util/IntrusiveList: add enum LinkMode

Compile-time code simplification.

src/RemoteTagCache.hxx

@@ -46,7 +46,7 @@ class RemoteTagCache final {
 
 	struct Item final
 		: public boost::intrusive::unordered_set_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>>,
-		  public IntrusiveListHook,
+		  public IntrusiveListHook<>,
 		  RemoteTagHandler
 	{
 		RemoteTagCache &parent;

src/client/Client.hxx

@@ -53,7 +53,7 @@ class Client final
 	friend struct ClientPerPartitionListHook;
 	friend class ClientList;
 
-	IntrusiveListHook list_siblings, partition_siblings;
+	IntrusiveListHook<> list_siblings, partition_siblings;
 
 	CoarseTimerEvent timeout_event;
 

src/db/plugins/simple/Directory.hxx

@@ -51,7 +51,7 @@ static constexpr unsigned DEVICE_PLAYLIST = -3;
 
 class SongFilter;
 
-struct Directory : IntrusiveListHook {
+struct Directory : IntrusiveListHook<> {
 	/* Note: the #IntrusiveListHook is protected with the global
 	   #db_mutex.  Read access in the update thread does not need
 	   protection. */

src/db/plugins/simple/Song.hxx

@@ -39,7 +39,7 @@ class ArchiveFile;
  * A song file inside the configured music directory.  Internal
  * #SimpleDatabase class.
  */
-struct Song : IntrusiveListHook {
+struct Song : IntrusiveListHook<> {
 	/* Note: the #IntrusiveListHook is protected with the global
 	   #db_mutex.  Read access in the update thread does not need
 	   protection. */

src/event/SocketEvent.hxx

@@ -40,7 +40,9 @@ class EventLoop;
  * thread that runs the #EventLoop, except where explicitly documented
  * as thread-safe.
  */
-class SocketEvent final : IntrusiveListHook, public EventPollBackendEvents
+class SocketEvent final
+	: IntrusiveListHook<IntrusiveHookMode::NORMAL>,
+	  public EventPollBackendEvents
 {
 	friend class EventLoop;
 	friend struct IntrusiveListBaseHookTraits<SocketEvent>;

src/input/cache/Lease.hxx

@@ -29,7 +29,7 @@
  * A lease for an #InputCacheItem.
  */
 class InputCacheLease
-	: public IntrusiveListHook
+	: public IntrusiveListHook<>
 {
 	InputCacheItem *item = nullptr;
 

src/io/uring/CancellableOperation.hxx

@@ -40,7 +40,8 @@
 
 namespace Uring {
 
-class CancellableOperation : public IntrusiveListHook
+class CancellableOperation
+	: public IntrusiveListHook<IntrusiveHookMode::NORMAL>
 {
 	Operation *operation;
 

src/lib/nfs/Cancellable.hxx

@@ -27,7 +27,7 @@
 
 template<typename T>
 class CancellablePointer
-	: public IntrusiveListHook
+	: public IntrusiveListHook<>
 {
 public:
 	typedef T *pointer;

src/lib/upnp/Discovery.hxx

@@ -79,7 +79,7 @@ class UPnPDeviceDirectory final : UpnpCallback {
 	};
 
 	class Downloader final
-		: public IntrusiveListHook, CurlResponseHandler
+		: public IntrusiveListHook<>, CurlResponseHandler
 	{
 		InjectEvent defer_start_event;
 

src/output/plugins/httpd/HttpdClient.hxx

@@ -34,7 +34,7 @@ class HttpdOutput;
 
 class HttpdClient final
 	: BufferedSocket,
-	  public IntrusiveListHook
+	  public IntrusiveListHook<>
 {
 	/**
 	 * The httpd output object this client is connected to.

src/output/plugins/snapcast/Client.hxx

@@ -33,7 +33,7 @@ struct SnapcastTime;
 class SnapcastOutput;
 class UniqueSocketDescriptor;
 
-class SnapcastClient final : BufferedSocket, public IntrusiveListHook
+class SnapcastClient final : BufferedSocket, public IntrusiveListHook<>
 {
 	SnapcastOutput &output;
 

src/util/IntrusiveHookMode.hxx

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2022 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+/**
+ * Specifies the mode in which a hook for intrusive containers
+ * operates.  This is meant to be used as a template argument to the
+ * hook class (e.g. #IntrusiveListHook).
+ */
+enum class IntrusiveHookMode {
+	/**
+	 * No implicit initialization.
+	 */
+	NORMAL,
+
+	/**
+	 * Keep track of whether the item is currently linked, allows
+	 * using method is_linked().  This requires implicit
+	 * initialization and requires iterating all items when
+	 * deleting them which adds a considerable amount of overhead.
+	 */
+	TRACK,
+
+	/**
+	 * Automatically unlinks the item in the destructor.  This
+	 * implies #TRACK and adds code to the destructor.
+	 */
+	AUTO_UNLINK,
+};

src/util/IntrusiveList.hxx

@@ -30,6 +30,7 @@
 #pragma once
 
 #include "Cast.hxx"
+#include "IntrusiveHookMode.hxx"
 #include "MemberPointer.hxx"
 #include "OptionalCounter.hxx"
 
@@ -47,6 +48,7 @@ struct IntrusiveListNode {
 	}
 };
 
+template<IntrusiveHookMode _mode=IntrusiveHookMode::NORMAL>
 class IntrusiveListHook {
 	template<typename T> friend struct IntrusiveListBaseHookTraits;
 	template<auto member> friend struct IntrusiveListMemberHookTraits;
@@ -56,13 +58,33 @@ protected:
 	IntrusiveListNode siblings;
 
 public:
-	IntrusiveListHook() noexcept = default;
+	static constexpr IntrusiveHookMode mode = _mode;
+
+	IntrusiveListHook() noexcept {
+		if constexpr (mode >= IntrusiveHookMode::TRACK)
+			siblings.next = nullptr;
+	}
+
+	~IntrusiveListHook() noexcept {
+		if constexpr (mode >= IntrusiveHookMode::AUTO_UNLINK)
+			if (is_linked())
+				unlink();
+	}
 
 	IntrusiveListHook(const IntrusiveListHook &) = delete;
 	IntrusiveListHook &operator=(const IntrusiveListHook &) = delete;
 
 	void unlink() noexcept {
 		IntrusiveListNode::Connect(*siblings.prev, *siblings.next);
+
+		if constexpr (mode >= IntrusiveHookMode::TRACK)
+			siblings.next = nullptr;
+	}
+
+	bool is_linked() const noexcept {
+		static_assert(mode >= IntrusiveHookMode::TRACK);
+
+		return siblings.next != nullptr;
 	}
 
 private:
@@ -75,52 +97,27 @@ private:
 	}
 };
 
-/**
+using SafeLinkIntrusiveListHook =
- * A variant of #IntrusiveListHook which keeps track of whether it is
+	IntrusiveListHook<IntrusiveHookMode::TRACK>;
- * currently in a list.
+using AutoUnlinkIntrusiveListHook =
- */
+	IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>;
-class SafeLinkIntrusiveListHook : public IntrusiveListHook {
-public:
-	SafeLinkIntrusiveListHook() noexcept {
-		siblings.next = nullptr;
-	}
-
-	void unlink() noexcept {
-		IntrusiveListHook::unlink();
-		siblings.next = nullptr;
-	}
-
-	bool is_linked() const noexcept {
-		return siblings.next != nullptr;
-	}
-};
 
 /**
- * A variant of #IntrusiveListHook which auto-unlinks itself from the
+ * Detect the hook type which is embedded in the given type as a base
- * list upon destruction.  As a side effect, it has an is_linked()
+ * class.  This is a template to postpone the type checks, to allow
- * method.
- */
-class AutoUnlinkIntrusiveListHook : public SafeLinkIntrusiveListHook {
-public:
-	~AutoUnlinkIntrusiveListHook() noexcept {
-		if (is_linked())
-			unlink();
-	}
-};
-
-/**
- * Detect the hook type; this is important because
- * SafeLinkIntrusiveListHook::unlink() needs to clear the "next"
- * pointer.  This is a template to postpone the type checks, to allow
  * forward-declared types.
  */
 template<typename U>
 struct IntrusiveListHookDetection {
-	static_assert(std::is_base_of_v<IntrusiveListHook, U>);
+	/* TODO can this be simplified somehow, without checking for
-
+	   all possible enum values? */
-	using type = std::conditional_t<std::is_base_of_v<SafeLinkIntrusiveListHook, U>,
+	using type = std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::NORMAL>, U>,
-					SafeLinkIntrusiveListHook,
+					IntrusiveListHook<IntrusiveHookMode::NORMAL>,
-					IntrusiveListHook>;
+					std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::TRACK>, U>,
+							   IntrusiveListHook<IntrusiveHookMode::TRACK>,
+							   std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>, U>,
+									      IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>,
+									      void>>>;
 };
 
 /**
@@ -131,10 +128,6 @@ struct IntrusiveListBaseHookTraits {
 	template<typename U>
 	using Hook = typename IntrusiveListHookDetection<U>::type;
 
-	static constexpr bool IsAutoUnlink() noexcept {
-		return std::is_base_of_v<AutoUnlinkIntrusiveListHook, T>;
-	}
-
 	static constexpr T *Cast(IntrusiveListNode *node) noexcept {
 		auto *hook = &Hook<T>::Cast(*node);
 		return static_cast<T *>(hook);
@@ -152,14 +145,12 @@ template<auto member>
 struct IntrusiveListMemberHookTraits {
 	using T = MemberPointerContainerType<decltype(member)>;
 	using _Hook = MemberPointerType<decltype(member)>;
-	using Hook = typename IntrusiveListHookDetection<_Hook>::type;
 
-	static constexpr bool IsAutoUnlink() noexcept {
+	template<typename Dummy>
-		return std::is_base_of_v<AutoUnlinkIntrusiveListHook, _Hook>;
+	using Hook = _Hook;
-	}
 
 	static constexpr T *Cast(IntrusiveListNode *node) noexcept {
-		auto &hook = Hook::Cast(*node);
+		auto &hook = Hook<T>::Cast(*node);
 		return &ContainerCast(hook, member);
 	}
 
@@ -176,14 +167,15 @@ template<typename T,
 	 typename HookTraits=IntrusiveListBaseHookTraits<T>,
 	 bool constant_time_size=false>
 class IntrusiveList {
-	template<typename U>
-	using Hook = typename IntrusiveListHookDetection<U>::type;
-
 	IntrusiveListNode head{&head, &head};
 
 	[[no_unique_address]]
 	OptionalCounter<constant_time_size> counter;
 
+	static constexpr auto GetHookMode() noexcept {
+		return HookTraits::template Hook<T>::mode;
+	}
+
 	static constexpr T *Cast(IntrusiveListNode *node) noexcept {
 		return HookTraits::Cast(node);
 	}
@@ -234,7 +226,7 @@ public:
 	}
 
 	~IntrusiveList() noexcept {
-		if constexpr (std::is_base_of_v<SafeLinkIntrusiveListHook, T>)
+		if constexpr (GetHookMode() >= IntrusiveHookMode::TRACK)
 			clear();
 	}
 
@@ -283,7 +275,7 @@ public:
 	}
 
 	void clear() noexcept {
-		if constexpr (std::is_base_of_v<SafeLinkIntrusiveListHook, T>) {
+		if constexpr (GetHookMode() >= IntrusiveHookMode::TRACK) {
 			/* for SafeLinkIntrusiveListHook, we need to
 			   remove each item manually, or else its
 			   is_linked() method will not work */
@@ -504,7 +496,7 @@ public:
 
 	void insert(iterator p, reference t) noexcept {
 		static_assert(!constant_time_size ||
-			      !HookTraits::IsAutoUnlink(),
+			      GetHookMode() < IntrusiveHookMode::AUTO_UNLINK,
 			      "Can't use auto-unlink hooks with constant_time_size");
 
 		auto &existing_node = ToNode(*p);

test/util/TestIntrusiveList.cxx

@@ -36,17 +36,17 @@
 
 namespace {
 
-template<typename Hook>
+template<IntrusiveHookMode mode>
-struct CharItem final : Hook {
+struct CharItem final : IntrusiveListHook<mode> {
 	char ch;
 
 	constexpr CharItem(char _ch) noexcept:ch(_ch) {}
 };
 
-template<typename Hook>
+template<IntrusiveHookMode mode>
 static std::string
-ToString(const IntrusiveList<CharItem<Hook>> &list,
+ToString(const IntrusiveList<CharItem<mode>> &list,
-	 typename IntrusiveList<CharItem<Hook>>::const_iterator it,
+	 typename IntrusiveList<CharItem<mode>>::const_iterator it,
 	 std::size_t n) noexcept
 {
 	std::string result;
@@ -55,10 +55,10 @@ ToString(const IntrusiveList<CharItem<Hook>> &list,
 	return result;
 }
 
-template<typename Hook>
+template<IntrusiveHookMode mode>
 static std::string
-ToStringReverse(const IntrusiveList<CharItem<Hook>> &list,
+ToStringReverse(const IntrusiveList<CharItem<mode>> &list,
-		typename IntrusiveList<CharItem<Hook>>::const_iterator it,
+		typename IntrusiveList<CharItem<mode>>::const_iterator it,
 		std::size_t n) noexcept
 {
 	std::string result;
@@ -71,7 +71,7 @@ ToStringReverse(const IntrusiveList<CharItem<Hook>> &list,
 
 TEST(IntrusiveList, Basic)
 {
-	using Item = CharItem<IntrusiveListHook>;
+	using Item = CharItem<IntrusiveHookMode::NORMAL>;
 
 	Item items[]{'a', 'b', 'c'};
 
@@ -103,9 +103,9 @@ TEST(IntrusiveList, Basic)
 	ASSERT_EQ(ToStringReverse(list, list.begin(), 6), "a_cfea");
 }
 
-TEST(IntrusiveList, SafeLink)
+TEST(IntrusiveList, Track)
 {
-	using Item = CharItem<SafeLinkIntrusiveListHook>;
+	using Item = CharItem<IntrusiveHookMode::TRACK>;
 
 	Item items[]{'a', 'b', 'c'};
 
@@ -162,7 +162,7 @@ TEST(IntrusiveList, SafeLink)
 
 TEST(IntrusiveList, AutoUnlink)
 {
-	using Item = CharItem<AutoUnlinkIntrusiveListHook>;
+	using Item = CharItem<IntrusiveHookMode::AUTO_UNLINK>;
 
 	Item a{'a'};
 	ASSERT_FALSE(a.is_linked());
@@ -194,7 +194,7 @@ TEST(IntrusiveList, AutoUnlink)
 
 TEST(IntrusiveList, Merge)
 {
-	using Item = CharItem<IntrusiveListHook>;
+	using Item = CharItem<IntrusiveHookMode::NORMAL>;
 
 	const auto predicate = [](const Item &a, const Item &b){
 		return a.ch < b.ch;
@@ -229,7 +229,7 @@ TEST(IntrusiveList, Merge)
 
 TEST(IntrusiveList, Sort)
 {
-	using Item = CharItem<IntrusiveListHook>;
+	using Item = CharItem<IntrusiveHookMode::NORMAL>;
 
 	const auto predicate = [](const Item &a, const Item &b){
 		return a.ch < b.ch;