[PATCH 1/3] uiautomationcore: Implement UiaGetReservedNotSupportedValue.

[Connor McAdams]

On Wed, Oct 27, 2021 at 11:49:51PM +0300, Nikolay Sivov wrote:
> 
> 
> 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.

Do you mean something like, when the reserved object is QI'd for an
IMarshal interface, we check if we've already created one, and if not,
we create one? And then subsequent QI's for IMarshal will return the
same interface?

I guess that wouldn't be a problem, although it might cause weirdness if
someone called Release too many times on the shared IMarshal and caused
it to get freed while another thread/instance is using it. I can't think
of a circumstance where this would happen, but that would fail with the
approach I believe you're suggesting.