Bug 368869, suspect all native wrappers for cycle collection. Relanding. Patch by graydon@mozilla.com. r=brendan, sr=jst

js/src/xpconnect/src/xpcwrappednativescope.cpp

@@ -272,13 +272,17 @@ WrappedNativeJSGCThingTracer(JSDHashTable *table, JSDHashEntryHdr *hdr,
         JS_CALL_OBJECT_TRACER(trc, wrapper->GetFlatJSObject(),
                               "XPCWrappedNative::mFlatJSObject");
 
-        // FIXME: this call appears to do more harm than good, but
-        // there is reason to imagine it might clean up some cycles
-        // formed by a poor order between C++ and JS garbage cycle
-        // formations. See Bug 368869.
-        //
-        // if (JS_IsGCMarkingTracer(trc))
-        //   nsCycleCollector_suspectCurrent(wrapper);
+        if (JS_IsGCMarkingTracer(trc)) 
+        {
+            XPCWrappedNativeProto* proto = wrapper->GetProto();
+            if (proto && 
+                proto->ClassIsMainThreadOnly()) 
+            {
+                NS_ASSERTION(NS_IsMainThread(), 
+                             "Tracing wrapped natives from non-main thread");
+                nsCycleCollector_suspectCurrent(wrapper);
+            }
+        }
     }
     return JS_DHASH_NEXT;
 }