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.

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;