Exactly-once pin cleanup

Exactly-once pin cleanup gives one owner responsibility for ending a particular pinning lifetime, despite several possible completion paths. The resource must remain pinned until its consumers finish, and its cleanup must occur once after that condition is met. This combines the safety requirement of asynchronous pin release with a liveness requirement: a pin left permanently owned by nobody can obstruct later memory compaction.

A useful state machine has states “submission in progress”, “completion pending” and “released”. A submission error that guarantees no future consumer can access the address may take the first state directly to released. A pending result transfers cleanup responsibility to a completion path. Immediate completion must follow the API's stated rule about whether a callback will also arrive; it must not independently trigger two owners to dispose of the same handle. Microsoft's native-call example explicitly handles submission exceptions and funnels immediate or deferred completion into cleanup.^[ref]

For an original race example, suppose a completion callback and an error-reporting path can both observe an operation as finished. A plain sequence “test flag; if clear, set flag; release pin” can race: both callers read clear before either writes set, and both release. A synchronized ownership claim, using a mutex or an appropriate compare-and-swap transition, can ensure that only one caller changes pending to released. The losing caller performs no cleanup. This is a schematic ownership protocol, not a claim that every pin API tolerates arbitrary racing callbacks.

For a count $r$ of releases on one acquired handle, the at-most-once condition is $r\le 1$. The requirement that cleanup eventually occurs after all promised access has ended supplies $r\ge 1$ at final completion, so together they give $r=1$. A one-time flag alone establishes neither the end of access nor eventual execution of the winning path.

The operation's state must also stay alive while any callback can inspect it. Reference counting can protect that state independently of the pin being released. Marking a handle released does not permit another caller to access freed callback state. Keeping resource ownership, callback-state lifetime and data synchronization separate makes the protocol easier to verify. The primary task-based memory rule similarly requires all consumer access to finish by task termination.^[ref]