[PATCH 1/3] uiautomationcore: Implement UiaGetReservedNotSupportedValue.

[Nikolay Sivov]

On 10/27/21 11:24 PM, Connor McAdams wrote:
> On Wed, Oct 27, 2021 at 11:12:31PM +0300, Nikolay Sivov wrote:
>>
>> On 10/27/21 10:55 PM, Connor McAdams wrote:
>>> +/*
>>> + * When passing the ReservedNotSupportedValue/ReservedMixedAttributeValue
>>> + * interface pointers across apartments within the same process, use the
>>> + * free threaded marshaler to preserve the pointer value. Wrap the IMarshal
>>> + * interface received by CoCreateFreeThreadedMarshaler so that we can maintain
>>> + * our own reference count and release the object when it hits 0.
>>> + */
>>> +static HRESULT uia_reserved_val_create_ftm(IUnknown *outer, IMarshal **marshaler)
>> I don't understand this part, if every QI for IMarshal returns new
>> instance, why do need a wrapper? And which object do you mean to
>> release, with our own reference count?
>>
> CoCreateFreeThreadedMarshaler with a non-NULL outer argument returns an
> IMarshal interface that has its reference count tied to the reference
> count of the outer object. In this case, AddRef/Release on the reserved
> object is pointless, because it is a static object whose reference count
> doesn't (and probably shouldn't) change.
>
> Normally, when the outer objects reference count hits 0, you'd release
> the unique inner IUnknown interface created by CoCreateFreeThreadedMarshaler
> there. We can't do that here, so my only idea was to create a unique
> IMarshal 'wrapper' interface with its own reference count, so that when
> it hits 0, we release the inner IUnknown object and free the memory.
I see. Is that really important that QI returns new instance every time?
It seems fine to ignore that detail, and if you do, you can maintain one
marshaler instance per static object type, releasing it on 0 refcount.
Could be some cleanup function instead to release then all at once, if
it's just one per "reserved" object, and not bother with artificial
refcounting at all.