class IsAlternateFor

Check whether the calling [`fuchsia.sysmem2/Node`] is in a subtree

rooted at a different child token of a common parent

[`fuchsia.sysmem2/BufferCollectionTokenGroup`], in relation to the

passed-in `node_ref`.

This call is for assisting with admission control de-duplication, and

with debugging.

The `node_ref` must be obtained using

[`fuchsia.sysmem2/Node.GetNodeRef`].

The `node_ref` can be a duplicated handle; it's not necessary to call

`GetNodeRef` for every call to [`fuchsia.sysmem2/Node.IsAlternateFor`].

If a calling token may not actually be a valid token at all due to a

potentially hostile/untrusted provider of the token, call

[`fuchsia.sysmem2/Allocator.ValidateBufferCollectionToken`] first

instead of potentially getting stuck indefinitely if `IsAlternateFor`

never responds due to a calling token not being a real token (not really

talking to sysmem). Another option is to call

[`fuchsia.sysmem2/Allocator.BindSharedCollection`] with this token first

which also validates the token along with converting it to a

[`fuchsia.sysmem2/BufferCollection`], then call `IsAlternateFor`.

All table fields are currently required.

- response `is_alternate`

- true: The first parent node in common between the calling node and

the `node_ref` `Node` is a `BufferCollectionTokenGroup`. This means

that the calling `Node` and the `node_ref` `Node` will not have both

their constraints apply - rather sysmem will choose one or the other

of the constraints - never both. This is because only one child of

a `BufferCollectionTokenGroup` is selected during logical

allocation, with only that one child's subtree contributing to

constraints aggregation.

- false: The first parent node in common between the calling `Node`

and the `node_ref` `Node` is not a `BufferCollectionTokenGroup`.

Currently, this means the first parent node in common is a

`BufferCollectionToken` or `BufferCollection` (regardless of not

`Release`ed). This means that the calling `Node` and the `node_ref`

`Node` may have both their constraints apply during constraints

aggregation of the logical allocation, if both `Node`(s) are

selected by any parent `BufferCollectionTokenGroup`(s) involved. In

this case, there is no `BufferCollectionTokenGroup` that will

directly prevent the two `Node`(s) from both being selected and

their constraints both aggregated, but even when false, one or both

`Node`(s) may still be eliminated from consideration if one or both

`Node`(s) has a direct or indirect parent

`BufferCollectionTokenGroup` which selects a child subtree other

than the subtree containing the calling `Node` or `node_ref` `Node`.

* error `[fuchsia.sysmem2/Error.NOT_FOUND]` The node_ref wasn't

associated with the same buffer collection as the calling `Node`.

Another reason for this error is if the `node_ref` is an

[`zx.Handle.EVENT`] handle with sufficient rights, but isn't actually

a real `node_ref` obtained from `GetNodeRef`.

* error `[fuchsia.sysmem2/Error.PROTOCOL_DEVIATION]` The caller passed a

`node_ref` that isn't a [`zx.Handle:EVENT`] handle , or doesn't have

the needed rights expected on a real `node_ref`.

* No other failing status codes are returned by this call. However,

sysmem may add additional codes in future, so the client should have

sensible default handling for any failing status code.