Increase health check channel buffer (#17821)

go.mod

@@ -180,6 +180,7 @@ require (
 	github.com/tidwall/pretty v1.2.0 // indirect
 	go.opencensus.io v0.23.0 // indirect
 	go.uber.org/atomic v1.7.0 // indirect
+	go.uber.org/goleak v1.3.0 // indirect
 	go.uber.org/multierr v1.6.0 // indirect
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 	google.golang.org/appengine v1.6.7 // indirect

go.sum

@@ -799,8 +799,8 @@ go.uber.org/atomic v1.3.2/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.4.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.7.0 h1:ADUqmZGgLDDfbSL9ZmPxKTybcoEYHgpYfELNoN+7hsw=
 go.uber.org/atomic v1.7.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
-go.uber.org/goleak v1.1.11 h1:wy28qYRKZgnJTxGxvye5/wgWr1EKjmUDGYox5mGlRlI=
-go.uber.org/goleak v1.1.11/go.mod h1:cwTWslyiVhfpKIDGSZEM2HlOvcqm+tG4zioyIeLoqMQ=
+go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
+go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 go.uber.org/mock v0.4.0 h1:VcM4ZOtdbR4f6VXfiOpwpVJDL6lCReaZ6mw31wqh7KU=
 go.uber.org/mock v0.4.0/go.mod h1:a6FSlNadKUHUa9IP5Vyt1zh4fC7uAwxMutEAscFbkZc=
 go.uber.org/multierr v1.1.0/go.mod h1:wR5kodmAFQ0UK8QlbwjlSNy0Z68gJhDJUG5sjR94q/0=

go/vt/discovery/healthcheck.go

@@ -58,6 +58,7 @@ import (
 	"vitess.io/vitess/go/vt/servenv"
 	"vitess.io/vitess/go/vt/topo"
 	"vitess.io/vitess/go/vt/topo/topoproto"
+	"vitess.io/vitess/go/vt/topotools"
 	"vitess.io/vitess/go/vt/vterrors"
 	"vitess.io/vitess/go/vt/vttablet/queryservice"
 )
@@ -68,6 +69,9 @@ var (
 	hcPrimaryPromotedCounters = stats.NewCountersWithMultiLabels("HealthcheckPrimaryPromoted", "Primary promoted in keyspace/shard name because of health check errors", []string{"Keyspace", "ShardName"})
 	healthcheckOnce           sync.Once
 
+	// counter that tells us how many healthcheck messages have been dropped
+	hcChannelFullCounter = stats.NewCounter("HealthCheckChannelFullErrors", "Number of times the healthcheck broadcast channel was full")
+
 	// TabletURLTemplateString is a flag to generate URLs for the tablets that vtgate discovers.
 	TabletURLTemplateString = "http://{{.GetTabletHostPort}}"
 	tabletURLTemplate       *template.Template
@@ -598,7 +602,7 @@ func (hc *HealthCheckImpl) recomputeHealthy(key KeyspaceShardTabletType) {
 func (hc *HealthCheckImpl) Subscribe() chan *TabletHealth {
 	hc.subMu.Lock()
 	defer hc.subMu.Unlock()
-	c := make(chan *TabletHealth, 2)
+	c := make(chan *TabletHealth, 2048)
 	hc.subscribers[c] = struct{}{}
 	return c
 }
@@ -617,6 +621,9 @@ func (hc *HealthCheckImpl) broadcast(th *TabletHealth) {
 		select {
 		case c <- th:
 		default:
+			// If the channel is full, we drop the message.
+			hcChannelFullCounter.Add(1)
+			log.Warningf("HealthCheck broadcast channel is full, dropping message for %s", topotools.TabletIdent(th.Tablet))
 		}
 	}
 }

go/vt/discovery/healthcheck_test.go

@@ -24,6 +24,7 @@ import (
 	"io"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -1307,6 +1308,44 @@ func TestDebugURLFormatting(t *testing.T) {
 	require.Contains(t, wr.String(), expectedURL, "output missing formatted URL")
 }
 
+// TestConcurrentUpdates tests that concurrent updates from the HealthCheck implementation aren't dropped.
+// Added in response to https://github.com/vitessio/vitess/issues/17629.
+func TestConcurrentUpdates(t *testing.T) {
+	// reset error counters
+	hcErrorCounters.ResetAll()
+	ts := memorytopo.NewServer("cell")
+	defer ts.Close()
+	hc := createTestHc(ts)
+	// close healthcheck
+	defer hc.Close()
+
+	// Subscribe to the healthcheck
+	// Make the receiver keep track of the updates received.
+	ch := hc.Subscribe()
+	var totalCount atomic.Int32
+	go func() {
+		for range ch {
+			totalCount.Add(1)
+			// Simulate a somewhat slow consumer.
+			time.Sleep(100 * time.Millisecond)
+		}
+	}()
+
+	// Run multiple updates really quickly
+	// one after the other.
+	totalUpdates := 10
+	for i := 0; i < totalUpdates; i++ {
+		hc.broadcast(&TabletHealth{})
+	}
+	// Unsubscribe from the healthcheck
+	// and verify we process all the updates eventually.
+	hc.Unsubscribe(ch)
+	defer close(ch)
+	require.Eventuallyf(t, func() bool {
+		return totalUpdates == int(totalCount.Load())
+	}, 5*time.Second, 100*time.Millisecond, "expected all updates to be processed")
+}
+
 func tabletDialer(tablet *topodatapb.Tablet, _ grpcclient.FailFast) (queryservice.QueryService, error) {
 	connMapMu.Lock()
 	defer connMapMu.Unlock()