domain: adjust initialization sequence of domain-reload and ddl-start…

… to avoid infoCache nil (pingcap#28945)

domain/domain.go

@@ -737,24 +737,31 @@ func (do *Domain) Init(ddlLease time.Duration, sysFactory func(*Domain) (pools.R
 		ddl.WithHook(callback),
 		ddl.WithLease(ddlLease),
 	)
-	err = do.ddl.Start(sysCtxPool)
-	if err != nil {
-		return err
-	}
 	failpoint.Inject("MockReplaceDDL", func(val failpoint.Value) {
 		if val.(bool) {
-			if err := do.ddl.Stop(); err != nil {
-				logutil.BgLogger().Error("stop DDL failed", zap.Error(err))
-			}
 			do.ddl = d
 		}
 	})
-
+	// step 1: prepare the info/schema syncer which domain reload needed.
 	skipRegisterToDashboard := config.GetGlobalConfig().SkipRegisterToDashboard
+	do.info, err = infosync.GlobalInfoSyncerInit(ctx, do.ddl.GetID(), do.ServerID, do.etcdClient, skipRegisterToDashboard)
+	if err != nil {
+		return err
+	}
 	err = do.ddl.SchemaSyncer().Init(ctx)
 	if err != nil {
 		return err
 	}
+	// step 2: domain reload the infoSchema.
+	err = do.Reload()
+	if err != nil {
+		return err
+	}
+	// step 3: start the ddl after the domain reload, avoiding some internal sql running before infoSchema construction.
+	err = do.ddl.Start(sysCtxPool)
+	if err != nil {
+		return err
+	}
 
 	if config.GetGlobalConfig().Experimental.EnableGlobalKill {
 		if do.etcdClient != nil {
@@ -774,15 +781,6 @@ func (do *Domain) Init(ddlLease time.Duration, sysFactory func(*Domain) (pools.R
 		}
 	}
 
-	do.info, err = infosync.GlobalInfoSyncerInit(ctx, do.ddl.GetID(), do.ServerID, do.etcdClient, skipRegisterToDashboard)
-	if err != nil {
-		return err
-	}
-	err = do.Reload()
-	if err != nil {
-		return err
-	}
-
 	// Only when the store is local that the lease value is 0.
 	// If the store is local, it doesn't need loadSchemaInLoop.
 	if ddlLease > 0 {

domain/domain_test.go

@@ -85,7 +85,6 @@ func SubTestInfo(t *testing.T) {
 		ddl.WithInfoCache(dom.infoCache),
 		ddl.WithLease(ddlLease),
 	)
-	require.NoError(t, dom.ddl.Start(nil))
 	require.NoError(t, failpoint.Enable("github.com/pingcap/tidb/domain/MockReplaceDDL", `return(true)`))
 	require.NoError(t, dom.Init(ddlLease, sysMockFactory))
 	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/domain/MockReplaceDDL"))