tell us what heartbeats happen, before they happen

StackExchange.Redis.Tests/TestBase.cs

@@ -276,6 +276,7 @@ protected IServer GetAnyMaster(ConnectionMultiplexer muxer)
                 };
                 muxer.Connecting += (e, t) => Writer.WriteLine($"Connecting to {Format.ToString(e)} as {t}");
                 muxer.TransactionLog += msg => { lock (Writer) { Writer.WriteLine("tran: " + msg); } };
+                muxer.Heartbeat += msg => Writer.WriteLine($"{Time()}: {msg}");
                 muxer.Resurrecting += (e, t) => Writer.WriteLine($"Resurrecting {Format.ToString(e)} as {t}");
                 muxer.Closing += complete => Writer.WriteLine(complete ? "Closed" : "Closing...");
                 return muxer;

StackExchange.Redis.Tests/TestInfoReplicationChecks.cs

@@ -22,7 +22,7 @@ public async Task Exec()
                 await Task.Delay(7000).ForAwait();
                 var after = conn.GetCounters();
                 int done = (int)(after.Interactive.CompletedSynchronously - before.Interactive.CompletedSynchronously);
-                Assert.True(done >= 2);
+                Assert.True(done >= 2, $"expected >=2, got {done}");
             }
         }
     }

StackExchange.Redis/StackExchange/Redis/PhysicalBridge.cs

@@ -237,6 +237,7 @@ internal void KeepAlive()
         {
             var commandMap = Multiplexer.CommandMap;
             Message msg = null;
+            var features = ServerEndPoint.GetFeatures();
             switch (ConnectionType)
             {
                 case ConnectionType.Interactive:
@@ -244,7 +245,7 @@ internal void KeepAlive()
                     msg.SetSource(ResultProcessor.Tracer, null);
                     break;
                 case ConnectionType.Subscription:
-                    if (commandMap.IsAvailable(RedisCommand.PING) && ServerEndPoint.GetFeatures().PingOnSubscriber)
+                    if (commandMap.IsAvailable(RedisCommand.PING) && features.PingOnSubscriber)
                     {
                         msg = Message.Create(-1, CommandFlags.FireAndForget, RedisCommand.PING);
                         msg.SetSource(ResultProcessor.Tracer, null);
@@ -261,6 +262,7 @@ internal void KeepAlive()
             {
                 msg.SetInternalCall();
                 Multiplexer.Trace("Enqueue: " + msg);
+                Multiplexer.OnHeartbeat($"heartbeat '{msg.CommandAndKey}' on '{PhysicalName}' (v{features.Version})");
                 var result = TryWrite(msg, ServerEndPoint.IsSlave);
 
                 if (result != WriteResult.Success)

StackExchange.Redis/StackExchange/Redis/RedisSubscriber.cs

@@ -254,7 +254,7 @@ internal string GetConnectionName(EndPoint endPoint, ConnectionType connectionTy
 
         internal event Action<string, Exception, string> MessageFaulted;
         internal event Action<bool> Closing;
-        internal event Action<string> PreTransactionExec, TransactionLog;
+        internal event Action<string> PreTransactionExec, TransactionLog, Heartbeat;
         internal event Action<EndPoint, ConnectionType> Connecting;
         internal event Action<EndPoint, ConnectionType> Resurrecting;
 
@@ -262,6 +262,10 @@ internal void OnMessageFaulted(Message msg, Exception fault, [CallerMemberName]
         {
             MessageFaulted?.Invoke(msg?.CommandAndKey, fault, $"{origin} ({path}#{lineNumber})");
         }
+        internal void OnHeartbeat(string message)
+        {
+            Heartbeat?.Invoke(message);
+        }
 
         internal void OnClosing(bool complete)
         {