server: make the span stats fan-out more fault tolerant

This commit adds improved fault tolerance to the span stats fan-out:

1. Errors encountered during the fan-out will not invalidate the
entire request. Now, a span stats fan-out will always return a
roachpb.SpanStatsResponse that has been updated by values from nodes that
service their requests without error. In the extreme case where there's
a failure encountered on every node, an empty response is returned.

Errors that are encountered are logged, and then appended to the response
in the newly added `Errors` field.

2. Nodes must service requests within the timeout passed to `iterateNodes`.
For span stats, the value comes from a new cluster setting:
'server.span_stats.node.timeout', with a default value of 1 minute.

Resolves #106097
Epic: none
Release note (ops change): Span stats requests will return a partial
result if the request encounters any errors. Errors that would have
previously terminated the request are now included in the response.

pkg/roachpb/span_stats.go

@@ -12,6 +12,7 @@ package roachpb
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/settings"
 )
@@ -30,6 +31,15 @@ var SpanStatsBatchLimit = settings.RegisterIntSetting(
 	settings.PositiveInt,
 )
 
+var SpanStatsNodeTimeout = settings.RegisterDurationSetting(
+	settings.TenantWritable,
+	"server.span_stats.node.timeout",
+	"the duration allowed for a single node to return span stats data before"+
+		" the request is cancelled; if set to 0, there is no timeout",
+	time.Minute,
+	settings.NonNegativeDuration,
+)
+
 const defaultRangeStatsBatchLimit = 100
 
 // RangeStatsBatchLimit registers the maximum number of ranges to be batched

pkg/roachpb/span_stats.proto

@@ -67,5 +67,7 @@ message SpanStatsResponse {
 
   map<string, SpanStats> span_to_stats = 4;
 
-  // NEXT ID: 5.
+  string errors = 5;
+
+  // NEXT ID: 6.
 }

pkg/server/admin.go

@@ -3201,6 +3201,7 @@ func (s *systemAdminServer) EnqueueRange(
 	if err := timeutil.RunWithTimeout(ctx, "enqueue range", time.Minute, func(ctx context.Context) error {
 		return s.server.status.iterateNodes(
 			ctx, fmt.Sprintf("enqueue r%d in queue %s", req.RangeID, req.Queue),
+			noTimeout,
 			dialFn, nodeFn, responseFn, errorFn,
 		)
 	}); err != nil {

pkg/server/api_v2_ranges.go

@@ -247,7 +247,11 @@ func (a *apiV2Server) listRange(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if err := a.status.iterateNodes(
-		ctx, fmt.Sprintf("details about range %d", rangeID), dialFn, nodeFn, responseFn, errorFn,
+		ctx,
+		fmt.Sprintf("details about range %d", rangeID),
+		noTimeout,
+		dialFn, nodeFn,
+		responseFn, errorFn,
 	); err != nil {
 		srverrors.APIV2InternalError(ctx, err, w)
 		return

pkg/server/index_usage_stats.go

@@ -100,6 +100,7 @@ func (s *statusServer) IndexUsageStatistics(
 	// yields an incorrect result.
 	if err := s.iterateNodes(ctx,
 		"requesting index usage stats",
+		noTimeout,
 		dialFn, fetchIndexUsageStats, aggFn, errFn); err != nil {
 		return nil, err
 	}
@@ -196,6 +197,7 @@ func (s *statusServer) ResetIndexUsageStats(
 
 	if err := s.iterateNodes(ctx,
 		"Resetting index usage stats",
+		noTimeout,
 		dialFn, resetIndexUsageStats, aggFn, errFn); err != nil {
 		return nil, err
 	}

pkg/server/key_visualizer_server.go

@@ -110,8 +110,13 @@ func (s *KeyVisualizerServer) getSamplesFromFanOut(
 	}
 
 	err := s.status.iterateNodes(ctx,
-		"iterating nodes for key visualizer samples", dialFn, nodeFn,
-		responseFn, errorFn)
+		"iterating nodes for key visualizer samples",
+		noTimeout,
+		dialFn,
+		nodeFn,
+		responseFn,
+		errorFn,
+	)
 	if err != nil {
 		return nil, err
 	}

pkg/server/server.go

@@ -976,6 +976,11 @@ func NewServer(cfg Config, stopper *stop.Stopper) (serverctl.ServerStartupInterf
 	}
 
 	// Instantiate the status API server.
+	var serverTestingKnobs *TestingKnobs
+	if cfg.TestingKnobs.Server != nil {
+		serverTestingKnobs = cfg.TestingKnobs.Server.(*TestingKnobs)
+	}
+
 	sStatus := newSystemStatusServer(
 		cfg.AmbientCtx,
 		st,
@@ -998,6 +1003,7 @@ func NewServer(cfg Config, stopper *stop.Stopper) (serverctl.ServerStartupInterf
 		clock,
 		rangestats.NewFetcher(db),
 		node,
+		serverTestingKnobs,
 	)
 
 	keyVisualizerServer := &KeyVisualizerServer{

pkg/server/span_stats_server.go

@@ -12,7 +12,9 @@ package server
 
 import (
 	"context"
+	"fmt"
 	"strconv"
+	"strings"
 
 	"github.com/cockroachdb/cockroach/pkg/keys"
 	"github.com/cockroachdb/cockroach/pkg/kv"
@@ -37,8 +39,15 @@ func (s *systemStatusServer) spanStatsFanOut(
 	res := &roachpb.SpanStatsResponse{
 		SpanToStats: make(map[string]*roachpb.SpanStats),
 	}
-	// Response level error
-	var respErr error
+
+	errorMessages := make([]string, 0)
+
+	// Populate SpanToStats with empty values for each span,
+	// so that clients may still access stats for a specific span
+	// in the extreme case of an error encountered on every node.
+	for _, sp := range req.Spans {
+		res.SpanToStats[sp.String()] = &roachpb.SpanStats{}
+	}
 
 	spansPerNode, err := s.getSpansPerNode(ctx, req)
 	if err != nil {
@@ -51,13 +60,29 @@ func (s *systemStatusServer) spanStatsFanOut(
 		ctx context.Context,
 		nodeID roachpb.NodeID,
 	) (interface{}, error) {
+		if s.knobs != nil {
+			if s.knobs.IterateNodesDialCallback != nil {
+				if err := s.knobs.IterateNodesDialCallback(nodeID); err != nil {
+					return nil, err
+				}
+			}
+		}
+
 		if _, ok := spansPerNode[nodeID]; ok {
 			return s.dialNode(ctx, nodeID)
 		}
 		return nil, nil
 	}
 
 	nodeFn := func(ctx context.Context, client interface{}, nodeID roachpb.NodeID) (interface{}, error) {
+		if s.knobs != nil {
+			if s.knobs.IterateNodesNodeCallback != nil {
+				if err := s.knobs.IterateNodesNodeCallback(ctx, nodeID); err != nil {
+					return nil, err
+				}
+			}
+		}
+
 		// `smartDial` may skip this node, so check to see if the client is nil.
 		// If it is, return nil response.
 		if client == nil {
@@ -81,23 +106,26 @@ func (s *systemStatusServer) spanStatsFanOut(
 		nodeResponse := resp.(*roachpb.SpanStatsResponse)
 
 		for spanStr, spanStats := range nodeResponse.SpanToStats {
-			_, exists := res.SpanToStats[spanStr]
-			if !exists {
-				res.SpanToStats[spanStr] = spanStats
-			} else {
-				res.SpanToStats[spanStr].Add(spanStats)
+			// We are not counting replicas, so only consider range count
+			// if it has not been set.
+			if res.SpanToStats[spanStr].RangeCount == 0 {
+				res.SpanToStats[spanStr].RangeCount = spanStats.RangeCount
 			}
+			res.SpanToStats[spanStr].Add(spanStats)
 		}
 	}
 
 	errorFn := func(nodeID roachpb.NodeID, err error) {
 		log.Errorf(ctx, nodeErrorMsgPlaceholder, nodeID, err)
-		respErr = err
+		errorMessage := fmt.Sprintf("%v", err)
+		errorMessages = append(errorMessages, errorMessage)
 	}
 
+	timeout := roachpb.SpanStatsNodeTimeout.Get(&s.st.SV)
 	if err := s.statusServer.iterateNodes(
 		ctx,
 		"iterating nodes for span stats",
+		timeout,
 		smartDial,
 		nodeFn,
 		responseFn,
@@ -106,7 +134,8 @@ func (s *systemStatusServer) spanStatsFanOut(
 		return nil, err
 	}
 
-	return res, respErr
+	res.Errors = strings.Join(errorMessages, ",")
+	return res, nil
 }
 
 func (s *systemStatusServer) getLocalStats(

pkg/server/span_stats_test.go

@@ -20,6 +20,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/keys"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
+	"github.com/cockroachdb/cockroach/pkg/server"
 	"github.com/cockroachdb/cockroach/pkg/server/serverpb"
 	"github.com/cockroachdb/cockroach/pkg/testutils"
 	"github.com/cockroachdb/cockroach/pkg/testutils/serverutils"
@@ -191,6 +192,63 @@ func TestSpanStatsFanOut(t *testing.T) {
 
 }
 
+func TestSpanStatsFanOutFaultTolerance(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+	ctx := context.Background()
+	const numNodes = 5
+
+	serverArgs := base.TestServerArgs{}
+	serverArgs.Knobs.Server = &server.TestingKnobs{
+		IterateNodesDialCallback: func(nodeID roachpb.NodeID) error {
+			// On the 1st and 2nd node, simulate a connection error.
+			if nodeID == 1 || nodeID == 2 {
+				return errors.Newf("error dialing node %d", nodeID)
+			}
+			return nil
+		},
+		IterateNodesNodeCallback: func(ctx context.Context, nodeID roachpb.NodeID) error {
+			// On the 3rd node, simulate some sort of KV error.
+			if nodeID == 3 {
+				return errors.Newf("kv error on node %d", nodeID)
+			}
+
+			// On the 4th and 5th node, simulate a request that takes a very long time.
+			// In this case, nodeFn will block until the context is cancelled
+			// i.e. if iterateNodes respects the timeout cluster setting.
+			if nodeID == 4 || nodeID == 5 {
+				<-ctx.Done()
+				// Return an error that mimics the error returned
+				// when a rpc's context is cancelled:
+				return errors.Newf("node %d timed out", nodeID)
+			}
+			return nil
+		},
+	}
+
+	tc := testcluster.StartTestCluster(t, numNodes, base.TestClusterArgs{ServerArgs: serverArgs})
+	defer tc.Stopper().Stop(ctx)
+
+	sqlDB := tc.Server(0).SQLConn(t, "defaultdb")
+	_, err := sqlDB.Exec("SET CLUSTER SETTING server.span_stats.node.timeout = '3s'")
+	require.NoError(t, err)
+
+	res, err := tc.GetStatusClient(t, 0).SpanStats(ctx, &roachpb.SpanStatsRequest{
+		NodeID: "0",
+		Spans:  []roachpb.Span{keys.EverythingSpan},
+	})
+	require.NoError(t, err)
+	// Expect to still be able to access SpanToStats for keys.EverythingSpan
+	// without panicking, even though there was a failure on every node.
+	require.Equal(t, int64(0), res.SpanToStats[keys.EverythingSpan.String()].TotalStats.LiveCount)
+	require.Contains(t, res.Errors, "error dialing node 1")
+	require.Contains(t, res.Errors, "error dialing node 2")
+	require.Contains(t, res.Errors, "kv error on node 3")
+	require.Contains(t, res.Errors, "node 4 timed out")
+	require.Contains(t, res.Errors, "node 5 timed out")
+
+}
+
 // BenchmarkSpanStats measures the cost of collecting span statistics.
 func BenchmarkSpanStats(b *testing.B) {
 	skip.UnderShort(b)

pkg/server/sql_stats.go

@@ -79,6 +79,7 @@ func (s *statusServer) ResetSQLStats(
 	var fanoutError error
 
 	if err := s.iterateNodes(ctx, "reset SQL statistics",
+		noTimeout,
 		dialFn,
 		resetSQLStats,
 		func(nodeID roachpb.NodeID, resp interface{}) {

pkg/server/statements.go

@@ -85,6 +85,7 @@ func (s *statusServer) Statements(
 	}
 
 	if err := s.iterateNodes(ctx, "statement statistics",
+		noTimeout,
 		dialFn,
 		nodeStatement,
 		func(nodeID roachpb.NodeID, resp interface{}) {