Fix flaky tbuffered server test (#2635)

* Fix flaky tbuffered server test

Signed-off-by: Pavel Kositsyn <kositsyn.pa@phystech.edu>

* Apply suggestions from code review - more readable comments

Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>
Signed-off-by: Pavel Kositsyn <kositsyn.pa@phystech.edu>

Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>

cmd/agent/app/servers/tbuffered_server_test.go

@@ -96,13 +96,19 @@ type fakeTransport struct {
 	wg     sync.WaitGroup
 }
 
+// Read simulates three packets received, then blocks until semaphore is released at the end of the test.
+// First packet is returned as normal.
+// Second packet is simulated as error.
+// Third packet is returned as normal, but will be dropped as overflow by the server whose queue size = 1.
 func (t *fakeTransport) Read(p []byte) (n int, err error) {
 	packet := t.packet.Inc()
-	if packet > 2 {
-		if packet > 3 {
-			// return error once when packet==3, otherwise block
-			t.wg.Wait()
-		}
+	if packet == 2 {
+		// return some error packet, followed by valid one
+		return 0, io.ErrNoProgress
+	}
+	if packet > 3 {
+		// block after 3 packets until the server is shutdown and semaphore released
+		t.wg.Wait()
 		return 0, io.EOF
 	}
 	for i := range p {
@@ -128,9 +134,9 @@ func TestTBufferedServer_Metrics(t *testing.T) {
 	go server.Serve()
 	defer server.Stop()
 
-	// The fakeTransport will allow the server to read exactly two packets and one error.
+	// The fakeTransport will allow the server to read exactly two packets and one error in between.
 	// Since we use the server with queue size == 1, the first packet will be
-	// sent to channel, and the second one dropped.
+	// sent to channel, the error will increment the metric, and the second valid packet dropped.
 
 	packetDropped := false
 	for i := 0; i < 5000; i++ {