Since #34288, we might hit deadlock if the FollowTask has more fetchers
than writers. This can happen in the following scenario:
Suppose the leader has two operations [seq#0, seq#1]; the FollowTask has
two fetchers and one writer.
1. The FollowTask issues two concurrent fetch requests: {from_seq_no: 0,
num_ops:1} and {from_seq_no: 1, num_ops:1} to read seq#0 and seq#1
respectively.
2. The second request which fetches seq#1 completes before, and then it
triggers a write request containing only seq#1.
3. The primary of a follower fails after it has replicated seq#1 to
replicas.
4. Since the old primary did not respond, the FollowTask issues another
write request containing seq#1 (resend the previous write request).
5. The new primary has seq#1 already; thus it won't replicate seq#1 to
replicas but will wait for the global checkpoint to advance at least
seq#1.
The problem is that the FollowTask has only one writer and that writer
is waiting for seq#0 which won't be delivered until the writer completed.
This PR proposes to replicate existing operations with the old primary
term (instead of the current term) on the follower. In particular, when
the following primary detects that it has processed an process already,
it will look up the term of an existing operation with the same seq_no
in the Lucene index, then rewrite that operation with the old term
before replicating it to the following replicas. This approach is
wait-free but requires soft-deletes on the follower.
Relates #34288
bd92a28cfc