[Filebeat][CometD] Resolve flaky unit test (#34903)

* update unit test

* add change log entry

CHANGELOG.next.asciidoc

@@ -103,6 +103,7 @@ https://github.com/elastic/beats/compare/v8.2.0\...main[Check the HEAD diff]
 - Add missing Basic Authentication support to CEL input {issue}34609[34609] {pull}34689[34689]
 - [Gcs Input] - Added missing locks for safe concurrency {pull}34914[34914]
 - Fix the ignore_inactive option being ignored in Filebeat's filestream input {pull}34770[34770]
+- Fix TestMultiEventForEOFRetryHandlerInput unit test of CometD input {pull}34903[34903]
 
 *Heartbeat*
 

x-pack/filebeat/input/cometd/input_test.go

@@ -393,18 +393,16 @@ func assertEventMatches(t *testing.T, expected bay.MaybeMsg, got beat.Event) {
 func TestMultiEventForEOFRetryHandlerInput(t *testing.T) {
 	var err error
 
-	errorAfterEvent := 2
-	expectedHTTPEventCount := 6
-	expectedEventCount := 4
+	expectedEventCount := 2
 
-	eventsCh := make(chan beat.Event)
-	defer close(eventsCh)
+	eventsCh := make(chan beat.Event, expectedEventCount)
 	signal := make(chan struct{}, 1)
 	defer close(signal)
 
 	outlet := &mockedOutleter{
 		onEventHandler: func(event beat.Event) bool {
 			eventsCh <- event
+			signal <- struct{}{}
 			return true
 		},
 	}
@@ -442,16 +440,11 @@ func TestMultiEventForEOFRetryHandlerInput(t *testing.T) {
 			_, _ = w.Write([]byte(`[{"ext":{"replay":true,"payload.format":true},"minimumVersion":"1.0","clientId":"client_id","supportedConnectionTypes":["long-polling"],"channel":"/meta/handshake","version":"1.0","successful":true}]`))
 			return
 		case "/meta/connect":
-			if i < expectedHTTPEventCount {
-				if i == errorAfterEvent {
-					// stop server to produce EOF errors
-					signal <- struct{}{}
-				}
-				i++
+			if i == 0 {
 				_, _ = w.Write([]byte(`[{"data": {"payload": {"CountryIso": "IN"}, "event": {"replayId":1234}}, "channel": "channel_name"}]`))
+				i++
 				return
 			}
-			i++
 			_, _ = w.Write([]byte(`{}`))
 			return
 		case "/meta/subscribe":
@@ -474,31 +467,33 @@ func TestMultiEventForEOFRetryHandlerInput(t *testing.T) {
 	require.NotNil(t, input)
 
 	input.Run()
-	go func() {
-		j := 0
-		for event := range eventsCh {
-			if j >= expectedEventCount {
-				signal <- struct{}{}
-				break
-			}
-			assertEventMatches(t, expected, event)
-			j++
-		}
-	}()
 
-	<-signal
 	// close previous connection
+	<-signal
 	server.CloseClientConnections()
 	server.Close()
 	time.Sleep(100 * time.Millisecond)
 
 	// restart connection for new events
+	i = 0
 	server, err = newTestServer(strings.Split(serverURL, "http://")[1], r)
-	assert.NoError(t, err)
-	serverURL = server.URL
-	defer server.Close()
+	for err != nil {
+		server, err = newTestServer(strings.Split(serverURL, "http://")[1], r)
+	}
 	<-signal
+	server.CloseClientConnections()
+	server.Close()
 
+	close(eventsCh)
+
+	go func() {
+		for j := 0; j < expectedEventCount; j++ {
+			event := <-eventsCh
+			assertEventMatches(t, expected, event)
+		}
+		signal <- struct{}{}
+	}()
+	<-signal
 	input.Stop()
 }