on message, track what the *physical* is doing when we add it to backlog

src/StackExchange.Redis/ExceptionFactory.cs

@@ -212,6 +212,7 @@ void add(string lk, string sk, string v)
                 try
                 {
                     if (message.QueuePosition >= 0) add("QueuePosition", null, message.QueuePosition.ToString()); // the position the item was when added to the queue
+                    if ((int)message.ConnectionWriteState >= 0) add("WriteState", null, message.ConnectionWriteState.ToString()); // what the physical was doing when it was 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

@@ -51,7 +51,14 @@ protected override void WriteImpl(PhysicalConnection physical)
     internal abstract class Message : ICompletable
     {
         public readonly int Db;
-        internal int QueuePosition { get; set; }
+        internal int QueuePosition { get; private set; }
+        internal PhysicalConnection.WriteStatus ConnectionWriteState { get; private set; }
+
+        internal void SetBacklogState(int position, PhysicalConnection physical)
+        {
+            QueuePosition = position;
+            ConnectionWriteState = physical?.Status ?? (PhysicalConnection.WriteStatus)(-1);
+        }
 
         internal const CommandFlags InternalCallFlag = (CommandFlags)128;
 
@@ -617,6 +624,7 @@ internal bool TrySetResult<T>(T value)
         internal void SetEnqueued(PhysicalConnection connection)
         {
             QueuePosition = -1;
+            ConnectionWriteState = (PhysicalConnection.WriteStatus)(-1);
             SetWriteTime();
             performance?.SetEnqueued();
             _enqueuedTo = connection;

src/StackExchange.Redis/PhysicalBridge.cs

@@ -140,7 +140,7 @@ private WriteResult QueueOrFailMessage(Message message)
                 message.SetEnqueued(null);
                 lock (_backlog)
                 {
-                    message.QueuePosition = _backlog.Count;
+                    message.SetBacklogState(_backlog.Count, null);
                     _backlog.Enqueue(message);
                 }
                 return WriteResult.Success; // we'll take it...
@@ -728,7 +728,7 @@ private bool PushToBacklog(Message message, bool onlyIfExists)
                 wasEmpty = count == 0;
                 if (wasEmpty & onlyIfExists) return false;
 
-                message.QueuePosition = count;
+                message.SetBacklogState(count, physical);
                 _backlog.Enqueue(message);
             }
             if (wasEmpty) StartBacklogProcessor();