[browser][wasm] Implement JavaScript managed object reference counting

[kjpou1]

Wrap the javascript handle in a SafeHandleZeroOrMinusOneIsInvalid implementation.

• AnyRef

  • Modify AnyRef to be abstract and deriving from SafeHandleZeroOrMinusOneIsInvalid
  • Add extra parameter for ownsHandle
  • The GCHandle.Alloc creates a normal allocation or a weak allocation depending on the ownsHandle value passed.
    • true - GCHandleType.Weak
    • false - GCHandleType.Normal

• JSObject

  • Implements ReleaseHandle that is overridden from AnyRef
  • This removes the Dispose and Finalizers definitions from the objects. This will be taken care of by AnyRef which derives from SafeHandleZeroOrMinusOneIsInvalid
  • Add extra parameter for ownsHandle

• Runtime

  • The _boundObjects dictionary that tracks all the objects that have been bound from JavaScript objects to managed code is now a WeakReference.
  • Implement methods for obtaining a handle, adding a reference to a handle and releasing a handle. These are called from the javascript binding_support lib.
  • Refactored the releasing a JSObject.
    • Involved multiple calls to the bindings and managed code.
    • Streamlined to only call the bindings code once to free handles.

Add support for marshaling the AnyRef SafeHandle.

• When calling into the bindings from managed code the JSObject reference handle is incremented.
• After execution of the call the JSObject reference is decremented.
• Special processing for the GlobalObject where the object is set with ownsHandle = false so that it is not released.
• Release reference counts on javascript exceptions.

Support freeing delegates marshaled to the bindings.

Every delegate that was passed to the bindings code was kept forever causing leaks.

• Delegates are now not kept alive when GC'd.

• Delegates using lambda expressions forced the developer to explicitly free the delegate.

  • If a local is captured (used) by a lambda it becomes heap memory as we translate them into fields on an object.
  • A delegate that used lambda expressions was never collected and never released the heap so it could be collected.
  • This was a cause of memory leaks forcing the developer to explicitly call FreeObject (foreachAction);

• If a delegate is collected and the javascript code calls that delegate function an exception is thrown instead of inexplicably crashing with null exception error.

  • throw new Error("The delegate target that is being invoked is no longer available. Please check if it has been prematurely GC'd.");
  • Uncaught Error: The delegate target that is being invoked is no longer available. Please check if it has been prematurely GC'd.

[marek-safar]

Why are these not asserts?