class WireWeakOnewayBufferClientImpl

On a BufferCollectionToken channel:

Normally a participant will convert a BufferCollectionToken into a

BufferCollection view, but a participant is also free to Close() the

token (and then close the channel immediately or shortly later in

response to server closing its end), which avoids causing logical buffer

collection failure.  Normally an unexpected token channel close will

cause logical buffer collection failure (the only exceptions being

certain cases involving AttachToken() or SetDispensable()).

On a BufferCollection channel:

By default the server handles unexpected failure of a BufferCollection

by failing the whole logical buffer collection. Partly this is to

expedite closing VMO handles to reclaim memory when any participant

fails. If a participant would like to cleanly close a BufferCollection

view without causing logical buffer collection failure, the participant

can send Close() before closing the client end of the BufferCollection

channel. If this is the last BufferCollection view, the logical buffer

collection will still go away. The Close() can occur before or after

SetConstraints(). If before SetConstraints(), the buffer collection

won't require constraints from this node in order to allocate. If

after SetConstraints(), the constraints are retained and aggregated

along with any subsequent logical allocation(s), despite the lack of

channel connection.

On a BufferCollectionTokenGroup channel:

By default, unexpected failure of a BufferCollectionTokenGroup will

trigger failure of the logical BufferCollectionTokenGroup and will

propagate failure to its parent. To close a BufferCollectionTokenGroup

channel without failing the logical group or propagating failure, send

Close() before closing the channel client endpoint.

If Close() occurs before AllChildrenPresent(), the logical buffer

collection will still fail despite the Close() (because sysmem can't be

sure whether all relevant children were created, so it's ambiguous

whether all relevant constraints will be provided to sysmem). If

Close() occurs after AllChildrenPresent(), the children and all their

constraints remain intact (just as they would if the

BufferCollectionTokenGroup channel had remained open), and the close

doesn't trigger or propagate failure.

Caller provides the backing storage for FIDL message.

Set a name for VMOs in this buffer collection. The name may be truncated

shorter. The name only affects VMOs allocated after it's set - this call

does not rename existing VMOs. If multiple clients set different names

then the larger priority value will win.

Caller provides the backing storage for FIDL message.

Set information about the current client that can be used by sysmem to

help debug leaking memory and hangs waiting for constraints. |name| can

be an arbitrary string, but the current process name (see

fsl::GetCurrentProcessName()) is a good default. |id| can be an

arbitrary id, but the current process ID (see

fsl::GetCurrentProcessKoid()) is a good default.

Also used when verbose logging is enabled (see SetVerboseLogging()) to

indicate which client is closing their channel first, leading to

sub-tree failure (which can be normal if the purpose of the sub-tree is

over, but if happening earlier than expected, the

client-channel-specific name can help diagnose where the failure is

first coming from, from sysmem's point of view).

By default (unless overriden by this message or using

Allocator.SetDebugClientInfo()), a Node will copy info from its

parent Node at the time the child Node is created. While this can be

better than nothing, it's often better for each participant to use

Node.SetDebugClientInfo() or Allocator.SetDebugClientInfo() to keep the

info directly relevant to the current client. Also, SetVerboseLogging()

can be used to help disambiguate if a Node is suspected of having info

that was copied from its parent.

Caller provides the backing storage for FIDL message.

Sysmem logs a warning if not all clients have set constraints 5 seconds

after creating a collection. Clients can call this method to change

when the log is printed. If multiple client set the deadline, it's

unspecified which deadline will take effect.

Caller provides the backing storage for FIDL message.

Verbose logging includes constraints set via SetConstraints() from each

client along with info set via SetDebugClientInfo() and the structure of

the tree of Node(s).

Normally sysmem prints only a single line complaint when aggregation

fails, with just the specific detailed reason that aggregation failed,

with minimal context. While this is often enough to diagnose a problem

if only a small change was made and the system had been working before

the small change, it's often not particularly helpful for getting a new

buffer collection to work for the first time. Especially with more

complex trees of nodes, involving things like AttachToken(),

SetDispensable(), BufferCollectionTokenGroup nodes, and associated

sub-trees of nodes, verbose logging may help in diagnosing what the tree

looks like and why it's failing a logical allocation, or why a tree or

sub-tree is failing sooner than expected.

The intent of the extra logging is to be acceptable from a performance

point of view, if only enabled on a low number of buffer collections.

If we're not tracking down a bug, we shouldn't send this message.

If too many participants leave verbose logging enabled, we may end up

needing to require that system-wide sysmem verbose logging be permitted

via some other setting, to avoid sysmem spamming the log too much due to

this message.

This may be a NOP for some nodes due to intentional policy associated

with the node, if we don't trust a node enough to let it turn on verbose

logging.

Caller provides the backing storage for FIDL message.

Provide BufferCollectionConstraints to the logical BufferCollection.

A participant may only call SetConstraints() once.

Sometimes the initiator is a participant only in the sense of wanting to

keep an eye on success/failure to populate with buffers, and zx.Status

on failure. In that case, `has_constraints` can be false, and

`constraints` will be ignored.

VMO handles will not be provided to the client that sends null

constraints - that can be intentional for an initiator that doesn't need

VMO handles. Not having VMO handles doesn't prevent the initator from

adjusting which portion of a buffer is considered valid and similar, but

the initiator can't hold a VMO handle open to prevent the logical

BufferCollection from cleaning up if the logical BufferCollection needs

to go away regardless of the initiator's degree of involvement for

whatever reason.

For population of buffers to be attempted, all holders of a

BufferCollection client channel need to call SetConstraints() before

sysmem will attempt to allocate buffers.

`has_constraints` if false, the constraints are effectively null, and

`constraints` are ignored. The sender of null constraints won't get any

VMO handles in BufferCollectionInfo, but can still find out how many

buffers were allocated and can still refer to buffers by their

buffer_index.

`constraints` are constraints on the buffer collection.

Caller provides the backing storage for FIDL message.

Create a new token, for trying to add a new participant to an existing

collection, if the existing collection's buffer counts, constraints,

and participants allow.

This can be useful in replacing a failed participant, and/or in

adding/re-adding a participant after buffers have already been

allocated.

Failure of an attached token / collection does not propagate to the

parent of the attached token. Failure does propagate from a normal

child of a dispensable token to the dispensable token. Failure

of a child is blocked from reaching its parent if the child is attached,

or if the child is dispensable and the failure occurred after logical

allocation.

An initiator may in some scenarios choose to initially use a dispensable

token for a given instance of a participant, and then later if the first

instance of that participant fails, a new second instance of that

participant my be given a token created with AttachToken().

From the point of view of the client end of the BufferCollectionToken

channel, the token acts like any other token. The client can

Duplicate() the token as needed, and can send the token to a different

process. The token should be converted to a BufferCollection channel

as normal by calling BindSharedCollection(). SetConstraints() should

be called on that BufferCollection channel.

A success result from WaitForBuffersAllocated() means the new

participant's constraints were satisfiable using the already-existing

buffer collection, the already-established BufferCollectionInfo

including image format constraints, and the already-existing other

participants and their buffer counts. A failure result means the new

participant's constraints cannot be satisfied using the existing

buffer collection and its already-logically-allocated participants.

Creating a new collection instead may allow all participant's

constraints to be satisfied, assuming SetDispensable() is used in place

of AttachToken(), or a normal token is used.

A token created with AttachToken() performs constraints aggregation with

all constraints currently in effect on the buffer collection, plus the

attached token under consideration plus child tokens under the attached

token which are not themselves an attached token or under such a token.

Allocation of buffer_count to min_buffer_count_for_camping etc is

first-come first-served, but a child can't logically allocate before

all its parents have sent SetConstraints().

See also SetDispensable(), which in contrast to AttachToken(), has the

created token + children participate in constraints aggregation along

with its parent.

The newly created token needs to be Sync()ed to sysmem before the new

token can be passed to BindSharedCollection(). The Sync() of the new

token can be accomplished with BufferCollection.Sync() on this

BufferCollection. Alternately BufferCollectionToken.Sync() on the new

token also works. A BufferCollectionToken.Sync() can be started after

any BufferCollectionToken.Duplicate() messages have been sent via the

newly created token, to also sync those additional tokens to sysmem

using a single round-trip.

These values for rights_attenuation_mask result in no attenuation (note

that 0 is not on this list; 0 will output an ERROR to the system log

to help diagnose the bug in client code):

* ZX_RIGHT_SAME_RIGHTS (preferred)

* 0xFFFFFFFF (this is reasonable when an attenuation mask is computed)

Caller provides the backing storage for FIDL message.

AttachLifetimeTracking:

AttachLifetimeTracking() is intended to allow a client to wait until an

old logical buffer collection is fully or mostly deallocated before

attempting allocation of a new logical buffer collection.

Attach an eventpair endpoint to the logical buffer collection, so that

the server_end will be closed when the number of buffers allocated

drops to 'buffers_remaining'. The server_end won't close until after

logical allocation has completed.

If logical allocation fails, such as for an attached sub-tree (using

AttachToken()), the server_end will close during that failure regardless

of the number of buffers potenitally allocated in the overall logical

buffer collection.

The lifetime signalled by this event includes asynchronous cleanup of

allocated buffers, and this asynchronous cleanup cannot occur until all

holders of VMO handles to the buffers have closed those VMO handles.

Therefore clients should take care not to become blocked forever waiting

for ZX_EVENTPAIR_PEER_CLOSED to be signalled, especially if any of the

participants using the logical buffer collection are less trusted or

less reliable.

The buffers_remaining parameter allows waiting for all but

buffers_remaining buffers to be fully deallocated. This can be useful

in situations where a known number of buffers are intentionally not

closed so that the data can continue to be used, such as for keeping the

last available video picture displayed in the UI even if the video

stream was using protected output buffers. It's outside the scope of

the BufferCollection interface (at least for now) to determine how many

buffers may be held without closing, but it'll typically be in the range

0-2.

This mechanism is meant to be compatible with other protocols providing

a similar AttachLifetimeTracking() mechanism, in that duplicates of the

same event can be sent to more than one AttachLifetimeTracking(), and

the ZX_EVENTPAIR_PEER_CLOSED will be signalled when all the lifetime

over conditions are met (all holders of duplicates have closed their

handle(s)).

There is no way to cancel an attach. Closing the client end of the

eventpair doesn't subtract from the number of pending attach(es).

Closing the client's end doesn't result in any action by the server.

If the server listens to events from the client end at all, it is for

debug logging only.

The server intentionally doesn't "trust" any bits signalled by the

client. This mechanism intentionally uses only ZX_EVENTPAIR_PEER_CLOSED

which can't be triggered early, and is only triggered when all handles

to server_end are closed. No meaning is associated with any of the

other signal bits, and clients should functionally ignore any other

signal bits on either end of the eventpair or its peer.

The server_end may lack ZX_RIGHT_SIGNAL or ZX_RIGHT_SIGNAL_PEER, but

must have ZX_RIGHT_DUPLICATE (and must have ZX_RIGHT_TRANSFER to

transfer without causing CodecFactory channel failure).

Caller provides the backing storage for FIDL message.