
+
 	/**
 	 * Send "idle" response to this client.
 	 */

src/client/ProtocolFeature.cxx

@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright The Music Player Daemon Project
+
+#include "ProtocolFeature.hxx"
+#include "Client.hxx"
+#include "Response.hxx"
+#include "util/StringAPI.hxx"
+
+#include <cassert>
+#include <fmt/format.h>
+
+
+struct feature_type_table {
+	const char *name;
+
+	ProtocolFeatureType type;
+};
+
+static constexpr struct feature_type_table protocol_feature_names_init[] = {
+	{"hide_playlists_in_root", PF_HIDE_PLAYLISTS_IN_ROOT},
+};
+
+/**
+ * This function converts the #tag_item_names_init array to an
+ * associative array at compile time.  This is a kludge because C++20
+ * doesn't support designated initializers for arrays, unlike C99.
+ */
+static constexpr auto
+MakeProtocolFeatureNames() noexcept
+{
+	std::array<const char *, PF_NUM_OF_ITEM_TYPES> result{};
+
+	static_assert(std::size(protocol_feature_names_init) == result.size());
+
+	for (const auto &i : protocol_feature_names_init) {
+		/* no duplicates allowed */
+		assert(result[i.type] == nullptr);
+
+		result[i.type] = i.name;
+	}
+
+	return result;
+}
+
+constinit const std::array<const char *, PF_NUM_OF_ITEM_TYPES> protocol_feature_names = MakeProtocolFeatureNames();
+
+void
+protocol_features_print(Client &client, Response &r) noexcept
+{
+	const auto protocol_feature = client.GetProtocolFeatures();
+	for (unsigned i = 0; i < PF_NUM_OF_ITEM_TYPES; i++)
+		if (protocol_feature.Test(ProtocolFeatureType(i)))
+			r.Fmt(FMT_STRING("feature: {}\n"), protocol_feature_names[i]);
+}
+
+void
+protocol_features_print_all(Response &r) noexcept
+{
+	for (unsigned i = 0; i < PF_NUM_OF_ITEM_TYPES; i++)
+		r.Fmt(FMT_STRING("feature: {}\n"), protocol_feature_names[i]);
+}
+
+ProtocolFeatureType
+protocol_feature_parse_i(const char *name) noexcept
+{
+	for (unsigned i = 0; i < PF_NUM_OF_ITEM_TYPES; ++i) {
+		assert(protocol_feature_names[i] != nullptr);
+
+		if (StringIsEqualIgnoreCase(name, protocol_feature_names[i]))
+			return (ProtocolFeatureType)i;
+	}
+
+	return PF_NUM_OF_ITEM_TYPES;
+}

src/client/ProtocolFeature.hxx

@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright The Music Player Daemon Project
+
+#pragma once
+
+#include <string_view>
+#include <cstdint>
+
+class Client;
+class Response;
+
+/**
+ * Codes for the type of a protocol feature.
+ */
+enum ProtocolFeatureType : uint8_t {
+	PF_HIDE_PLAYLISTS_IN_ROOT,
+
+	PF_NUM_OF_ITEM_TYPES
+};
+
+class ProtocolFeature {
+	using protocol_feature_t = uint_least8_t;
+
+	/* must have enough bits to represent all protocol features
+	   supported by MPD */
+	static_assert(PF_NUM_OF_ITEM_TYPES <= sizeof(protocol_feature_t) * 8);
+
+	protocol_feature_t value;
+
+	explicit constexpr ProtocolFeature(protocol_feature_t _value) noexcept
+		:value(_value) {}
+
+public:
+	constexpr ProtocolFeature() noexcept = default;
+
+	constexpr ProtocolFeature(ProtocolFeatureType _value) noexcept
+		:value(protocol_feature_t(1) << protocol_feature_t(_value)) {}
+
+	static constexpr ProtocolFeature None() noexcept {
+		return ProtocolFeature(protocol_feature_t(0));
+	}
+
+	static constexpr ProtocolFeature All() noexcept {
+		return ~None();
+	}
+
+	constexpr ProtocolFeature operator~() const noexcept {
+		return ProtocolFeature(~value);
+	}
+
+	constexpr ProtocolFeature operator&(ProtocolFeature other) const noexcept {
+		return ProtocolFeature(value & other.value);
+	}
+
+	constexpr ProtocolFeature operator|(ProtocolFeature other) const noexcept {
+		return ProtocolFeature(value | other.value);
+	}
+
+	constexpr ProtocolFeature operator^(ProtocolFeature other) const noexcept {
+		return ProtocolFeature(value ^ other.value);
+	}
+
+	constexpr ProtocolFeature &operator&=(ProtocolFeature other) noexcept {
+		value &= other.value;
+		return *this;
+	}
+
+	constexpr ProtocolFeature &operator|=(ProtocolFeature other) noexcept {
+		value |= other.value;
+		return *this;
+	}
+
+	constexpr ProtocolFeature &operator^=(ProtocolFeature other) noexcept {
+		value ^= other.value;
+		return *this;
+	}
+
+	constexpr bool TestAny() const noexcept {
+		return value != 0;
+	}
+
+	constexpr bool Test(ProtocolFeatureType feature) const noexcept {
+		return (*this & feature).TestAny();
+	}
+
+	constexpr void Set(ProtocolFeature features) noexcept {
+		*this |= features;
+	}
+
+	constexpr void Unset(ProtocolFeature features) noexcept {
+		*this &= ~ProtocolFeature(features);
+	}
+
+	constexpr void SetAll() noexcept {
+		*this = ProtocolFeature::All();
+	}
+
+	constexpr void Clear() noexcept {
+		*this = ProtocolFeature::None();
+	}
+};
+
+void
+protocol_features_print(Client &client, Response &r) noexcept;
+
+void
+protocol_features_print_all(Response &r) noexcept;
+
+ProtocolFeatureType
+protocol_feature_parse_i(const char *name) noexcept;

src/command/AllCommands.cxx

@@ -156,6 +156,7 @@ static constexpr struct command commands[] = {
 	{ "previous", PERMISSION_PLAYER, 0, 0, handle_previous },
 	{ "prio", PERMISSION_PLAYER, 2, -1, handle_prio },
 	{ "prioid", PERMISSION_PLAYER, 2, -1, handle_prioid },
+	{ "protocol", PERMISSION_NONE, 0, -1, handle_protocol },
 	{ "random", PERMISSION_PLAYER, 1, 1, handle_random },
 	{ "rangeid", PERMISSION_ADD, 2, 2, handle_rangeid },
 	{ "readcomments", PERMISSION_READ, 1, 1, handle_read_comments },

src/command/ClientCommands.cxx

@@ -109,3 +109,67 @@ handle_tagtypes(Client &client, Request request, Response &r)
 		return CommandResult::ERROR;
 	}
 }
+
+static ProtocolFeature
+ParseProtocolFeature(Request request)
+{
+	if (request.empty())
+		throw ProtocolError(ACK_ERROR_ARG, "Not enough arguments");
+
+	ProtocolFeature result = ProtocolFeature::None();
+
+	for (const char *name : request) {
+		auto type = protocol_feature_parse_i(name);
+		if (type == PF_NUM_OF_ITEM_TYPES)
+			throw ProtocolError(ACK_ERROR_ARG, "Unknown protcol feature");
+
+		result |= type;
+	}
+
+	return result;
+}
+
+CommandResult
+handle_protocol(Client &client, Request request, Response &r)
+{
+	if (request.empty()) {
+		protocol_features_print(client, r);
+		return CommandResult::OK;
+	}
+
+	const char *cmd = request.shift();
+	if (StringIsEqual(cmd, "all")) {
+		if (!request.empty()) {
+			r.Error(ACK_ERROR_ARG, "Too many arguments");
+			return CommandResult::ERROR;
+		}
+
+		client.AllProtocolFeatures();
+		return CommandResult::OK;
+	} else if (StringIsEqual(cmd, "clear")) {
+		if (!request.empty()) {
+			r.Error(ACK_ERROR_ARG, "Too many arguments");
+			return CommandResult::ERROR;
+		}
+
+		client.ClearProtocolFeatures();
+		return CommandResult::OK;
+	} else if (StringIsEqual(cmd, "enable")) {
+		client.SetProtocolFeatures(ParseProtocolFeature(request), true);
+		return CommandResult::OK;
+	} else if (StringIsEqual(cmd, "disable")) {
+		client.SetProtocolFeatures(ParseProtocolFeature(request), false);
+		return CommandResult::OK;
+	} else if (StringIsEqual(cmd, "available")) {
+		if (!request.empty()) {
+			r.Error(ACK_ERROR_ARG, "Too many arguments");
+			return CommandResult::ERROR;
+		}
+
+		protocol_features_print_all(r);
+		return CommandResult::OK;
+	} else {
+		r.Error(ACK_ERROR_ARG, "Unknown sub command");
+		return CommandResult::ERROR;
+	}
+}

src/command/ClientCommands.hxx

@@ -25,4 +25,7 @@ handle_password(Client &client, Request request, Response &response);
 CommandResult
 handle_tagtypes(Client &client, Request request, Response &response);
 
+CommandResult
+handle_protocol(Client &client, Request request, Response &response);
+
 #endif

src/command/OtherCommands.cxx

@@ -160,7 +160,7 @@ handle_lsinfo_relative(Client &client, Response &r, const char *uri)
 	(void)client;
 #endif
 
-	if (isRootDirectory(uri)) {
+	if (!client.ProtocolFeatureEnabled(PF_HIDE_PLAYLISTS_IN_ROOT) && isRootDirectory(uri)) {
 		try {
 			print_spl_list(r, ListPlaylistFiles());
 		} catch (...) {