replica selector refactor (#1142)

* init

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* add ReplicaSelector interface

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* add replica_selector_v2, todo: fix test

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* fix all test

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* fix test in another way to compatible with old version about stale-read request

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* tiny refactor

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* refactor to remove duplicate code

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* add more test

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* refine onServerIsBusy region error handing logic

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* support forwarding by proxy

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* support busyThreshold and tiny reractor code

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* add config

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* fix test

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* fix proxy bug

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* fix test and tiny refactor

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* add some test

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* add more test

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* tidy refine

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* address comment

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* fix test in v1

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* add backoff-cnt check and timeout test

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* refine test

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* refine code

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* refine code

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* add benchmark test

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* make test stale

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* fix golangci

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* add more test and refine code

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* fix race test

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* add more comprehensive enumeration testing

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* refine test

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* fix test

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* fix accessFollower with label retry bug

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* refine test and fix some bug

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* fix test and add more test

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* add more test

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* refine test

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* refine test

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* refine onNotLeader logic when new leader is not available

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* refine calculateScore logic, if the replica already tried, decrease the score

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* refine logic: replica-read request with mixed strategy and with label, should be able to retry all remain replicas

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* prefer-leader for mixed read with label for non stale-read req

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* add more test case

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* refine code

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* resolve conflict

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* remove some duplicate test

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* refactor test

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* refine code

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* refine test

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* add busy_threshold test and fix bug

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* fix proxy bug and add more test

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* refine code and add test

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* add learner test

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* add more test

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* remove old test and refine test

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* move test

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* address comment

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* use new score calculation by flag bit

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* make test stable

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* refine comment,license and fix lint

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* refine comment

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* refine comment

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* fix race test timeout

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* add flagNotAttemp in score

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* address comment

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---------

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config/config.go

@@ -82,23 +82,26 @@ type Config struct {
 	RegionsRefreshInterval uint64
 	// EnablePreload indicates whether to preload region info when initializing the client.
 	EnablePreload bool
+	// EnableReplicaSelectorV2 indicates whether to enable ReplicaSelectorV2.
+	EnableReplicaSelectorV2 bool
 }
 
 // DefaultConfig returns the default configuration.
 func DefaultConfig() Config {
 	return Config{
-		CommitterConcurrency:  128,
-		MaxTxnTTL:             60 * 60 * 1000, // 1hour
-		TiKVClient:            DefaultTiKVClient(),
-		PDClient:              DefaultPDClient(),
-		TxnLocalLatches:       DefaultTxnLocalLatches(),
-		StoresRefreshInterval: DefStoresRefreshInterval,
-		OpenTracingEnable:     false,
-		Path:                  "",
-		EnableForwarding:      false,
-		TxnScope:              "",
-		EnableAsyncCommit:     false,
-		Enable1PC:             false,
+		CommitterConcurrency:    128,
+		MaxTxnTTL:               60 * 60 * 1000, // 1hour
+		TiKVClient:              DefaultTiKVClient(),
+		PDClient:                DefaultPDClient(),
+		TxnLocalLatches:         DefaultTxnLocalLatches(),
+		StoresRefreshInterval:   DefStoresRefreshInterval,
+		OpenTracingEnable:       false,
+		Path:                    "",
+		EnableForwarding:        false,
+		TxnScope:                "",
+		EnableAsyncCommit:       false,
+		Enable1PC:               false,
+		EnableReplicaSelectorV2: true,
 	}
 }