class GetBufferCollectionId

Get the buffer collection ID. This ID is also available from

[`fuchsia.sysmem2/Allocator.GetVmoInfo`] (along with the `buffer_index`

within the collection).

This call is mainly useful in situations where we can't convey a

[`fuchsia.sysmem2/BufferCollectionToken`] or

[`fuchsia.sysmem2/BufferCollection`] directly, but can only convey a VMO

handle, which can be joined back up with a `BufferCollection` client end

that was created via a different path. Prefer to convey a

`BufferCollectionToken` or `BufferCollection` directly when feasible.

Trusting a `buffer_collection_id` value from a source other than sysmem

is analogous to trusting a koid value from a source other than zircon.

Both should be avoided unless really necessary, and both require

caution. In some situations it may be reasonable to refer to a

pre-established `BufferCollection` by `buffer_collection_id` via a

protocol for efficiency reasons, but an incoming value purporting to be

a `buffer_collection_id` is not sufficient alone to justify granting the

sender of the `buffer_collection_id` any capability. The sender must

first prove to a receiver that the sender has/had a VMO or has/had a

`BufferCollectionToken` to the same collection by sending a handle that

sysmem confirms is a valid sysmem handle and which sysmem maps to the

`buffer_collection_id` value. The receiver should take care to avoid

assuming that a sender had a `BufferCollectionToken` in cases where the

sender has only proven that the sender had a VMO.

- response `buffer_collection_id` This ID is unique per buffer

collection per boot. Each buffer is uniquely identified by the

`buffer_collection_id` and `buffer_index` together.