ddl: initialize the mock session in advanced (#41227)

close pingcap/tidb#40879

ddl/backfilling.go

@@ -861,6 +861,9 @@ func initSessCtx(sessCtx sessionctx.Context, sqlMode mysql.SQLMode, tzLocation *
 	sessCtx.GetSessionVars().StmtCtx.DividedByZeroAsWarning = !sqlMode.HasStrictMode()
 	sessCtx.GetSessionVars().StmtCtx.IgnoreZeroInDate = !sqlMode.HasStrictMode() || sqlMode.HasAllowInvalidDatesMode()
 	sessCtx.GetSessionVars().StmtCtx.NoZeroDate = sqlMode.HasStrictMode()
+	// Prevent initializing the mock context in the workers concurrently.
+	// For details, see https://github.com/pingcap/tidb/issues/40879.
+	_ = sessCtx.GetDomainInfoSchema()
 	return nil
 }