Fix DeferredFuture crashes by using QWeakPointer

[jsravn]

This replaces the custom ref counting with QWeakPointer. It fixes several issues with the code that lead to crashes.

In the original code there is an inherent race between the QSharedPointer<DeferredFuture>'s deleter and decWeakRefCount() being called in the various watch lambdas:

1. If there is even a 1ms pause after line

   asyncfuture/asyncfuture.h

   Line 712 in f2c773b

   object->decStrongRef();

   it always crashes when there are watchers. This is because the strongRefCount is decremented, which means deferred can be deleted by a watcher lambda when called. The deleter then crashes on the next line as object is no longer valid.

2. If there is a pause before

   asyncfuture/asyncfuture.h

   Line 689 in f2c773b

   QMetaObject::invokeMethod(this, "deleteLater");

   , it is possible for the deleteLater to be invoked by the shared pointer deleter, and the object deleted in the main thread. This then crashes as invokeMethod can't call deleteLater on a deleted object.

3. Calling raw delete this is not thread safe (

   asyncfuture/asyncfuture.h

   Line 691 in f2c773b

   delete this;

   ), it should always be doing deleteLater in the presence of multiple threads calling deleteLater.

There is not really a good way to fix this with the existing approach. The best way I found is to use the QSharedPointer semantics to handle ordered deletion. We can maintain a weakRef internally which can be used to obtain a shared pointer reference whenever we need to create a new watcher.

In my testing, this leads to deferred being correctly deleted in all cases without any race condition.

[jsravn]

@vpicaver appreciate if you could have a look. All unit tests pass.

[vpicaver]

Do you mind moving this pull request onto vpicaver/asyncfuture?