diff -r 000000000000 -r 6474c204b198 xpcom/ds/nsExpirationTracker.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/ds/nsExpirationTracker.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,383 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#ifndef NSEXPIRATIONTRACKER_H_
+#define NSEXPIRATIONTRACKER_H_
+
+#include "mozilla/Attributes.h"
+
+#include "prlog.h"
+#include "nsTArray.h"
+#include "nsITimer.h"
+#include "nsCOMPtr.h"
+#include "nsAutoPtr.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "mozilla/Services.h"
+#include "mozilla/Attributes.h"
+
+/**
+ * Data used to track the expiration state of an object. We promise that this
+ * is 32 bits so that objects that includes this as a field can pad and align
+ * efficiently.
+ */
+struct nsExpirationState {
+  enum { NOT_TRACKED = (1U << 4) - 1,
+         MAX_INDEX_IN_GENERATION = (1U << 28) - 1 };
+
+  nsExpirationState() : mGeneration(NOT_TRACKED) {}
+  bool IsTracked() { return mGeneration != NOT_TRACKED; }
+
+  /**
+   * The generation that this object belongs to, or NOT_TRACKED.
+   */
+  uint32_t mGeneration:4;
+  uint32_t mIndexInGeneration:28;
+};
+
+/**
+ * nsExpirationTracker can track the lifetimes and usage of a large number of
+ * objects, and send a notification some window of time after a live object was
+ * last used. This is very useful when you manage a large number of objects
+ * and want to flush some after they haven't been used for a while.
+ * nsExpirationTracker is designed to be very space and time efficient.
+ * 
+ * The type parameter T is the object type that we will track pointers to. T
+ * must include an accessible method GetExpirationState() that returns a
+ * pointer to an nsExpirationState associated with the object (preferably,
+ * stored in a field of the object).
+ * 
+ * The parameter K is the number of generations that will be used. Increasing
+ * the number of generations narrows the window within which we promise
+ * to fire notifications, at a slight increase in space cost for the tracker.
+ * We require 2 <= K <= nsExpirationState::NOT_TRACKED (currently 15).
+ * 
+ * To use this class, you need to inherit from it and override the
+ * NotifyExpired() method.
+ * 
+ * The approach is to track objects in K generations. When an object is accessed
+ * it moves from its current generation to the newest generation. Generations
+ * are stored in a cyclic array; when a timer interrupt fires, we advance
+ * the current generation pointer to effectively age all objects very efficiently.
+ * By storing information in each object about its generation and index within its
+ * generation array, we make removal of objects from a generation very cheap.
+ * 
+ * Future work:
+ * -- Add a method to change the timer period?
+ */
+template <class T, uint32_t K> class nsExpirationTracker {
+  public:
+    /**
+     * Initialize the tracker.
+     * @param aTimerPeriod the timer period in milliseconds. The guarantees
+     * provided by the tracker are defined in terms of this period. If the
+     * period is zero, then we don't use a timer and rely on someone calling
+     * AgeOneGeneration explicitly.
+     */
+    nsExpirationTracker(uint32_t aTimerPeriod)
+      : mTimerPeriod(aTimerPeriod), mNewestGeneration(0),
+        mInAgeOneGeneration(false) {
+      static_assert(K >= 2 && K <= nsExpirationState::NOT_TRACKED,
+                    "Unsupported number of generations (must be 2 <= K <= 15)");
+      mObserver = new ExpirationTrackerObserver();
+      mObserver->Init(this);
+    }
+    ~nsExpirationTracker() {
+      if (mTimer) {
+        mTimer->Cancel();
+      }
+      mObserver->Destroy();
+    }
+
+    /**
+     * Add an object to be tracked. It must not already be tracked. It will
+     * be added to the newest generation, i.e., as if it was just used.
+     * @return an error on out-of-memory
+     */
+    nsresult AddObject(T* aObj) {
+      nsExpirationState* state = aObj->GetExpirationState();
+      NS_ASSERTION(!state->IsTracked(), "Tried to add an object that's already tracked");
+      nsTArray<T*>& generation = mGenerations[mNewestGeneration];
+      uint32_t index = generation.Length();
+      if (index > nsExpirationState::MAX_INDEX_IN_GENERATION) {
+        NS_WARNING("More than 256M elements tracked, this is probably a problem");
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+      if (index == 0) {
+        // We might need to start the timer
+        nsresult rv = CheckStartTimer();
+        if (NS_FAILED(rv))
+          return rv;
+      }
+      if (!generation.AppendElement(aObj))
+        return NS_ERROR_OUT_OF_MEMORY;
+      state->mGeneration = mNewestGeneration;
+      state->mIndexInGeneration = index;
+      return NS_OK;
+    }
+
+    /**
+     * Remove an object from the tracker. It must currently be tracked.
+     */
+    void RemoveObject(T* aObj) {
+      nsExpirationState* state = aObj->GetExpirationState();
+      NS_ASSERTION(state->IsTracked(), "Tried to remove an object that's not tracked");
+      nsTArray<T*>& generation = mGenerations[state->mGeneration];
+      uint32_t index = state->mIndexInGeneration;
+      NS_ASSERTION(generation.Length() > index &&
+                   generation[index] == aObj, "Object is lying about its index");
+      // Move the last object to fill the hole created by removing aObj
+      uint32_t last = generation.Length() - 1;
+      T* lastObj = generation[last];
+      generation[index] = lastObj;
+      lastObj->GetExpirationState()->mIndexInGeneration = index;
+      generation.RemoveElementAt(last);
+      state->mGeneration = nsExpirationState::NOT_TRACKED;
+      // We do not check whether we need to stop the timer here. The timer
+      // will check that itself next time it fires. Checking here would not
+      // be efficient since we'd need to track all generations. Also we could
+      // thrash by incessantly creating and destroying timers if someone
+      // kept adding and removing an object from the tracker.
+    }
+
+    /**
+     * Notify that an object has been used.
+     * @return an error if we lost the object from the tracker...
+     */
+    nsresult MarkUsed(T* aObj) {
+      nsExpirationState* state = aObj->GetExpirationState();
+      if (mNewestGeneration == state->mGeneration)
+        return NS_OK;
+      RemoveObject(aObj);
+      return AddObject(aObj);
+    }
+
+    /**
+     * The timer calls this, but it can also be manually called if you want
+     * to age objects "artifically". This can result in calls to NotifyExpired.
+     */
+    void AgeOneGeneration() {
+      if (mInAgeOneGeneration) {
+        NS_WARNING("Can't reenter AgeOneGeneration from NotifyExpired");
+        return;
+      }
+      
+      mInAgeOneGeneration = true;
+      uint32_t reapGeneration = 
+        mNewestGeneration > 0 ? mNewestGeneration - 1 : K - 1;
+      nsTArray<T*>& generation = mGenerations[reapGeneration];
+      // The following is rather tricky. We have to cope with objects being
+      // removed from this generation either because of a call to RemoveObject
+      // (or indirectly via MarkUsed) inside NotifyExpired. Fortunately no
+      // objects can be added to this generation because it's not the newest
+      // generation. We depend on the fact that RemoveObject can only cause
+      // the indexes of objects in this generation to *decrease*, not increase.
+      // So if we start from the end and work our way backwards we are guaranteed
+      // to see each object at least once.
+      uint32_t index = generation.Length();
+      for (;;) {
+        // Objects could have been removed so index could be outside
+        // the array
+        index = XPCOM_MIN(index, generation.Length());
+        if (index == 0)
+          break;
+        --index;
+        NotifyExpired(generation[index]);
+      }
+      // Any leftover objects from reapGeneration just end up in the new
+      // newest-generation. This is bad form, though, so warn if there are any.
+      if (!generation.IsEmpty()) {
+        NS_WARNING("Expired objects were not removed or marked used");
+      }
+      // Free excess memory used by the generation array, since we probably
+      // just removed most or all of its elements.
+      generation.Compact();
+      mNewestGeneration = reapGeneration;
+      mInAgeOneGeneration = false;
+    }
+
+    /**
+     * This just calls AgeOneGeneration K times. Under normal circumstances this
+     * will result in all objects getting NotifyExpired called on them, but
+     * if NotifyExpired itself marks some objects as used, then those objects
+     * might not expire. This would be a good thing to call if we get into
+     * a critically-low memory situation.
+     */
+    void AgeAllGenerations() {
+      uint32_t i;
+      for (i = 0; i < K; ++i) {
+        AgeOneGeneration();
+      }
+    }
+    
+    class Iterator {
+    private:
+      nsExpirationTracker<T,K>* mTracker;
+      uint32_t                  mGeneration;
+      uint32_t                  mIndex;
+    public:
+      Iterator(nsExpirationTracker<T,K>* aTracker)
+        : mTracker(aTracker), mGeneration(0), mIndex(0) {}
+      T* Next() {
+        while (mGeneration < K) {
+          nsTArray<T*>* generation = &mTracker->mGenerations[mGeneration];
+          if (mIndex < generation->Length()) {
+            ++mIndex;
+            return (*generation)[mIndex - 1];
+          }
+          ++mGeneration;
+          mIndex = 0;
+        }
+        return nullptr;
+      }
+    };
+    
+    friend class Iterator;
+
+    bool IsEmpty() {
+      for (uint32_t i = 0; i < K; ++i) {
+        if (!mGenerations[i].IsEmpty())
+          return false;
+      }
+      return true;
+    }
+
+  protected:
+    /**
+     * This must be overridden to catch notifications. It is called whenever
+     * we detect that an object has not been used for at least (K-1)*mTimerPeriod
+     * milliseconds. If timer events are not delayed, it will be called within
+     * roughly K*mTimerPeriod milliseconds after the last use. (Unless AgeOneGeneration
+     * or AgeAllGenerations have been called to accelerate the aging process.)
+     * 
+     * NOTE: These bounds ignore delays in timer firings due to actual work being
+     * performed by the browser. We use a slack timer so there is always at least
+     * mTimerPeriod milliseconds between firings, which gives us (K-1)*mTimerPeriod
+     * as a pretty solid lower bound. The upper bound is rather loose, however.
+     * If the maximum amount by which any given timer firing is delayed is D, then
+     * the upper bound before NotifyExpired is called is K*(mTimerPeriod + D).
+     * 
+     * The NotifyExpired call is expected to remove the object from the tracker,
+     * but it need not. The object (or other objects) could be "resurrected"
+     * by calling MarkUsed() on them, or they might just not be removed.
+     * Any objects left over that have not been resurrected or removed
+     * are placed in the new newest-generation, but this is considered "bad form"
+     * and should be avoided (we'll issue a warning). (This recycling counts
+     * as "a use" for the purposes of the expiry guarantee above...)
+     * 
+     * For robustness and simplicity, we allow objects to be notified more than
+     * once here in the same timer tick.
+     */
+    virtual void NotifyExpired(T* aObj) = 0;
+
+  private:
+    class ExpirationTrackerObserver;
+    nsRefPtr<ExpirationTrackerObserver> mObserver;
+    nsTArray<T*>       mGenerations[K];
+    nsCOMPtr<nsITimer> mTimer;
+    uint32_t           mTimerPeriod;
+    uint32_t           mNewestGeneration;
+    bool               mInAgeOneGeneration;
+
+    /**
+     * Whenever "memory-pressure" is observed, it calls AgeAllGenerations()
+     * to minimize memory usage.
+     */
+    class ExpirationTrackerObserver MOZ_FINAL : public nsIObserver {
+    public:
+      void Init(nsExpirationTracker<T,K> *obj) {
+        mOwner = obj;
+        nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+        if (obs) {
+          obs->AddObserver(this, "memory-pressure", false);
+        }
+      }
+      void Destroy() {
+        mOwner = nullptr;
+        nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+        if (obs)
+          obs->RemoveObserver(this, "memory-pressure");
+      }
+      NS_DECL_ISUPPORTS
+      NS_DECL_NSIOBSERVER
+    private:
+      nsExpirationTracker<T,K> *mOwner;
+    };
+  
+    static void TimerCallback(nsITimer* aTimer, void* aThis) {
+      nsExpirationTracker* tracker = static_cast<nsExpirationTracker*>(aThis);
+      tracker->AgeOneGeneration();
+      // Cancel the timer if we have no objects to track
+      if (tracker->IsEmpty()) {
+        tracker->mTimer->Cancel();
+        tracker->mTimer = nullptr;
+      }
+    }
+
+    nsresult CheckStartTimer() {
+      if (mTimer || !mTimerPeriod)
+        return NS_OK;
+      mTimer = do_CreateInstance("@mozilla.org/timer;1");
+      if (!mTimer)
+        return NS_ERROR_OUT_OF_MEMORY;
+      mTimer->InitWithFuncCallback(TimerCallback, this, mTimerPeriod,
+                                   nsITimer::TYPE_REPEATING_SLACK);
+      return NS_OK;
+    }
+};
+
+template<class T, uint32_t K>
+NS_IMETHODIMP
+nsExpirationTracker<T, K>::ExpirationTrackerObserver::Observe(nsISupports     *aSubject,
+                                                              const char      *aTopic,
+                                                              const char16_t *aData)
+{
+  if (!strcmp(aTopic, "memory-pressure") && mOwner)
+    mOwner->AgeAllGenerations();
+  return NS_OK;
+}
+
+template <class T, uint32_t K>
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsExpirationTracker<T,K>::ExpirationTrackerObserver::AddRef(void)
+{
+  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+  NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+  ++mRefCnt;
+  NS_LOG_ADDREF(this, mRefCnt, "ExpirationTrackerObserver", sizeof(*this));
+  return mRefCnt;
+}
+
+template <class T, uint32_t K>
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsExpirationTracker<T,K>::ExpirationTrackerObserver::Release(void)
+{
+  MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
+  NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+  --mRefCnt;
+  NS_LOG_RELEASE(this, mRefCnt, "ExpirationTrackerObserver");
+  if (mRefCnt == 0) {
+    NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+    mRefCnt = 1; /* stabilize */
+    delete (this);
+    return 0;
+  }
+  return mRefCnt;
+}
+
+template <class T, uint32_t K>
+NS_IMETHODIMP
+nsExpirationTracker<T,K>::ExpirationTrackerObserver::QueryInterface(REFNSIID aIID, 
+                                                                    void** aInstancePtr)
+{
+  NS_ASSERTION(aInstancePtr,
+               "QueryInterface requires a non-NULL destination!");            
+  nsresult rv = NS_ERROR_FAILURE;
+  NS_INTERFACE_TABLE(ExpirationTrackerObserver, nsIObserver)
+  return rv;
+}
+
+#endif /*NSEXPIRATIONTRACKER_H_*/