The Tor Browser: mfbt/LinkedList.h@97036ab72558 (annotated)

mfbt/LinkedList.h@97036ab72558 (annotated)

mfbt/LinkedList.h

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 /* A type-safe doubly-linked list class. */
 /*
  * The classes LinkedList<T> and LinkedListElement<T> together form a
  * convenient, type-safe doubly-linked list implementation.
  *
  * The class T which will be inserted into the linked list must inherit from
  * LinkedListElement<T>.  A given object may be in only one linked list at a
  * time.
  *
  * A LinkedListElement automatically removes itself from the list upon
  * destruction, and a LinkedList will fatally assert in debug builds if it's
  * non-empty when it's destructed.
  *
  * For example, you might use LinkedList in a simple observer list class as
  * follows.
  *
  *   class Observer : public LinkedListElement<Observer>
  *   {
  *     public:
  *       void observe(char* topic) { ... }
  *   };
  *
  *   class ObserverContainer
  *   {
  *     private:
  *       LinkedList<Observer> list;
  *
  *     public:
  *       void addObserver(Observer* observer) {
  *         // Will assert if |observer| is part of another list.
  *         list.insertBack(observer);
  *       }
  *
  *       void removeObserver(Observer* observer) {
  *         // Will assert if |observer| is not part of some list.
  *         observer.remove();
  *         // Or, will assert if |observer| is not part of |list| specifically.
  *         // observer.removeFrom(list);
  *       }
  *
  *       void notifyObservers(char* topic) {
  *         for (Observer* o = list.getFirst(); o != nullptr; o = o->getNext())
  *           o->observe(topic);
  *       }
  *   };
  *
  */
 #ifndef mozilla_LinkedList_h
 #define mozilla_LinkedList_h
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Move.h"
 #include "mozilla/NullPtr.h"
 #ifdef __cplusplus
 namespace mozilla {
 template<typename T>
 class LinkedList;
 template<typename T>
 class LinkedListElement
 {
     /*
      * It's convenient that we return nullptr when getNext() or getPrevious()
      * hits the end of the list, but doing so costs an extra word of storage in
      * each linked list node (to keep track of whether |this| is the sentinel
      * node) and a branch on this value in getNext/getPrevious.
      *
      * We could get rid of the extra word of storage by shoving the "is
      * sentinel" bit into one of the pointers, although this would, of course,
      * have performance implications of its own.
      *
      * But the goal here isn't to win an award for the fastest or slimmest
      * linked list; rather, we want a *convenient* linked list.  So we won't
      * waste time guessing which micro-optimization strategy is best.
      *
      *
      * Speaking of unnecessary work, it's worth addressing here why we wrote
      * mozilla::LinkedList in the first place, instead of using stl::list.
      *
      * The key difference between mozilla::LinkedList and stl::list is that
      * mozilla::LinkedList stores the prev/next pointers in the object itself,
      * while stl::list stores the prev/next pointers in a list element which
      * itself points to the object being stored.
      *
      * mozilla::LinkedList's approach makes it harder to store an object in more
      * than one list.  But the upside is that you can call next() / prev() /
      * remove() directly on the object.  With stl::list, you'd need to store a
      * pointer to its iterator in the object in order to accomplish this.  Not
      * only would this waste space, but you'd have to remember to update that
      * pointer every time you added or removed the object from a list.
      *
      * In-place, constant-time removal is a killer feature of doubly-linked
      * lists, and supporting this painlessly was a key design criterion.
      */
   private:
     LinkedListElement* next;
     LinkedListElement* prev;
     const bool isSentinel;
   public:
     LinkedListElement()
       : next(MOZ_THIS_IN_INITIALIZER_LIST()),
         prev(MOZ_THIS_IN_INITIALIZER_LIST()),
         isSentinel(false)
     { }
     LinkedListElement(LinkedListElement<T>&& other)
       : isSentinel(other.isSentinel)
     {
       if (!other.isInList()) {
         next = this;
         prev = this;
         return;
       }
       MOZ_ASSERT(other.next->prev == &other);
       MOZ_ASSERT(other.prev->next == &other);
       /*
        * Initialize |this| with |other|'s prev/next pointers, and adjust those
        * element to point to this one.
        */
       next = other.next;
       prev = other.prev;
       next->prev = this;
       prev->next = this;
       /*
        * Adjust |other| so it doesn't think it's in a list.  This makes it
        * safely destructable.
        */
       other.next = &other;
       other.prev = &other;
     }
     ~LinkedListElement() {
       if (!isSentinel && isInList())
         remove();
     }
     /*
      * Get the next element in the list, or nullptr if this is the last element
      * in the list.
      */
     T* getNext() {
       return next->asT();
     }
     const T* getNext() const {
       return next->asT();
     }
     /*
      * Get the previous element in the list, or nullptr if this is the first
      * element in the list.
      */
     T* getPrevious() {
       return prev->asT();
     }
     const T* getPrevious() const {
       return prev->asT();
     }
     /*
      * Insert elem after this element in the list.  |this| must be part of a
      * linked list when you call setNext(); otherwise, this method will assert.
      */
     void setNext(T* elem) {
       MOZ_ASSERT(isInList());
       setNextUnsafe(elem);
     }
     /*
      * Insert elem before this element in the list.  |this| must be part of a
      * linked list when you call setPrevious(); otherwise, this method will
      * assert.
      */
     void setPrevious(T* elem) {
       MOZ_ASSERT(isInList());
       setPreviousUnsafe(elem);
     }
     /*
      * Remove this element from the list which contains it.  If this element is
      * not currently part of a linked list, this method asserts.
      */
     void remove() {
       MOZ_ASSERT(isInList());
       prev->next = next;
       next->prev = prev;
       next = this;
       prev = this;
     }
     /*
      * Identical to remove(), but also asserts in debug builds that this element
      * is in list.
      */
     void removeFrom(const LinkedList<T>& list) {
       list.assertContains(asT());
       remove();
     }
     /*
      * Return true if |this| part is of a linked list, and false otherwise.
      */
     bool isInList() const {
       MOZ_ASSERT((next == this) == (prev == this));
       return next != this;
     }
   private:
     friend class LinkedList<T>;
     enum NodeKind {
       NODE_KIND_NORMAL,
       NODE_KIND_SENTINEL
     };
     LinkedListElement(NodeKind nodeKind)
       : next(MOZ_THIS_IN_INITIALIZER_LIST()),
         prev(MOZ_THIS_IN_INITIALIZER_LIST()),
         isSentinel(nodeKind == NODE_KIND_SENTINEL)
     { }
     /*
      * Return |this| cast to T* if we're a normal node, or return nullptr if
      * we're a sentinel node.
      */
     T* asT() {
       if (isSentinel)
         return nullptr;
       return static_cast<T*>(this);
     }
     const T* asT() const {
       if (isSentinel)
         return nullptr;
       return static_cast<const T*>(this);
     }
     /*
      * Insert elem after this element, but don't check that this element is in
      * the list.  This is called by LinkedList::insertFront().
      */
     void setNextUnsafe(T* elem) {
       LinkedListElement *listElem = static_cast<LinkedListElement*>(elem);
       MOZ_ASSERT(!listElem->isInList());
       listElem->next = this->next;
       listElem->prev = this;
       this->next->prev = listElem;
       this->next = listElem;
     }
     /*
      * Insert elem before this element, but don't check that this element is in
      * the list.  This is called by LinkedList::insertBack().
      */
     void setPreviousUnsafe(T* elem) {
       LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(elem);
       MOZ_ASSERT(!listElem->isInList());
       listElem->next = this;
       listElem->prev = this->prev;
       this->prev->next = listElem;
       this->prev = listElem;
     }
   private:
     LinkedListElement& operator=(const LinkedListElement<T>& other) MOZ_DELETE;
     LinkedListElement(const LinkedListElement<T>& other) MOZ_DELETE;
 };
 template<typename T>
 class LinkedList
 {
   private:
     LinkedListElement<T> sentinel;
   public:
     LinkedList() : sentinel(LinkedListElement<T>::NODE_KIND_SENTINEL) { }
     LinkedList(LinkedList<T>&& other)
       : sentinel(mozilla::Move(other.sentinel))
     { }
     ~LinkedList() {
       MOZ_ASSERT(isEmpty());
     }
     /*
      * Add elem to the front of the list.
      */
     void insertFront(T* elem) {
       /* Bypass setNext()'s this->isInList() assertion. */
       sentinel.setNextUnsafe(elem);
     }
     /*
      * Add elem to the back of the list.
      */
     void insertBack(T* elem) {
       sentinel.setPreviousUnsafe(elem);
     }
     /*
      * Get the first element of the list, or nullptr if the list is empty.
      */
     T* getFirst() {
       return sentinel.getNext();
     }
     const T* getFirst() const {
       return sentinel.getNext();
     }
     /*
      * Get the last element of the list, or nullptr if the list is empty.
      */
     T* getLast() {
       return sentinel.getPrevious();
     }
     const T* getLast() const {
       return sentinel.getPrevious();
     }
     /*
      * Get and remove the first element of the list.  If the list is empty,
      * return nullptr.
      */
     T* popFirst() {
       T* ret = sentinel.getNext();
       if (ret)
         static_cast<LinkedListElement<T>*>(ret)->remove();
       return ret;
     }
     /*
      * Get and remove the last element of the list.  If the list is empty,
      * return nullptr.
      */
     T* popLast() {
       T* ret = sentinel.getPrevious();
       if (ret)
         static_cast<LinkedListElement<T>*>(ret)->remove();
       return ret;
     }
     /*
      * Return true if the list is empty, or false otherwise.
      */
     bool isEmpty() const {
       return !sentinel.isInList();
     }
     /*
      * Remove all the elements from the list.
      *
      * This runs in time linear to the list's length, because we have to mark
      * each element as not in the list.
      */
     void clear() {
       while (popFirst())
         continue;
     }
     /*
      * Measures the memory consumption of the list excluding |this|.  Note that
      * it only measures the list elements themselves.  If the list elements
      * contain pointers to other memory blocks, those blocks must be measured
      * separately during a subsequent iteration over the list.
      */
     size_t sizeOfExcludingThis(MallocSizeOf mallocSizeOf) const {
       size_t n = 0;
       for (const T* t = getFirst(); t; t = t->getNext())
         n += mallocSizeOf(t);
       return n;
     }
     /*
      * Like sizeOfExcludingThis(), but measures |this| as well.
      */
     size_t sizeOfIncludingThis(MallocSizeOf mallocSizeOf) const {
       return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
     }
     /*
      * In a debug build, make sure that the list is sane (no cycles, consistent
      * next/prev pointers, only one sentinel).  Has no effect in release builds.
      */
     void debugAssertIsSane() const {
 #ifdef DEBUG
       const LinkedListElement<T>* slow;
       const LinkedListElement<T>* fast1;
       const LinkedListElement<T>* fast2;
       /*
        * Check for cycles in the forward singly-linked list using the
        * tortoise/hare algorithm.
        */
       for (slow = sentinel.next,
            fast1 = sentinel.next->next,
            fast2 = sentinel.next->next->next;
            slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
            slow = slow->next, fast1 = fast2->next, fast2 = fast1->next)
       {
         MOZ_ASSERT(slow != fast1);
         MOZ_ASSERT(slow != fast2);
       }
       /* Check for cycles in the backward singly-linked list. */
       for (slow = sentinel.prev,
            fast1 = sentinel.prev->prev,
            fast2 = sentinel.prev->prev->prev;
            slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
            slow = slow->prev, fast1 = fast2->prev, fast2 = fast1->prev)
       {
         MOZ_ASSERT(slow != fast1);
         MOZ_ASSERT(slow != fast2);
       }
       /*
        * Check that |sentinel| is the only node in the list with
        * isSentinel == true.
        */
       for (const LinkedListElement<T>* elem = sentinel.next;
            elem != &sentinel;
            elem = elem->next)
       {
         MOZ_ASSERT(!elem->isSentinel);
       }
       /* Check that the next/prev pointers match up. */
       const LinkedListElement<T>* prev = &sentinel;
       const LinkedListElement<T>* cur = sentinel.next;
       do {
           MOZ_ASSERT(cur->prev == prev);
           MOZ_ASSERT(prev->next == cur);
           prev = cur;
           cur = cur->next;
       } while (cur != &sentinel);
 #endif /* ifdef DEBUG */
     }
   private:
     friend class LinkedListElement<T>;
     void assertContains(const T* t) const {
 #ifdef DEBUG
       for (const T* elem = getFirst();
            elem;
            elem = elem->getNext())
       {
         if (elem == t)
           return;
       }
       MOZ_CRASH("element wasn't found in this list!");
 #endif
     }
     LinkedList& operator=(const LinkedList<T>& other) MOZ_DELETE;
     LinkedList(const LinkedList<T>& other) MOZ_DELETE;
 };
 } /* namespace mozilla */
 #endif /* __cplusplus */
 #endif /* mozilla_LinkedList_h */

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mfbt/LinkedList.h@97036ab72558 (annotated)

mfbt/LinkedList.h