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Signed-off-by: Connor1996 <zbk602423539@gmail.com>

server/schedule/operator.go

@@ -504,19 +504,20 @@ func matchPeerSteps(cluster Cluster, source *core.RegionInfo, target *core.Regio
  return nil, kind, errors.New("mismatch count of peer")
  }
 
- // there is a case that a follower is added and transfer leader to it,
+ // There is a case that a follower is added and transfer leader to it,
  // and the apply process of it is slow so leader regards it as voter
  // but actually it is still learner. Once that, the follower can't be leader,
  // but old leader can't know that so there is no leader to serve for a while.
+ // So target leader should be the first added follower if there is no transection stores.
  var targetLeader uint64
  var toAdds [][]OperatorStep
 
- // Check whether to transfer leader or not
+ // get overlapped part of the peers of two regions
  intersection := getIntersectionStores(sourcePeers, targetPeers)
  for _, peer := range targetPeers {
  storeID := peer.GetStoreId()
+ // find missing peers.
  if _, found := intersection[storeID]; !found {
- // Find missing peers.
  var addSteps []OperatorStep
 
  peer, err := cluster.AllocPeer(storeID)
@@ -534,6 +535,7 @@ func matchPeerSteps(cluster Cluster, source *core.RegionInfo, target *core.Regio
  }
  toAdds = append(toAdds, addSteps)
 
+ // record the first added peer
  if targetLeader == 0 {
  targetLeader = storeID
  }
@@ -543,23 +545,23 @@ func matchPeerSteps(cluster Cluster, source *core.RegionInfo, target *core.Regio
 
  leaderID := source.GetLeader().GetStoreId()
  for storeID := range intersection {
+ // if leader belongs to overlapped part, no need to transfer
  if storeID == leaderID {
  targetLeader = 0
  break
  }
  targetLeader = storeID
  }
 
+ // if intersection is not empty and leader doesn't belong to intersection, transfer leader to store in overlapped part
  if len(intersection) != 0 && targetLeader != 0 {
- // if intersection is not empty and leader doesn't belong to intersection, transfer leader to store in intersection
  steps = append(steps, TransferLeader{FromStore: source.GetLeader().GetStoreId(), ToStore: targetLeader})
  kind |= OpLeader
  targetLeader = 0
  }
 
- // target leader should be the first added follower if there is no transection stores.
  index := 0
- // Remove redundant peers.
+ // remove redundant peers.
  for _, peer := range sourcePeers {
  if _, found := intersection[peer.GetStoreId()]; found {
  continue
@@ -576,7 +578,7 @@ func matchPeerSteps(cluster Cluster, source *core.RegionInfo, target *core.Regio
  index++
  }
 
- // remove leader
+ // transfer leader before remove leader
  if targetLeader != 0 {
  steps = append(steps, toAdds[index]...)
  steps = append(steps, TransferLeader{FromStore: leaderID, ToStore: targetLeader})