diff --git a/src/util/IntrusiveList.hxx b/src/util/IntrusiveList.hxx
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+++ b/src/util/IntrusiveList.hxx
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+/*
+ * Copyright 2020 Max Kellermann <max.kellermann@gmail.com>
+ * All rights reserved.
+ *
+ * author: Max Kellermann <mk@cm4all.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
+
+#include "Cast.hxx"
+
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+struct IntrusiveListNode {
+	IntrusiveListNode *next, *prev;
+};
+
+class IntrusiveListHook {
+	template<typename T> friend class IntrusiveList;
+
+protected:
+	IntrusiveListNode siblings;
+
+public:
+	void unlink() noexcept {
+		siblings.next->prev = siblings.prev;
+		siblings.prev->next = siblings.next;
+	}
+
+private:
+	static constexpr auto &Cast(IntrusiveListNode &node) noexcept {
+		return ContainerCast(node, &IntrusiveListHook::siblings);
+	}
+
+	static constexpr const auto &Cast(const IntrusiveListNode &node) noexcept {
+		return ContainerCast(node, &IntrusiveListHook::siblings);
+	}
+};
+
+/**
+ * A variant of #IntrusiveListHook which auto-unlinks itself from the
+ * list upon destruction.  As a side effect, it has an is_linked()
+ * method.
+ */
+class AutoUnlinkIntrusiveListHook : public IntrusiveListHook {
+public:
+	AutoUnlinkIntrusiveListHook() noexcept {
+		siblings.next = nullptr;
+	}
+
+	~AutoUnlinkIntrusiveListHook() noexcept {
+		if (is_linked())
+			unlink();
+	}
+
+	void unlink() noexcept {
+		IntrusiveListHook::unlink();
+		siblings.next = nullptr;
+	}
+
+	bool is_linked() const noexcept {
+		return siblings.next != nullptr;
+	}
+};
+
+template<typename T>
+class IntrusiveList {
+	IntrusiveListNode head{&head, &head};
+
+	static constexpr T *Cast(IntrusiveListNode *node) noexcept {
+		static_assert(std::is_base_of<IntrusiveListHook, T>::value);
+		auto *hook = &IntrusiveListHook::Cast(*node);
+		return static_cast<T *>(hook);
+	}
+
+	static constexpr const T *Cast(const IntrusiveListNode *node) noexcept {
+		static_assert(std::is_base_of<IntrusiveListHook, T>::value);
+		const auto *hook = &IntrusiveListHook::Cast(*node);
+		return static_cast<const T *>(hook);
+	}
+
+	static constexpr IntrusiveListHook &ToHook(T &t) noexcept {
+		static_assert(std::is_base_of<IntrusiveListHook, T>::value);
+		return t;
+	}
+
+	static constexpr const IntrusiveListHook &ToHook(const T &t) noexcept {
+		static_assert(std::is_base_of<IntrusiveListHook, T>::value);
+		return t;
+	}
+
+	static constexpr IntrusiveListNode &ToNode(T &t) noexcept {
+		return ToHook(t).siblings;
+	}
+
+	static constexpr const IntrusiveListNode &ToNode(const T &t) noexcept {
+		return ToHook(t).siblings;
+	}
+
+public:
+	IntrusiveList() = default;
+
+	IntrusiveList(IntrusiveList &&src) noexcept {
+		if (src.empty())
+			return;
+
+		head = src.head;
+		head.next->prev = &head;
+		head.prev->next = &head;
+
+		src.head.next = &src.head;
+		src.head.prev = &src.head;
+	}
+
+	~IntrusiveList() noexcept {
+		if (std::is_base_of<AutoUnlinkIntrusiveListHook, T>::value)
+			clear();
+	}
+
+	IntrusiveList &operator=(IntrusiveList &&) = delete;
+
+	constexpr bool empty() const noexcept {
+		return head.next == &head;
+	}
+
+	void clear() noexcept {
+		if (std::is_base_of<AutoUnlinkIntrusiveListHook, T>::value) {
+			/* for AutoUnlinkIntrusiveListHook, we need to
+			   remove each item manually, or else its
+			   is_linked() method will not work */
+			while (!empty())
+				pop_front();
+		} else
+			head = {&head, &head};
+	}
+
+	template<typename D>
+	void clear_and_dispose(D &&disposer) noexcept {
+		while (!empty()) {
+			auto *item = &front();
+			pop_front();
+			disposer(item);
+		}
+	}
+
+	template<typename P, typename D>
+	void remove_and_dispose_if(P &&pred, D &&dispose) noexcept {
+		auto *n = head.next;
+
+		while (n != &head) {
+			auto *i = Cast(n);
+			n = n->next;
+
+			if (pred(*i)) {
+				i->unlink();
+				dispose(i);
+			}
+		}
+	}
+
+	const T &front() const noexcept {
+		return *Cast(head.next);
+	}
+
+	T &front() noexcept {
+		return *Cast(head.next);
+	}
+
+	void pop_front() noexcept {
+		front().unlink();
+	}
+
+	template<typename D>
+	void pop_front_and_dispose(D &&disposer) noexcept {
+		auto &i = front();
+		i.unlink();
+		disposer(&i);
+	}
+
+	T &back() noexcept {
+		return *Cast(head.prev);
+	}
+
+	void pop_back() noexcept {
+		back().unlink();
+	}
+
+	class const_iterator;
+
+	class iterator final
+		: public std::iterator<std::forward_iterator_tag, T> {
+
+		friend IntrusiveList;
+		friend const_iterator;
+
+		IntrusiveListNode *cursor;
+
+		constexpr iterator(IntrusiveListNode *_cursor) noexcept
+			:cursor(_cursor) {}
+
+	public:
+		iterator() = default;
+
+		constexpr bool operator==(const iterator &other) const noexcept {
+			return cursor == other.cursor;
+		}
+
+		constexpr bool operator!=(const iterator &other) const noexcept {
+			return !(*this == other);
+		}
+
+		constexpr T &operator*() const noexcept {
+			return *Cast(cursor);
+		}
+
+		constexpr T *operator->() const noexcept {
+			return Cast(cursor);
+		}
+
+		iterator &operator++() noexcept {
+			cursor = cursor->next;
+			return *this;
+		}
+	};
+
+	constexpr iterator begin() noexcept {
+		return {head.next};
+	}
+
+	constexpr iterator end() noexcept {
+		return {&head};
+	}
+
+	static constexpr iterator iterator_to(T &t) noexcept {
+		return {&t};
+	}
+
+	class const_iterator final
+		: public std::iterator<std::forward_iterator_tag, const T> {
+
+		friend IntrusiveList;
+
+		const IntrusiveListNode *cursor;
+
+		constexpr const_iterator(const IntrusiveListNode *_cursor) noexcept
+			:cursor(_cursor) {}
+
+	public:
+		const_iterator() = default;
+
+		const_iterator(iterator src) noexcept
+			:cursor(src.cursor) {}
+
+		constexpr bool operator==(const const_iterator &other) const noexcept {
+			return cursor == other.cursor;
+		}
+
+		constexpr bool operator!=(const const_iterator &other) const noexcept {
+			return !(*this == other);
+		}
+
+		constexpr const T &operator*() const noexcept {
+			return *Cast(cursor);
+		}
+
+		constexpr const T *operator->() const noexcept {
+			return Cast(cursor);
+		}
+
+		const_iterator &operator++() noexcept {
+			cursor = cursor->next;
+			return *this;
+		}
+	};
+
+	constexpr const_iterator begin() const noexcept {
+		return {head.next};
+	}
+
+	constexpr const_iterator end() const noexcept {
+		return {&head};
+	}
+
+	static constexpr iterator iterator_to(const T &t) noexcept {
+		return {&t};
+	}
+
+	void push_front(T &t) noexcept {
+		auto &new_node = ToNode(t);
+		head.next->prev = &new_node;
+		new_node.next = head.next;
+		head.next = &new_node;
+		new_node.prev = &head;
+	}
+
+	void push_back(T &t) noexcept {
+		auto &new_node = ToNode(t);
+		head.prev->next = &new_node;
+		new_node.prev = head.prev;
+		head.prev = &new_node;
+		new_node.next = &head;
+	}
+};