replication: refactor replicaset_leader()

Firstly, rename it to replicaset_find_join_master(). Now, when
there is Raft with a concept of an actual leader, the function
name becomes confusing.

Secondly, do not access ballot member in struct applier in such a
long way - save the ballot pointer on the stack. This is going to
become useful when in one of the next patches the ballot will be
used more.

Part of #5613

(cherry picked from commit 2daec52)

src/box/box.cc

@@ -1369,7 +1369,7 @@ box_set_replication_anon(void)
 		 * Wait until the master has registered this
 		 * instance.
 		 */
-		struct replica *master = replicaset_leader();
+		struct replica *master = replicaset_find_join_master();
 		if (master == NULL || master->applier == NULL ||
 		    master->applier->state != APPLIER_CONNECTED) {
 			tnt_raise(ClientError, ER_CANNOT_REGISTER);
@@ -3114,8 +3114,7 @@ bootstrap(const struct tt_uuid *instance_uuid,
 	 */
 	box_sync_replication(true);
 
-	/* Use the first replica by URI as a bootstrap leader */
-	struct replica *master = replicaset_leader();
+	struct replica *master = replicaset_find_join_master();
 	assert(master == NULL || master->applier != NULL);
 
 	if (master != NULL && !tt_uuid_is_equal(&master->uuid, &INSTANCE_UUID)) {

src/box/replication.cc

@@ -968,33 +968,35 @@ replicaset_round(bool skip_ro)
 		struct applier *applier = replica->applier;
 		if (applier == NULL)
 			continue;
+		const struct ballot *ballot = &applier->ballot;
 		/**
 		 * While bootstrapping a new cluster, read-only
 		 * replicas shouldn't be considered as a leader.
 		 * The only exception if there is no read-write
 		 * replicas since there is still a possibility
 		 * that all replicas exist in cluster table.
 		 */
-		if (skip_ro && applier->ballot.is_ro)
+		if (skip_ro && ballot->is_ro)
 			continue;
 		if (leader == NULL) {
 			leader = replica;
 			continue;
 		}
+		const struct ballot *leader_ballot = &leader->applier->ballot;
 		/*
 		 * Try to find a replica which has already left
 		 * orphan mode.
 		 */
-		if (applier->ballot.is_loading && ! leader->applier->ballot.is_loading)
+		if (ballot->is_loading && !leader_ballot->is_loading)
 			continue;
 		/*
 		 * Choose the replica with the most advanced
 		 * vclock. If there are two or more replicas
 		 * with the same vclock, prefer the one with
 		 * the lowest uuid.
 		 */
-		int cmp = vclock_compare_ignore0(&applier->ballot.vclock,
-						 &leader->applier->ballot.vclock);
+		int cmp = vclock_compare_ignore0(&ballot->vclock,
+						 &leader_ballot->vclock);
 		if (cmp < 0)
 			continue;
 		if (cmp == 0 && tt_uuid_compare(&replica->uuid,
@@ -1006,7 +1008,7 @@ replicaset_round(bool skip_ro)
 }
 
 struct replica *
-replicaset_leader(void)
+replicaset_find_join_master(void)
 {
 	bool skip_ro = true;
 	/**

src/box/replication.h

@@ -356,10 +356,11 @@ struct replica *
 replica_by_id(uint32_t replica_id);
 
 /**
- * Return the replica set leader.
+ * Find a node in the replicaset on which the instance can try to register to
+ * join the replicaset.
  */
 struct replica *
-replicaset_leader(void);
+replicaset_find_join_master(void);
 
 struct replica *
 replicaset_first(void);