Write requests cannot be safely retried if some replicas respond with
accepts and others with rejects. In this case, the coordinator is
uncertain about the outcome of the LWT: a subsequent LWT may either
complete the Paxos round (if a quorum observed the accept) or overwrite it
(if a quorum did not). If the original LWT was actually completed by
later rounds and the coordinator retried it, the write could be applied
twice, potentially overwriting effects of other LWTs that slipped in
between. Read requests do not have this problem, so they
can be safely retried.
Before this commit, handler->accept_proposal was called with
timeout_if_partially_accepted := true. This caused both read and write
requests to throw an "uncertainty" timeout to the user in the case
of the contention described above. After this commit, we throw an
"uncertainty" timeout only for write requests, while read requests
are instead retried in the loop in sp::cas.
Closesscylladb/scylladb#25602
2e757d6de4