close

Closes this observation exactly once.

The source cleanup callback runs at most once and runs synchronously on the first caller. Other callers wait for it to finish. A cleanup failure without re-entry is propagated unchanged by this and every later call. Re-entry from the cleanup callback is rejected instead of deadlocking. The re-entry failure becomes primary even when cleanup catches it, and any distinct failure subsequently thrown by cleanup is suppressed on it. A successful return guarantees that the source can start no further callback for this subscription, although a callback already in progress may finish.

Throws