Does `fdb_future_set_callback` guarantee exactly once execution?

No, fdb_future_set_callback guarantees at most once execution.

A fdb_future is a C wrapper around a Flow future (Flow is the actor framework that FoundationDB is implemented in). A future can even be set to Never - in which case it is guaranteed that your callback will never be called. However, the callback is guaranteed to fire if one cancel or destroy the future at some point.

To alleviate the potential problemetic cases, for example the case where a wait on such a future can hang there for hours or even days, one can set the option transaction_timeout on each transaction started. Since 6.1, this option can be set on the database level so that each transation will by default have a transaction_timeout set.