raft: refresh election timer on recovery-path append_entries

A follower that is behind a live leader can get stuck in a perpetual
pre-vote storm. `should_skip_vote` suppresses elections only while
`_hbeat` (last leader contact) is within one election timeout, but in
`do_append_entries` the `_hbeat` refresh lives on the successful
batch-append path (a deferred update). The two early returns that fire
when the follower is behind, log gap (prev_log_index > last_log_offset)
and prev-log-term mismatch, call maybe_update_leader() and reply to the
leader but never refresh `_hbeat`.

So a follower in continuous contact with the current-term leader, but
unable to append because it is still being recovered, lets its election
timer expire, starts a (pre-)vote that deterministically fails the
longest-log check (log_ok=false), rearms, and repeats. The storm burns
CPU that further starves recovery, a self-reinforcing livelock that
leaves partitions under-replicated and stalls acks=all produce with
REQUEST_TIMED_OUT.

Refresh `_hbeat` as soon as an append_entries from the current-term
leader is accepted (right after maybe_update_leader), covering the
recovery early-return paths as well as the success path. This matches
the existing design intent noted in vote_request handling, where the
comment states `_hbeat` is updated by the leader. Liveness is preserved:
if the leader is actually gone, no append_entries arrive, `_hbeat` goes
stale, and elections proceed as before.

Fixes #30815

Signed-off-by: Steven Pall <mail@stevenpall.ca>

Author: stevenpall

src/v/raft/consensus.cc

@@ -2052,6 +2052,8 @@ consensus::do_append_entries(append_entries_request&& r) {
     // follower (§5.2)
     maybe_update_leader(r.source_node());
 
+    auto refresh_hbeat = ss::defer([this] { _hbeat = clock_type::now(); });
+
     // raft.pdf: Reply false if log doesn’t contain an entry at
     // prevLogIndex whose term matches prevLogTerm (§5.3)
     // broken into 3 sections
@@ -2307,11 +2309,6 @@ consensus::do_append_entries(append_entries_request&& r) {
     // success. copy entries for each subsystem
 
     try {
-        auto deferred = ss::defer([this] {
-            // we do not want to include our disk flush latency into
-            // the leader vote timeout
-            _hbeat = clock_type::now();
-        });
         validate_offset_translator_delta(request_metadata, lstats);
 
         // simulate disk error