box: refactor box_demote to make it more comprehensible

Suggested by Nikita Zheleztsov in the scope of tarantool#9855.

Needed for tarantool#9855

NO_CHANGELOG=<refactoring>
NO_DOC=<refactoring>
NO_TEST=<refactoring>

Co-authored-by: Nikita Zheleztsov <n.zheleztsov@proton.me>

Author: 2 people

src/box/box.cc

@@ -3080,36 +3080,35 @@ box_demote(void)
 		return 0;
 
 	const struct raft *raft = box_raft();
-	if (box_election_mode == ELECTION_MODE_OFF) {
-		assert(raft->state == RAFT_STATE_FOLLOWER);
-		if (raft->leader != REPLICA_ID_NIL) {
-			diag_set(ClientError, ER_NOT_LEADER, raft->leader);
-			return -1;
-		}
-		if (txn_limbo.owner_id == REPLICA_ID_NIL)
-			return 0;
-		/*
-		 * If the limbo term is up to date with Raft, then it might have
-		 * a valid owner right now. Demotion would disrupt it. In this
-		 * case the user has to explicitly overthrow the old owner with
-		 * local promote(), or call demote() on the actual owner.
-		 */
-		if (txn_limbo.promote_greatest_term == raft->term &&
-		    txn_limbo.owner_id != instance_id)
-			return 0;
-	} else {
+	if (box_election_mode != ELECTION_MODE_OFF) {
 		if (txn_limbo_replica_term(&txn_limbo, instance_id) !=
 		    raft->term)
 			return 0;
 		if (txn_limbo.owner_id != instance_id)
 			return 0;
 		if (raft->state != RAFT_STATE_LEADER)
 			return 0;
+		return box_trigger_elections();
 	}
-	if (box_trigger_elections() != 0)
+
+	assert(raft->state == RAFT_STATE_FOLLOWER);
+	if (raft->leader != REPLICA_ID_NIL) {
+		diag_set(ClientError, ER_NOT_LEADER, raft->leader);
 		return -1;
-	if (box_election_mode != ELECTION_MODE_OFF)
+	}
+	if (txn_limbo.owner_id == REPLICA_ID_NIL)
 		return 0;
+	/*
+	 * If the limbo term is up to date with Raft, then it might have
+	 * a valid owner right now. Demotion would disrupt it. In this
+	 * case the user has to explicitly overthrow the old owner with
+	 * local promote(), or call demote() on the actual owner.
+	 */
+	if (txn_limbo.promote_greatest_term == raft->term &&
+	    txn_limbo.owner_id != instance_id)
+		return 0;
+	if (box_trigger_elections() != 0)
+		return -1;
 	if (box_try_wait_confirm(2 * replication_synchro_timeout) < 0)
 		return -1;
 	int64_t wait_lsn = box_wait_limbo_acked(replication_synchro_timeout);