Bug fix: fix the Round-Robin algorithm when "ReadMode == ReplicaReadM…

…ixed". (#663)

Signed-off-by: Lucasliang <nkcs_lykx@hotmail.com>

internal/locate/region_request.go

@@ -523,15 +523,16 @@ type accessFollower struct {
 }
 
 func (state *accessFollower) next(bo *retry.Backoffer, selector *replicaSelector) (*RPCContext, error) {
+	replicaSize := len(selector.replicas)
 	if state.lastIdx < 0 {
 		if state.tryLeader {
-			state.lastIdx = AccessIndex(rand.Intn(len(selector.replicas)))
+			state.lastIdx = AccessIndex(rand.Intn(replicaSize))
 		} else {
-			if len(selector.replicas) <= 1 {
+			if replicaSize <= 1 {
 				state.lastIdx = state.leaderIdx
 			} else {
 				// Randomly select a non-leader peer
-				state.lastIdx = AccessIndex(rand.Intn(len(selector.replicas) - 1))
+				state.lastIdx = AccessIndex(rand.Intn(replicaSize - 1))
 				if state.lastIdx >= state.leaderIdx {
 					state.lastIdx++
 				}
@@ -547,8 +548,13 @@ func (state *accessFollower) next(bo *retry.Backoffer, selector *replicaSelector
 		state.lastIdx++
 	}
 
-	for i := 0; i < len(selector.replicas) && !state.option.leaderOnly; i++ {
-		idx := AccessIndex((int(state.lastIdx) + i) % len(selector.replicas))
+	for i := 0; i < replicaSize && !state.option.leaderOnly; i++ {
+		idx := AccessIndex((int(state.lastIdx) + i) % replicaSize)
+		// If the given store is abnormal to be accessed under `ReplicaReadMixed` mode, we should choose other followers or leader
+		// as candidates to serve the Read request. Meanwhile, we should make the choice of next() meet Uniform Distribution.
+		for cnt := 0; cnt < replicaSize && !state.isCandidate(idx, selector.replicas[idx]); cnt++ {
+			idx = AccessIndex((int(idx) + rand.Intn(replicaSize)) % replicaSize)
+		}
 		if state.isCandidate(idx, selector.replicas[idx]) {
 			state.lastIdx = idx
 			selector.targetIdx = idx