make RESP3 detection more accurate "what was" rather than "what might have been"

Author: mgravell

src/StackExchange.Redis/PhysicalBridge.cs

@@ -124,9 +124,12 @@ public enum State : byte
         public RedisCommand LastCommand { get; private set; }
 
         /// <summary>
-        /// If we have a connection, report the protocol being used.
+        /// If we have (or had) a connection, report the protocol being used.
         /// </summary>
-        public RedisProtocol? Protocol => physical?.Protocol;
+        /// <remarks>The value remains after disconnect, so that appropriate follow-up actions (pub/sub etc) can work reliably.</remarks>
+        public RedisProtocol? Protocol => _protocol == 0 ? default(RedisProtocol?) : _protocol;
+        private RedisProtocol _protocol; // note starts at zero, not RESP2
+        internal void SetProtocol(RedisProtocol protocol) => _protocol = protocol;
 
         public void Dispose()
         {

src/StackExchange.Redis/PhysicalConnection.cs

@@ -276,7 +276,11 @@ private enum ReadMode : byte
         private RedisProtocol _protocol; // note starts at **zero**, not RESP2
         public RedisProtocol? Protocol => _protocol == 0 ? null : _protocol;
 
-        internal void SetProtocol(RedisProtocol value) => _protocol = value;
+        internal void SetProtocol(RedisProtocol value)
+        {
+            _protocol = value;
+            BridgeCouldBeNull?.SetProtocol(value);
+        }
 
         [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2202:Do not dispose objects multiple times", Justification = "Trust me yo")]
         internal void Shutdown()

src/StackExchange.Redis/ServerEndPoint.cs

@@ -636,7 +636,10 @@ internal void OnDisconnected(PhysicalBridge bridge)
             if (bridge == interactive)
             {
                 CompletePendingConnectionMonitors("Disconnected");
-                if (KnowOrAssumeResp3()) Multiplexer.UpdateSubscriptions();
+                if (Protocol is RedisProtocol.Resp3)
+                {
+                    Multiplexer.UpdateSubscriptions();
+                }
             }
             else if (bridge == subscription)
             {