Bug 937818, part 3 - Add ScanIncrementalRoots(). r=smaug

Any object that has been stored away somewhere in the middle of incremental graph
building must be treated as live, because we can't trust that the CC graph has
accurate information about it. If such an object is truly garbage, we'll unlink it
in the next cycle collection instead.

xpcom/base/nsCycleCollector.cpp

@@ -1178,7 +1178,8 @@ private:
 
     void BeginCollection(ccType aCCType, nsICycleCollectorListener *aManualListener);
     void MarkRoots(SliceBudget &aBudget);
-    void ScanRoots();
+    void ScanRoots(bool aFullySynchGraphBuild);
+    void ScanIncrementalRoots();
     void ScanWeakMaps();
 
     // returns whether anything was collected
@@ -2303,6 +2304,16 @@ nsCycleCollector::ForgetSkippable(bool aRemoveChildlessNodes,
                                   bool aAsyncSnowWhiteFreeing)
 {
     CheckThreadSafety();
+
+    // If we remove things from the purple buffer during graph building, we may
+    // lose track of an object that was mutated during graph building. This should
+    // only happen when somebody calls nsJSContext::CycleCollectNow explicitly
+    // requesting extra forget skippable calls, during an incremental collection.
+    // See bug 950949 for fixing this so we actually run the forgetSkippable calls.
+    if (mIncrementalPhase != IdlePhase) {
+        return;
+    }
+
     if (mJSRuntime) {
         mJSRuntime->PrepareForForgetSkippable();
     }
@@ -2474,28 +2485,138 @@ nsCycleCollector::ScanWeakMaps()
     }
 }
 
+// Flood black from any objects in the purple buffer that are in the CC graph.
+class PurpleScanBlackVisitor
+{
+public:
+    PurpleScanBlackVisitor(GCGraph &aGraph, uint32_t &aCount, bool &aFailed)
+        : mGraph(aGraph), mCount(aCount), mFailed(aFailed)
+    {
+    }
+
+    void
+    Visit(nsPurpleBuffer &aBuffer, nsPurpleBufferEntry *aEntry)
+    {
+        MOZ_ASSERT(aEntry->mObject, "Entries with null mObject shouldn't be in the purple buffer.");
+        MOZ_ASSERT(aEntry->mRefCnt->get() != 0, "Snow-white objects shouldn't be in the purple buffer.");
+
+        void *obj = aEntry->mObject;
+        if (!aEntry->mParticipant) {
+            obj = CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj));
+            MOZ_ASSERT(obj, "Don't add objects that don't participate in collection!");
+        }
+
+        PtrInfo *pi = mGraph.FindNode(obj);
+        if (!pi) {
+            return;
+        }
+        MOZ_ASSERT(pi->mParticipant, "No dead objects should be in the purple buffer.");
+        if (pi->mColor == black) {
+            return;
+        }
+        GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mCount, mFailed)).Walk(pi);
+    }
+
+private:
+    GCGraph &mGraph;
+    uint32_t &mCount;
+    bool &mFailed;
+};
+
+// Objects that have been stored somewhere since the start of incremental graph building must
+// be treated as live for this cycle collection, because we may not have accurate information
+// about who holds references to them.
 void
-nsCycleCollector::ScanRoots()
+nsCycleCollector::ScanIncrementalRoots()
 {
     TimeLog timeLog;
+
+    // Reference counted objects:
+    // We cleared the purple buffer at the start of the current ICC, so if a
+    // refcounted object is purple, it may have been AddRef'd during the current
+    // ICC. (It may also have only been released.) If that is the case, we cannot
+    // be sure that the set of things pointing to the object in the CC graph
+    // is accurate. Therefore, for safety, we treat any purple objects as being
+    // live during the current CC. We don't remove anything from the purple
+    // buffer here, so these objects will be suspected and freed in the next CC
+    // if they are garbage.
+    bool failed = false;
+    PurpleScanBlackVisitor purpleScanBlackVisitor(mGraph, mWhiteNodeCount, failed);
+    mPurpleBuf.VisitEntries(purpleScanBlackVisitor);
+    timeLog.Checkpoint("ScanIncrementalRoots::fix purple");
+
+    // Garbage collected objects:
+    // If a GCed object was added to the graph with a refcount of zero, and is
+    // now marked black by the GC, it was probably gray before and was exposed
+    // to active JS, so it may have been stored somewhere, so it needs to be
+    // treated as live.
+    if (mJSRuntime) {
+        nsCycleCollectionParticipant *jsParticipant = mJSRuntime->GCThingParticipant();
+        nsCycleCollectionParticipant *zoneParticipant = mJSRuntime->ZoneParticipant();
+        NodePool::Enumerator etor(mGraph.mNodes);
+
+        while (!etor.IsDone()) {
+            PtrInfo *pi = etor.GetNext();
+
+            if (pi->mRefCount != 0 || pi->mColor == black) {
+                continue;
+            }
+
+            if (pi->mParticipant == jsParticipant) {
+                if (xpc_GCThingIsGrayCCThing(pi->mPointer)) {
+                    continue;
+                }
+            } else if (pi->mParticipant == zoneParticipant) {
+                JS::Zone *zone = static_cast<JS::Zone*>(pi->mPointer);
+                if (js::ZoneGlobalsAreAllGray(zone)) {
+                    continue;
+                }
+            } else {
+                MOZ_ASSERT(false, "Non-JS thing with 0 refcount? Treating as live.");
+            }
+
+            GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mWhiteNodeCount, failed)).Walk(pi);
+        }
+
+        timeLog.Checkpoint("ScanIncrementalRoots::fix JS");
+    }
+
+    if (failed) {
+        NS_ASSERTION(false, "Ran out of memory in ScanIncrementalRoots");
+        CC_TELEMETRY(_OOM, true);
+    }
+}
+
+void
+nsCycleCollector::ScanRoots(bool aFullySynchGraphBuild)
+{
     AutoRestore<bool> ar(mScanInProgress);
     MOZ_ASSERT(!mScanInProgress);
     mScanInProgress = true;
     mWhiteNodeCount = 0;
     MOZ_ASSERT(mIncrementalPhase == ScanAndCollectWhitePhase);
 
+    if (!aFullySynchGraphBuild) {
+        ScanIncrementalRoots();
+    }
+
+    TimeLog timeLog;
+
     // On the assumption that most nodes will be black, it's
     // probably faster to use a GraphWalker than a
     // NodePool::Enumerator.
     bool failed = false;
     GraphWalker<scanVisitor>(scanVisitor(mWhiteNodeCount, failed)).WalkFromRoots(mGraph);
+    timeLog.Checkpoint("ScanRoots::WalkFromRoots");
 
     if (failed) {
         NS_ASSERTION(false, "Ran out of memory in ScanRoots");
         CC_TELEMETRY(_OOM, true);
     }
 
+    // Scanning weak maps must be done last.
     ScanWeakMaps();
+    timeLog.Checkpoint("ScanRoots::ScanWeakMaps");
 
     if (mListener) {
         mListener->BeginResults();
@@ -2526,9 +2647,8 @@ nsCycleCollector::ScanRoots()
 
         mListener->End();
         mListener = nullptr;
+        timeLog.Checkpoint("ScanRoots::listener");
     }
-
-    timeLog.Checkpoint("ScanRoots()");
 }
 
 
@@ -2891,7 +3011,7 @@ nsCycleCollector::Collect(ccType aCCType,
             // promoted to a strong reference after ScanRoots has finished.
             // See bug 926533.
             PrintPhase("ScanRoots");
-            ScanRoots();
+            ScanRoots(startedIdle);
             PrintPhase("CollectWhite");
             collectedAny = CollectWhite();
             break;