track the position in the queue that an item was *when added*

src/StackExchange.Redis/ExceptionFactory.cs

@@ -211,6 +211,7 @@ void add(string lk, string sk, string v)
                 add("Timeout", "timeout", Format.ToString(mutiplexer.TimeoutMilliseconds));
                 try
                 {
+                    if (message.QueuePosition >= 0) add("QueuePosition", null, message.QueuePosition.ToString()); // the position the item was when added to the queue
                     if (message.TryGetPhysicalState(out var state, out var sentDelta, out var receivedDelta))
                     {
                         add("PhysicalState", "phys", state.ToString());

src/StackExchange.Redis/Message.cs

@@ -51,6 +51,7 @@ protected override void WriteImpl(PhysicalConnection physical)
     internal abstract class Message : ICompletable
     {
         public readonly int Db;
+        internal int QueuePosition { get; set; }
 
         internal const CommandFlags InternalCallFlag = (CommandFlags)128;
 
@@ -615,6 +616,7 @@ internal bool TrySetResult<T>(T value)
 
         internal void SetEnqueued(PhysicalConnection connection)
         {
+            QueuePosition = -1;
             SetWriteTime();
             performance?.SetEnqueued();
             _enqueuedTo = connection;

src/StackExchange.Redis/PhysicalBridge.cs

@@ -137,11 +137,12 @@ private WriteResult QueueOrFailMessage(Message message)
             {
                 // you can go in the queue, but we won't be starting
                 // a worker, because the handshake has not completed
+                message.SetEnqueued(null);
                 lock (_backlog)
                 {
+                    message.QueuePosition = _backlog.Count;
                     _backlog.Enqueue(message);
                 }
-                message.SetEnqueued(null);
                 return WriteResult.Success; // we'll take it...
             }
             else
@@ -723,9 +724,11 @@ private bool PushToBacklog(Message message, bool onlyIfExists)
             bool wasEmpty;
             lock (_backlog)
             {
-                wasEmpty = _backlog.Count == 0;
+                int count = _backlog.Count;
+                wasEmpty = count == 0;
                 if (wasEmpty & onlyIfExists) return false;
 
+                message.QueuePosition = count;
                 _backlog.Enqueue(message);
             }
             if (wasEmpty) StartBacklogProcessor();