lightning: fix dpanic log when init session vars (#37301)

close #37266

br/pkg/lightning/backend/kv/session.go

@@ -264,6 +264,16 @@ func newSession(options *SessionOptions, logger log.Logger) *session {
 	vars.SQLMode = sqlMode
 	if options.SysVars != nil {
 		for k, v := range options.SysVars {
+			// since 6.3(current master) tidb checks whether we can set a system variable
+			// lc_time_names is a read-only variable for now, but might be implemented later,
+			// so we not remove it from defaultImportantVariables and check it in below way.
+			if sv := variable.GetSysVar(k); sv == nil {
+				logger.DPanic("unknown system var", zap.String("key", k))
+				continue
+			} else if sv.ReadOnly {
+				logger.Debug("skip read-only variable", zap.String("key", k))
+				continue
+			}
 			if err := vars.SetSystemVar(k, v); err != nil {
 				logger.DPanic("new session: failed to set system var",
 					log.ShortError(err),

br/tests/lightning_local_backend/run.sh

@@ -20,6 +20,8 @@ check_cluster_version 4 0 0 'local backend' || exit 0
 
 ENGINE_COUNT=6
 
+res_file="$TEST_DIR/sql_res.$TEST_NAME.txt"
+
 # Test check table contains data
 rm -f "/tmp/tidb_lightning_checkpoint_local_backend_test.pb"
 rm -rf $TEST_DIR/lightning.log
@@ -80,7 +82,11 @@ set -e
 
 export GO_FAILPOINTS=''
 echo "******** Verify checkpoint no-op ********"
-run_lightning --backend local --enable-checkpoint=1 --log-file "$TEST_DIR/lightning-local.log" --config "tests/$TEST_NAME/config.toml"
+run_lightning --backend local --enable-checkpoint=1 --config "tests/$TEST_NAME/config.toml" --log-file $res_file -L debug
+check_not_contains "failed to set system var"
+check_not_contains "unknown system var"
+check_contains "skip read-only variable"
+check_contains "lc_time_names"
 
 run_sql 'SELECT count(*), sum(c) FROM cpeng.a'
 check_contains 'count(*): 4'