Bug 1509930 - Simplify CycleCollectWithLogsChild's lifetime, r=jld

Previously this actor would call `Send__delete__` in its destructor, meaning
that the lifetime of the actor is a bit hard to follow. This changes the actor
to instead use a separate object for XPCOM, and use refcounting universally.
This should also help with the transition to universally refcounted actors.

Differential Revision: https://phabricator.services.mozilla.com/D12955

dom/ipc/ContentChild.cpp

@@ -273,14 +273,19 @@ namespace dom {
 // IPC sender for remote GC/CC logging.
 class CycleCollectWithLogsChild final
   : public PCycleCollectWithLogsChild
-  , public nsICycleCollectorLogSink
 {
 public:
+  NS_INLINE_DECL_REFCOUNTING(CycleCollectWithLogsChild)
+
+  class Sink final : public nsICycleCollectorLogSink
+  {
     NS_DECL_ISUPPORTS
 
-  CycleCollectWithLogsChild(const FileDescriptor& aGCLog,
+    Sink(CycleCollectWithLogsChild* aActor,
+         const FileDescriptor& aGCLog,
          const FileDescriptor& aCCLog)
     {
+      mActor = aActor;
       mGCLog = FileDescriptorToFILE(aGCLog, "w");
       mCCLog = FileDescriptorToFILE(aCCLog, "w");
     }
@@ -300,7 +305,7 @@ public:
       MOZ_ASSERT(mGCLog);
       fclose(mGCLog);
       mGCLog = nullptr;
-    SendCloseGCLog();
+      mActor->SendCloseGCLog();
       return NS_OK;
     }
 
@@ -309,7 +314,7 @@ public:
       MOZ_ASSERT(mCCLog);
       fclose(mCCLog);
       mCCLog = nullptr;
-    SendCloseCCLog();
+      mActor->SendCloseCCLog();
       return NS_OK;
     }
 
@@ -344,7 +349,7 @@ public:
     }
 
   private:
-  ~CycleCollectWithLogsChild() override
+    ~Sink()
     {
       if (mGCLog) {
         fclose(mGCLog);
@@ -354,9 +359,8 @@ private:
         fclose(mCCLog);
         mCCLog = nullptr;
       }
-    // The XPCOM refcount drives the IPC lifecycle; see also
-    // DeallocPCycleCollectWithLogsChild.
-    Unused << Send__delete__(this);
+      // The XPCOM refcount drives the IPC lifecycle;
+      Unused << mActor->Send__delete__(mActor);
     }
 
     nsresult UnimplementedProperty()
@@ -366,11 +370,16 @@ private:
       return NS_ERROR_UNEXPECTED;
     }
 
+    RefPtr<CycleCollectWithLogsChild> mActor;
     FILE* mGCLog;
     FILE* mCCLog;
   };
 
-NS_IMPL_ISUPPORTS(CycleCollectWithLogsChild, nsICycleCollectorLogSink);
+private:
+  ~CycleCollectWithLogsChild() {}
+};
+
+NS_IMPL_ISUPPORTS(CycleCollectWithLogsChild::Sink, nsICycleCollectorLogSink);
 
 class AlertObserver
 {
@@ -1323,9 +1332,7 @@ ContentChild::AllocPCycleCollectWithLogsChild(const bool& aDumpAllTraces,
                                               const FileDescriptor& aGCLog,
                                               const FileDescriptor& aCCLog)
 {
-  auto* actor = new CycleCollectWithLogsChild(aGCLog, aCCLog);
-  // Return actor with refcount 0, which is safe because it has a non-XPCOM type.
-  return actor;
+  return do_AddRef(new CycleCollectWithLogsChild()).take();
 }
 
 mozilla::ipc::IPCResult
@@ -1334,23 +1341,23 @@ ContentChild::RecvPCycleCollectWithLogsConstructor(PCycleCollectWithLogsChild* a
                                                    const FileDescriptor& aGCLog,
                                                    const FileDescriptor& aCCLog)
 {
-  // Take a reference here, where the XPCOM type is regained.
-  RefPtr<CycleCollectWithLogsChild> sink = static_cast<CycleCollectWithLogsChild*>(aActor);
+  // The sink's destructor is called when the last reference goes away, which
+  // will cause the actor to be closed down.
+  auto* actor = static_cast<CycleCollectWithLogsChild*>(aActor);
+  RefPtr<CycleCollectWithLogsChild::Sink> sink =
+    new CycleCollectWithLogsChild::Sink(actor, aGCLog, aCCLog);
+
+  // Invoke the dumper, which will take a reference to the sink.
   nsCOMPtr<nsIMemoryInfoDumper> dumper = do_GetService("@mozilla.org/memory-info-dumper;1");
-
   dumper->DumpGCAndCCLogsToSink(aDumpAllTraces, sink);
-
-  // The actor's destructor is called when the last reference goes away...
   return IPC_OK();
 }
 
 bool
-ContentChild::DeallocPCycleCollectWithLogsChild(PCycleCollectWithLogsChild* /* aActor */)
+ContentChild::DeallocPCycleCollectWithLogsChild(PCycleCollectWithLogsChild* aActor)
 {
-  // ...so when we get here, there's nothing for us to do.
-  //
-  // Also, we're already in ~CycleCollectWithLogsChild (q.v.) at
-  // this point, so we shouldn't touch the actor in any case.
+  RefPtr<CycleCollectWithLogsChild> actor =
+    dont_AddRef(static_cast<CycleCollectWithLogsChild*>(aActor));
   return true;
 }