lightning: fix SET SESSION on connection pool (pingcap#54927)

close pingcap#49826

lightning/pkg/importer/meta_manager.go

@@ -185,7 +185,7 @@ func (m *dbTableMetaMgr) AllocTableRowIDs(ctx context.Context, rawRowIDMax int64
 	//nolint: errcheck
 	defer conn.Close()
 	exec := &common.SQLWithRetry{
-		DB:     m.session,
+		DB:     conn,
 		Logger: m.tr.logger,
 	}
 	var newRowIDBase, newRowIDMax int64
@@ -391,7 +391,7 @@ func (m *dbTableMetaMgr) CheckAndUpdateLocalChecksum(ctx context.Context, checks
 	//nolint: errcheck
 	defer conn.Close()
 	exec := &common.SQLWithRetry{
-		DB:     m.session,
+		DB:     conn,
 		Logger: m.tr.logger,
 	}
 	err = exec.Exec(ctx, "enable pessimistic transaction", "SET SESSION tidb_txn_mode = 'pessimistic';")
@@ -664,7 +664,7 @@ func (m *dbTaskMetaMgr) CheckTasksExclusively(ctx context.Context, action func(t
 	//nolint: errcheck
 	defer conn.Close()
 	exec := &common.SQLWithRetry{
-		DB:     m.session,
+		DB:     conn,
 		Logger: log.FromContext(ctx),
 	}
 	err = exec.Exec(ctx, "enable pessimistic transaction", "SET SESSION tidb_txn_mode = 'pessimistic';")
@@ -735,7 +735,7 @@ func (m *dbTaskMetaMgr) CheckAndPausePdSchedulers(ctx context.Context) (pdutil.U
 	//nolint: errcheck
 	defer conn.Close()
 	exec := &common.SQLWithRetry{
-		DB:     m.session,
+		DB:     conn,
 		Logger: log.FromContext(ctx),
 	}
 	err = exec.Exec(ctx, "enable pessimistic transaction", "SET SESSION tidb_txn_mode = 'pessimistic';")
@@ -875,7 +875,7 @@ func (m *dbTaskMetaMgr) CheckAndFinishRestore(ctx context.Context, finished bool
 	//nolint: errcheck
 	defer conn.Close()
 	exec := &common.SQLWithRetry{
-		DB:     m.session,
+		DB:     conn,
 		Logger: log.FromContext(ctx),
 	}
 	err = exec.Exec(ctx, "enable pessimistic transaction", "SET SESSION tidb_txn_mode = 'pessimistic';")