COM asynchronous interfaces, part 9: Asynchronous release, assembling a solution

Last time, we learned about the complex juggling required in order to accomplish a successful asynchronous release. Let’s try to put them together.

One of the things we need to do is aggregate the call object so that we can learn when the call has completed. This tells us when it’s safe to call Finish_Release and complete the client-side portion of the operation.

struct SyncForRelease :
    winrt::implements<SyncForRelease, ISynchronize>
{
  winrt::com_ptr<::IUnknown> m_inner;
  ::AsyncIUnknown* m_asyncUnknown;

  int32_t query_interface_tearoff(winrt::guid const& id, void** object)
    const noexcept override {
    if (m_inner) return m_inner.as(id, object);
    return E_NOINTERFACE;
  }

  auto Sync() noexcept { return m_inner.as<ISynchronize>(); }

  STDMETHODIMP Reset() { return Sync()->Reset(); }
  STDMETHODIMP Signal() {
    auto hr = Sync()->Signal();
    m_asyncUnknown->Finish_Release();
    m_inner.detach(); // don't Release it
    Release(); // I am dead to me
    return hr;
  }
  STDMETHODIMP Wait(DWORD flags, DWORD timeout) {
    return Sync()->Wait(flags, timeout);
  }
};

This is the object we’re going to use to aggregate the call. This follow the pattern we had seen earlier for aggregating the call object in order to override the ISynchronize method, and doing out bonus work inside the Signal method.

Actually, if the Signal and Wait calls fail, we fail to clean up or fail to wait for the operation to complete, and we have nowhere to report the failure. We may as well just fail fast. Instead of trying to catch the exception coming from the Sync() method, I just mark it as noexcept, which terminates the process if the query fails.

The stuff we do in the Signal won’t make sense until we understand how things are set up. So let’s set them up:

void ReleaseAsynchronously(IUnknown* unk)
{
  winrt::com_ptr<::ICallFactory> factory;
  unk->QueryInterface(IID_PPV_ARGS(factory.put()));
  unk->Release();
  if (!factory) return;

  winrt::com_ptr<SyncForRelease> sync;
  try {
    sync = winrt::make_self<SyncForRelease>();
  } catch (std::bad_alloc const&) { }
  if (!sync) return;

  factory->CreateCall(
    __uuidof(::AsyncIUnknown), sync.get(),
    __uuidof(::IUnknown), sync->m_inner.put());
  factory = nullptr;
  if (!sync->m_inner) return;

  sync->m_inner.as(IID_PPV_ARGS(&sync->m_asyncUnknown));
  if (!sync->m_asyncUnknown) return;

  // Release + AddRef cancel out

  sync->m_asyncUnknown->Begin_Release();
}

This function guarantees that the incoming IUnknown is released, one way or another: If we can’t release it asynchronously, then we’ll release it synchronously. This makes things easier for the caller, who can treat it as a fire-and-forget type of function.

First, we query the IUnknown for ICallFactory, and then immediately release the IUnknown. If the object is local, then the query will fail, and the Release will be a synchronous one. We detect this failure and return: The object has been release synchronously, and we’re done.

If the query for ICallFactory succeeds, then we have a proxy to a remote object. The release of the IUnknown won’t destroy the proxy because the ICallFactory is still outstanding.

Next up, we create the Sync­For­Release object, which we will use to aggregate the call so that we can be called back when the asynchronous method completes. We do it inside of a try block so we can handle the low-memory case and abandon the operation. The return will release the factory, which will be a synchronous release of the proxy. Sorry, we tried.

Assuming we have the Sync­For­Release object, we ask the factory to create a call (saving it as the aggregated inner object), and then immediately release the factory. This is a repeat of the previous pattern: If the Create­Call fails, then the release of the factory is a synchronous release, and we just return immediately. Otherwise, we keep going.

We ask the aggregated inner object for AsyncIUnknown so we can call the Begin_Release and Finish_Release methods. Again, if this fails, we just return, and the destructors will release the proxy synchronously.

Now, the normal pattern for querying an inner object for an interface is to perform the Query­Interface, and then perform a counteracting Release on the outer object. So in theory, there should be a Release() call here.

But we have a trick up our sleeve.

The next step would normally be to call AddRef() on the Sync­For­Release object so that it keeps itself alive while the call is in flight. This AddRef() cancels out the Release(), so the net result is that we don’t have to do anything! We are basically repurposing the reference count created by the Query­Interface call.

Now that everything is set up, we perform the Begin_Release(), which sets the asynchronous call into motion.

And then we wait.

Eventually, the asynchronous call completes, and the Sync­For­Release::Signal method is called. After asking the inner object to do the standard signaling work, we proceed with our custom response to the signal. We start by calling Finish_Release, which tells the call object that we have acknowledged the release of the object, and once Finish_Release returns, the object is truly released. The call to Finish_Release will not block because we forwarded the Signal call to the inner object, so the call is definitely complete.

When Finish_Release returns, the call object has been released, so we must throw away our references to it without calling Release. For our raw pointer, we can just abandon it. For our m_inner smart pointer, we use detach() to take ownership of the pointer. We just throw the pointer away, because it has already been released by the call to Finish_Release().

It took us a long time to get here, but we finally got it: A function for asynchronously releasing a COM pointer to a remote object.

Bonus chatter: Note in particular that our call to Sync()->Signal() was done with a temporary reference to the inner ISynchronize, so it got released when Signal() returned. If you do some tweaking of this method, make sure that you release the inner ISynchronize before calling Finish_Release(). Because Finish_Release() tears down the inner object, and all references to it become dead.