statistics: support global singleflight for sync load (#52796) (#53839)

close #52797

planner/core/casetest/correlated/main_test.go

@@ -33,6 +33,7 @@ func TestMain(m *testing.M) {
 		goleak.IgnoreTopFunction("gopkg.in/natefinch/lumberjack%2ev2.(*Logger).millRun"),
 		goleak.IgnoreTopFunction("github.com/tikv/client-go/v2/txnkv/transaction.keepAlive"),
 		goleak.IgnoreTopFunction("go.opencensus.io/stats/view.(*worker).start"),
+		goleak.IgnoreTopFunction("github.com/pingcap/tidb/pkg/statistics/handle/syncload.(*statsSyncLoad).SendLoadRequests.func1"), // For TestPlanStatsLoadTimeout
 	}
 	goleak.VerifyTestMain(m, opts...)
 }

sessionctx/stmtctx/BUILD.bazel

@@ -23,6 +23,7 @@ go_library(
         "@com_github_tikv_client_go_v2//util",
         "@org_golang_x_exp//maps",
         "@org_golang_x_exp//slices",
+        "@org_golang_x_sync//singleflight",
         "@org_uber_go_atomic//:atomic",
         "@org_uber_go_zap//:zap",
     ],

sessionctx/stmtctx/stmtctx.go

@@ -45,6 +45,7 @@ import (
 	"go.uber.org/zap"
 	"golang.org/x/exp/maps"
 	"golang.org/x/exp/slices"
+	"golang.org/x/sync/singleflight"
 )
 
 const (
@@ -347,7 +348,7 @@ type StatementContext struct {
 		// NeededItems stores the columns/indices whose stats are needed for planner.
 		NeededItems []model.TableItemID
 		// ResultCh to receive stats loading results
-		ResultCh chan StatsLoadResult
+		ResultCh []<-chan singleflight.Result
 		// LoadStartTime is to record the load start time to calculate latency
 		LoadStartTime time.Time
 	}

statistics/handle/handle_hist.go

@@ -40,6 +40,8 @@ import (
 // RetryCount is the max retry count for a sync load task.
 const RetryCount = 3
 
+var globalStatsSyncLoadSingleFlight singleflight.Group
+
 type statsWrapper struct {
 	col *statistics.Column
 	idx *statistics.Index
@@ -80,25 +82,26 @@ func (h *Handle) SendLoadRequests(sc *stmtctx.StatementContext, neededHistItems
 	}
 	sc.StatsLoad.Timeout = timeout
 	sc.StatsLoad.NeededItems = remainedItems
-	sc.StatsLoad.ResultCh = make(chan stmtctx.StatsLoadResult, len(remainedItems))
-	tasks := make([]*NeededItemTask, 0)
+	sc.StatsLoad.ResultCh = make([]<-chan singleflight.Result, 0, len(remainedItems))
 	for _, item := range remainedItems {
-		task := &NeededItemTask{
-			TableItemID: item,
-			ToTimeout:   time.Now().Local().Add(timeout),
-			ResultCh:    sc.StatsLoad.ResultCh,
-		}
-		tasks = append(tasks, task)
-	}
-	timer := time.NewTimer(timeout)
-	defer timer.Stop()
-	for _, task := range tasks {
-		select {
-		case h.StatsLoad.NeededItemsCh <- task:
-			continue
-		case <-timer.C:
-			return errors.New("sync load stats channel is full and timeout sending task to channel")
-		}
+		localItem := item
+		resultCh := globalStatsSyncLoadSingleFlight.DoChan(localItem.Key(), func() (any, error) {
+			timer := time.NewTimer(timeout)
+			defer timer.Stop()
+			task := &NeededItemTask{
+				TableItemID: localItem,
+				ToTimeout:   time.Now().Local().Add(timeout),
+				ResultCh:    make(chan stmtctx.StatsLoadResult, 1),
+			}
+			select {
+			case h.StatsLoad.NeededItemsCh <- task:
+				result := <-task.ResultCh
+				return result, nil
+			case <-timer.C:
+				return nil, errors.New("sync load stats channel is full and timeout sending task to channel")
+			}
+		})
+		sc.StatsLoad.ResultCh = append(sc.StatsLoad.ResultCh, resultCh)
 	}
 	sc.StatsLoad.LoadStartTime = time.Now()
 	return nil
@@ -124,26 +127,34 @@ func (h *Handle) SyncWaitStatsLoad(sc *stmtctx.StatementContext) error {
 	metrics.SyncLoadCounter.Inc()
 	timer := time.NewTimer(sc.StatsLoad.Timeout)
 	defer timer.Stop()
-	for {
+	for _, resultCh := range sc.StatsLoad.ResultCh {
 		select {
-		case result, ok := <-sc.StatsLoad.ResultCh:
-			if ok {
-				if result.HasError() {
-					errorMsgs = append(errorMsgs, result.ErrorMsg())
-				}
-				delete(resultCheckMap, result.Item)
-				if len(resultCheckMap) == 0 {
-					metrics.SyncLoadHistogram.Observe(float64(time.Since(sc.StatsLoad.LoadStartTime).Milliseconds()))
-					return nil
-				}
-			} else {
+		case result, ok := <-resultCh:
+			if !ok {
 				return errors.New("sync load stats channel closed unexpectedly")
 			}
+			// this error is from statsSyncLoad.SendLoadRequests which start to task and send task into worker,
+			// not the stats loading error
+			if result.Err != nil {
+				errorMsgs = append(errorMsgs, result.Err.Error())
+			} else {
+				val := result.Val.(stmtctx.StatsLoadResult)
+				// this error is from the stats loading error
+				if val.HasError() {
+					errorMsgs = append(errorMsgs, val.ErrorMsg())
+				}
+				delete(resultCheckMap, val.Item)
+			}
 		case <-timer.C:
 			metrics.SyncLoadTimeoutCounter.Inc()
 			return errors.New("sync load stats timeout")
 		}
 	}
+	if len(resultCheckMap) == 0 {
+		metrics.SyncLoadHistogram.Observe(float64(time.Since(sc.StatsLoad.LoadStartTime).Milliseconds()))
+		return nil
+	}
+	return nil
 }
 
 // removeHistLoadedColumns removed having-hist columns based on neededColumns and statsCache.

statistics/handle/handle_hist_test.go

@@ -208,34 +208,40 @@ func TestConcurrentLoadHistWithPanicAndFail(t *testing.T) {
 		task1, err1 := h.HandleOneTask(nil, readerCtx, testKit.Session().(sqlexec.RestrictedSQLExecutor), exitCh)
 		require.Error(t, err1)
 		require.NotNil(t, task1)
-		select {
-		case <-stmtCtx1.StatsLoad.ResultCh:
-			t.Logf("stmtCtx1.ResultCh should not get anything")
-			t.FailNow()
-		case <-stmtCtx2.StatsLoad.ResultCh:
-			t.Logf("stmtCtx2.ResultCh should not get anything")
-			t.FailNow()
-		case <-task1.ResultCh:
-			t.Logf("task1.ResultCh should not get anything")
-			t.FailNow()
-		default:
+		for _, resultCh := range stmtCtx1.StatsLoad.ResultCh {
+			select {
+			case <-resultCh:
+				t.Logf("stmtCtx1.ResultCh should not get anything")
+				t.FailNow()
+			default:
+			}
+		}
+		for _, resultCh := range stmtCtx2.StatsLoad.ResultCh {
+			select {
+			case <-resultCh:
+				t.Logf("stmtCtx1.ResultCh should not get anything")
+				t.FailNow()
+			default:
+			}
 		}
 
 		require.NoError(t, failpoint.Disable(fp.failPath))
 		task3, err3 := h.HandleOneTask(task1, readerCtx, testKit.Session().(sqlexec.RestrictedSQLExecutor), exitCh)
 		require.NoError(t, err3)
 		require.Nil(t, task3)
 
-		task, err3 := h.HandleOneTask(nil, readerCtx, testKit.Session().(sqlexec.RestrictedSQLExecutor), exitCh)
-		require.NoError(t, err3)
-		require.Nil(t, task)
-
-		rs1, ok1 := <-stmtCtx1.StatsLoad.ResultCh
-		require.True(t, ok1)
-		require.Equal(t, neededColumns[0], rs1.Item)
-		rs2, ok2 := <-stmtCtx2.StatsLoad.ResultCh
-		require.True(t, ok2)
-		require.Equal(t, neededColumns[0], rs2.Item)
+		for _, resultCh := range stmtCtx1.StatsLoad.ResultCh {
+			rs1, ok1 := <-resultCh
+			require.True(t, rs1.Shared)
+			require.True(t, ok1)
+			require.Equal(t, neededColumns[0].ID, rs1.Val.(stmtctx.StatsLoadResult).Item.ID)
+		}
+		for _, resultCh := range stmtCtx2.StatsLoad.ResultCh {
+			rs1, ok1 := <-resultCh
+			require.True(t, rs1.Shared)
+			require.True(t, ok1)
+			require.Equal(t, neededColumns[0].ID, rs1.Val.(stmtctx.StatsLoadResult).Item.ID)
+		}
 
 		stat = h.GetTableStats(tableInfo)
 		hg = stat.Columns[tableInfo.Columns[2].ID].Histogram
@@ -312,11 +318,11 @@ func TestRetry(t *testing.T) {
 	result, err1 := h.HandleOneTask(task1, readerCtx, testKit.Session().(sqlexec.RestrictedSQLExecutor), exitCh)
 	require.NoError(t, err1)
 	require.Nil(t, result)
-	select {
-	case <-task1.ResultCh:
-	default:
-		t.Logf("task1.ResultCh should get nothing")
-		t.FailNow()
+	for _, resultCh := range stmtCtx1.StatsLoad.ResultCh {
+		rs1, ok1 := <-resultCh
+		require.True(t, rs1.Shared)
+		require.True(t, ok1)
+		require.Error(t, rs1.Val.(stmtctx.StatsLoadResult).Error)
 	}
 	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/statistics/handle/mockReadStatsForOneFail"))
 }