Enrols a sender: checks the one-time registration code (hash, expiry, constant-time compare),
checks the proof of possession of the new key, then activates the registration with that key.
The code is single-use within this process: activation runs under the registration lock, and
inside it the row is reloaded from the database and the code is checked again. Two collector
processes that receive the same code at the same moment can still both accept it.
Enrols a sender: checks the one-time registration code (hash, expiry, constant-time compare), checks the proof of possession of the new key, then activates the registration with that key. The code is single-use within this process: activation runs under the registration lock, and inside it the row is reloaded from the database and the code is checked again. Two collector processes that receive the same code at the same moment can still both accept it.