Module kv_cas 

Optimistic-concurrency read-modify-write over a JetStream KV key.

A blind kv.get → mutate → kv.put loses updates when two writers race: both read the same snapshot, both put, and the second put silently erases the first writer’s change. JetStream KV’s update(key, value, revision) is a compare-and-swap — it fails unless the key is still at the revision the caller read — and create(key, value) is the matching CAS for “key must not exist yet”. read_modify_write wraps both behind a re-read retry loop so callers only supply the mutation.

Functions

read_modify_write

Read key (missing or deleted → T::default()), apply mutate, and write the result back guarded by the revision the read observed, retrying the whole round on a CAS conflict.