make async commit configurable (pingcap#3)

Author: Shouyan guo

br/pkg/lightning/config/config.go

@@ -75,6 +75,7 @@ const (
 	defaultChecksumTableConcurrency   = 2
 	defaultTableConcurrency           = 6
 	defaultIndexConcurrency           = 2
+	defaultEnableAsyncCommit          = "ON"
 
 	// defaultMetaSchemaName is the default database name used to store lightning metadata
 	defaultMetaSchemaName     = "lightning_metadata"
@@ -134,6 +135,7 @@ type DBStore struct {
 	BuildStatsConcurrency      int               `toml:"build-stats-concurrency" json:"build-stats-concurrency"`
 	IndexSerialScanConcurrency int               `toml:"index-serial-scan-concurrency" json:"index-serial-scan-concurrency"`
 	ChecksumTableConcurrency   int               `toml:"checksum-table-concurrency" json:"checksum-table-concurrency"`
+	EnableAsyncCommit          string            `toml:"enable-async-commit" json:"enable-async-commit"`
 	Vars                       map[string]string `toml:"-" json:"vars"`
 }
 
@@ -712,6 +714,7 @@ func NewConfig() *Config {
 			DistSQLScanConcurrency:     defaultDistSQLScanConcurrency,
 			IndexSerialScanConcurrency: defaultIndexSerialScanConcurrency,
 			ChecksumTableConcurrency:   defaultChecksumTableConcurrency,
+			EnableAsyncCommit:          defaultEnableAsyncCommit,
 		},
 		Cron: Cron{
 			SwitchMode:     Duration{Duration: 5 * time.Minute},
@@ -1045,6 +1048,9 @@ func (cfg *Config) DefaultVarsForImporterAndLocalBackend() {
 	if cfg.TiDB.ChecksumTableConcurrency == 0 {
 		cfg.TiDB.ChecksumTableConcurrency = defaultChecksumTableConcurrency
 	}
+	if len(cfg.TiDB.EnableAsyncCommit) == 0 {
+		cfg.TiDB.EnableAsyncCommit = defaultEnableAsyncCommit
+	}
 }
 
 func (cfg *Config) CheckAndAdjustTiDBPort(ctx context.Context, mustHaveInternalConnections bool) error {

br/pkg/lightning/restore/tidb.go

@@ -85,6 +85,7 @@ func DBFromConfig(ctx context.Context, dsn config.DBStore) (*sql.DB, error) {
 		"tidb_distsql_scan_concurrency":      strconv.Itoa(dsn.DistSQLScanConcurrency),
 		"tidb_index_serial_scan_concurrency": strconv.Itoa(dsn.IndexSerialScanConcurrency),
 		"tidb_checksum_table_concurrency":    strconv.Itoa(dsn.ChecksumTableConcurrency),
+		"tidb_enable_async_commit":           dsn.EnableAsyncCommit,
 
 		// after https://github.com/pingcap/tidb/pull/17102 merge,
 		// we need set session to true for insert auto_random value in TiDB Backend