raft: fix spurious split-vote

Due to a typo raft candidate counted a vote for another node as a vote
for self in its split-vote detector. This could lead to spurious
split-vote detection in cases when another node wins elections with a bare
minimum of votes for it (exactly a quorum of votes).

Closes tarantool#8698

NO_DOC=bugfix

changelogs/unreleased/gh-8698-spurious-split-vote.md

@@ -0,0 +1,5 @@
+## bugfix/replication
+
+* Fixed nodes configured with `election_mode = 'candidate'` spuriously detecting
+  a split-vote when another candidate should win with exactly a quorum of votes
+  for it (gh-8698).

src/lib/raft/raft.c

@@ -935,7 +935,7 @@ raft_sm_schedule_new_vote(struct raft *raft, uint32_t candidate_id,
 	assert(!raft->votes[raft->self].did_vote);
 	raft->volatile_vote = candidate_id;
 	vclock_copy(&raft->candidate_vclock, candidate_vclock);
-	raft_add_vote(raft, raft->self, raft->self);
+	raft_add_vote(raft, raft->self, candidate_id);
 	raft_sm_pause_and_dump(raft);
 	/* Nothing visible is changed - no broadcast. */
 }

test/unit/raft.c

@@ -1710,7 +1710,7 @@ raft_test_bump_term_before_cfg()
 static void
 raft_test_split_vote(void)
 {
-	raft_start_test(64);
+	raft_start_test(67);
 	struct raft_node node;
 	raft_node_create(&node);
 
@@ -2032,6 +2032,35 @@ raft_test_split_vote(void)
 
 	is(node.raft.timer.repeat, 0, "still waiting for yield");
 
+	/*
+	 * gh-8698: a candidate might erroneously discover a split vote when
+	 * simply voting for another node.
+	 */
+	raft_node_destroy(&node);
+	raft_node_create(&node);
+	raft_node_cfg_cluster_size(&node, 2);
+	raft_node_cfg_election_quorum(&node, 2);
+
+	is(raft_node_send_vote_request(
+		&node,
+		2 /* Term. */,
+		"{}" /* Vclock. */,
+		2 /* Source. */
+	), 0, "vote for 2");
+
+	ok(raft_node_check_full_state(
+		&node,
+		RAFT_STATE_FOLLOWER /* State. */,
+		0 /* Leader. */,
+		2 /* Term. */,
+		2 /* Vote. */,
+		2 /* Volatile term. */,
+		2 /* Volatile vote. */,
+		"{0: 2}" /* Vclock. */
+	), "term and vote are persisted");
+	ok(node.raft.timer.repeat >= node.raft.election_timeout,
+	   "no split vote");
+
 	raft_node_destroy(&node);
 	raft_finish_test();
 }