class ValidateBufferCollectionToken

Validate that a BufferCollectionToken is known to the sysmem server.

This can be used in cases where BindSharedCollection() won't be called

until after BufferCollectionToken.Duplicate() +

BufferCollectionToken.Sync(), when the client code wants to know earlier

whether an incoming token is valid (so far).

Calling BufferCollectionToken.Sync() on a token that isn't known to

sysmem risks the Sync() hanging forever.

Given that an incoming token can become invalid at any time if any

participant drops their BufferCollectionToken(s) or BufferCollection(s),

authors of client code are encouraged to consider not calling

ValidateBufferCollectionToken() and instead dealing with async failure

of the BufferCollection.Sync() after all the

BufferCollectionToken.Duplicate() and BindSharedCollection() (before

sending any duplicate tokens to other processes).

Regardless of the result of this call, this call has no effect on the

token with the referenced koid.

A true result from this call doesn't guarantee that the token remains

valid for any duration afterwards.

Client code will zx_object_get_info() on the client's token handle,

passing ZX_INFO_HANDLE_BASIC and getting back the related_koid

which then gets passed to ValidateBufferCollectionToken().

If ValidateBufferCollectionToken() returns true, the token was known at

the time the sysmem server processed the call, but may no longer be

valid/known by the time the client code receives the response.

If ValidateBufferCollectionToken() returns false, the token wasn't known

at the time the sysmem server processed the call, but the token may

become known by the time the client code receives the response. However

client code is not required to mitigate the possibility that the token

may become known late, since the source of the token should have synced

the token to sysmem before sending the token to the client code.

If calling ValidateBufferCollectionToken() fails in some way, there will

be a zx_status_t from the FIDL layer.

`token_server_koid` the koid of the server end of a channel that might

be a BufferCollectionToken channel. This can be obtained from

zx_object_get_info() ZX_INFO_HANDLE_BASIC related_koid.