Event-driven backstop for the terminal {:workflow_finished, _} notification.

Baton.Worker announces a workflow the instant its last step returns cleanly. But a step that crashes — raises, exits, times out, or otherwise exhausts its retries via an exception — never returns through the worker's result handler, so Baton.Completion.check/2 is not called for it. Historically the only thing that caught those workflows was Baton.Plugin's periodic sweep, which meant up to a full interval of latency before the finish event fired.

This module closes that gap by listening to Oban's own job telemetry. It attaches to [:oban, :job, :exception] and, when a workflow job reaches a terminal :discard, calls Baton.Completion.announce/2 immediately. Oban writes the job's terminal state to the database before emitting the event (ack_event precedes emit_event in the executor), so announce/2 reads the already-settled state with no overlay needed. The exactly-once claim in Baton.Completion means this never double-fires with the worker path.

With this attached, the plugin's sweep is a true backstop (for workflows killed entirely outside job execution, e.g. an external Oban.cancel_all_jobs that never runs perform/1) rather than the primary detector — so the sweep interval can be long without delaying the common crash case.

Wiring

Baton.Plugin attaches this in its init/1 and detaches on terminate/1, so hosts that run the plugin get it for free. Hosts that do not run the plugin can attach it from their own application start:

Baton.CompletionReporter.attach()