class ValidateBufferCollectionToken

Checks whether a [`fuchsia.sysmem2/BufferCollectionToken`] is known to

the sysmem server.

With this call, the client can determine whether an incoming token is a

real sysmem token that is known to the sysmem server, without any risk

of getting stuck waiting forever on a potentially fake token to complete

[`fuchsia.sysmem2/BufferCollectionToken.DuplicateSync`] or

[`fuchsia.sysmem2/BufferCollectionToken.Sync`] (or any other two-way

FIDL message). In cases where the client trusts the source of the token

to provide a real token, this call is not typically needed outside of

debugging.

If the validate fails sometimes but succeeds other times, the source of

the token may itself not be calling

[`fuchsia.sysmem2/BufferCollectionToken.Sync`] or

[`fuchsia.sysmem2/BufferCollection.Sync`] after creating/duplicating the

token but before sending the token to the current client. It may be more

convenient for the source to use

[`fuchsia.sysmem2/BufferCollectionToken.DuplicateSync`] to duplicate

token(s), since that call has the sync step built in. Or, the buffer

collection may be failing before this call is processed by the sysmem

server, as buffer collection failure cleans up sysmem's tracking of

associated tokens.

This call has no effect on any token.

+ request `token_server_koid` The koid of the server end of a channel

that might be a BufferCollectionToken channel. This can be obtained

via `zx_object_get_info` `ZX_INFO_HANDLE_BASIC` `related_koid`.

- response `is_known` true means sysmem knew of the token at the time

sysmem processed the request, but doesn't guarantee that the token is

still valid by the time the client receives the reply. What it does

guarantee is that the token at least was a real token, so a two-way

call to the token won't stall forever (will fail or succeed fairly

quickly, not stall). This can already be known implicitly if the

source of the token can be trusted to provide a real token. A false

value means the token wasn't known to sysmem at the time sysmem

processed this call, but the token may have previously been valid, or

may yet become valid. Or if the sender of the token isn't trusted to

provide a real token, the token may be fake. It's the responsibility

of the sender to sync with sysmem to ensure that previously

created/duplicated token(s) are known to sysmem, before sending the

token(s) to other participants.