*: fix panic when reusing OptimisticTxnContextProvider (#38952)

close #38951

session/txnmanager.go

@@ -51,7 +51,7 @@ type txnManager struct {
 	ctxProvider sessiontxn.TxnContextProvider
 
 	// We always reuse the same OptimisticTxnContextProvider in one session to reduce memory allocation cost for every new txn.
-	reservedOptimisticProvider isolation.OptimisticTxnContextProvider
+	reservedOptimisticProviders [2]isolation.OptimisticTxnContextProvider
 }
 
 func newTxnManager(sctx sessionctx.Context) *txnManager {
@@ -241,8 +241,13 @@ func (m *txnManager) newProviderWithRequest(r *sessiontxn.EnterNewTxnRequest) (s
 	switch txnMode {
 	case "", ast.Optimistic:
 		// When txnMode is 'OPTIMISTIC' or '', the transaction should be optimistic
-		m.reservedOptimisticProvider.ResetForNewTxn(m.sctx, r.CausalConsistencyOnly)
-		return &m.reservedOptimisticProvider, nil
+		provider := &m.reservedOptimisticProviders[0]
+		if old, ok := m.ctxProvider.(*isolation.OptimisticTxnContextProvider); ok && old == provider {
+			// We should make sure the new provider is not the same with the old one
+			provider = &m.reservedOptimisticProviders[1]
+		}
+		provider.ResetForNewTxn(m.sctx, r.CausalConsistencyOnly)
+		return provider, nil
 	case ast.Pessimistic:
 		// When txnMode is 'PESSIMISTIC', the provider should be determined by the isolation level
 		switch sessVars.IsolationLevelForNewTxn() {