fix: Rework packet loss metric tests to work with UTP 2.0 [MTT-4535] (Unity-Technologies#2174)

* Allow fixing the simulator RNG seed

* Fix packet loss test for UTP 2.0

* Add comment about random seed in packet loss test

Author: simon-lemay-unity

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -337,6 +337,8 @@ public struct SimulatorParameters
             PacketDropRate = 0
         };
 
+        internal uint DebugSimulatorRandomSeed { get; set; } = 0;
+
         private struct PacketLossCache
         {
             public int PacketsReceived;
@@ -1322,7 +1324,8 @@ private void ConfigureSimulator()
                 maxPacketSize: NetworkParameterConstants.MTU,
                 packetDelayMs: DebugSimulator.PacketDelayMS,
                 packetJitterMs: DebugSimulator.PacketJitterMS,
-                packetDropPercentage: DebugSimulator.PacketDropRate
+                packetDropPercentage: DebugSimulator.PacketDropRate,
+                randomSeed: DebugSimulatorRandomSeed
 #if UTP_TRANSPORT_2_0_ABOVE
                 , mode: ApplyMode.AllPackets
 #endif

com.unity.netcode.gameobjects/Tests/Runtime/Metrics/PacketLossMetricsTests.cs

@@ -16,7 +16,8 @@ public class PacketLossMetricsTests : NetcodeIntegrationTest
     {
         protected override int NumberOfClients => 1;
         private readonly int m_PacketLossRate = 25;
-        private readonly int m_PacketLossRangeDelta = 5;
+        private readonly int m_PacketLossRangeDelta = 3;
+        private readonly int m_MessageSize = 200;
 
         public PacketLossMetricsTests()
             : base(HostOrServer.Server)
@@ -27,6 +28,12 @@ protected override void OnServerAndClientsCreated()
             var clientTransport = (UnityTransport)m_ClientNetworkManagers[0].NetworkConfig.NetworkTransport;
             clientTransport.SetDebugSimulatorParameters(0, 0, m_PacketLossRate);
 
+            // Determined through trial and error. With both UTP 1.2 and 2.0, this random seed
+            // results in an effective packet loss percentage between 22% and 28%. Future UTP
+            // updates may change the RNG call patterns and cause this test to fail, in which
+            // case the value should be modified again.
+            clientTransport.DebugSimulatorRandomSeed = 4;
+
             base.OnServerAndClientsCreated();
         }
 
@@ -39,8 +46,8 @@ public IEnumerator TrackPacketLossAsServer()
 
             for (int i = 0; i < 1000; ++i)
             {
-                using var writer = new FastBufferWriter(sizeof(byte), Allocator.Persistent);
-                writer.WriteByteSafe(42);
+                using var writer = new FastBufferWriter(m_MessageSize, Allocator.Persistent);
+                writer.WriteBytesSafe(new byte[m_MessageSize]);
                 m_ServerNetworkManager.CustomMessagingManager.SendNamedMessage("Test", m_ServerNetworkManager.ConnectedClientsIds, writer);
             }
 
@@ -51,9 +58,6 @@ public IEnumerator TrackPacketLossAsServer()
         }
 
         [UnityTest]
-#if UTP_TRANSPORT_2_0_ABOVE
-        [Ignore("Pending adjustment for UTP 2.0")]
-#endif
         public IEnumerator TrackPacketLossAsClient()
         {
             double packetLossRateMinRange = (m_PacketLossRate - m_PacketLossRangeDelta) / 100d;
@@ -65,8 +69,8 @@ public IEnumerator TrackPacketLossAsClient()
 
             for (int i = 0; i < 1000; ++i)
             {
-                using var writer = new FastBufferWriter(sizeof(byte), Allocator.Persistent);
-                writer.WriteByteSafe(42);
+                using var writer = new FastBufferWriter(m_MessageSize, Allocator.Persistent);
+                writer.WriteBytesSafe(new byte[m_MessageSize]);
                 m_ServerNetworkManager.CustomMessagingManager.SendNamedMessage("Test", m_ServerNetworkManager.ConnectedClientsIds, writer);
             }