*: Support learner replica read (pingcap#39979)

close pingcap#40449

kv/option.go

@@ -113,6 +113,8 @@ const (
 	ReplicaReadClosest
 	// ReplicaReadClosestAdaptive stands for 'read from follower which locates in the same zone if the response size exceeds certain threshold'
 	ReplicaReadClosestAdaptive
+	// ReplicaReadLearner stands for 'read from learner'.
+	ReplicaReadLearner
 )
 
 // IsFollowerRead checks if follower is going to be used to read data.

sessionctx/variable/sysvar.go

@@ -1783,7 +1783,7 @@ var defaultSysVars = []*SysVar{
 		s.NoopFuncsMode = TiDBOptOnOffWarn(val)
 		return nil
 	}},
-	{Scope: ScopeGlobal | ScopeSession, Name: TiDBReplicaRead, Value: "leader", Type: TypeEnum, PossibleValues: []string{"leader", "follower", "leader-and-follower", "closest-replicas", "closest-adaptive"}, SetSession: func(s *SessionVars, val string) error {
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBReplicaRead, Value: "leader", Type: TypeEnum, PossibleValues: []string{"leader", "follower", "leader-and-follower", "closest-replicas", "closest-adaptive", "learner"}, SetSession: func(s *SessionVars, val string) error {
 		if strings.EqualFold(val, "follower") {
 			s.SetReplicaRead(kv.ReplicaReadFollower)
 		} else if strings.EqualFold(val, "leader-and-follower") {
@@ -1794,6 +1794,8 @@ var defaultSysVars = []*SysVar{
 			s.SetReplicaRead(kv.ReplicaReadClosest)
 		} else if strings.EqualFold(val, "closest-adaptive") {
 			s.SetReplicaRead(kv.ReplicaReadClosestAdaptive)
+		} else if strings.EqualFold(val, "learner") {
+			s.SetReplicaRead(kv.ReplicaReadLearner)
 		}
 		return nil
 	}},

store/driver/options/options.go

@@ -32,6 +32,8 @@ func GetTiKVReplicaReadType(t kv.ReplicaReadType) storekv.ReplicaReadType {
 		return storekv.ReplicaReadMixed
 	case kv.ReplicaReadClosestAdaptive:
 		return storekv.ReplicaReadMixed
+	case kv.ReplicaReadLearner:
+		return storekv.ReplicaReadLearner
 	}
 	return 0
 }