track the current message so we can give a good error in the case of fail;...

track the current message so we can give a good error in the case of fail; prefer Volatile.Read over CEX /null/null

StackExchange.Redis/ConnectionMultiplexer.cs

@@ -1216,7 +1216,7 @@ internal bool ReconfigureIfNeeded(EndPoint blame, bool fromBroadcast, string cau
             {
                 OnConfigurationChangedBroadcast(blame);
             }
-            string activeCause = Interlocked.CompareExchange(ref activeConfigCause, null, null);
+            string activeCause = Volatile.Read(ref activeConfigCause);
             if (activeCause == null)
             {
                 bool reconfigureAll = fromBroadcast || publishReconfigure;
@@ -2082,8 +2082,9 @@ internal Exception GetException(WriteResult result, Message message, ServerEndPo
                         var bridge = server.GetBridge(message.Command, false);
                         if (bridge != null)
                         {
+                            var active = bridge.GetActiveMessage();
                             bridge.GetOutstandingCount(out var inst, out var qs, out var @in);
-                            counters = $", inst={inst}, qs={qs}, in={@in}";
+                            counters = $", inst={inst}, qs={qs}, in={@in}, active={active}";
                         }
                     }
                     catch { }
@@ -2271,8 +2272,7 @@ private static int GetThreadPoolStats(out string iocp, out string worker)
         /// </summary>
         public string GetStormLog()
         {
-            var result = Interlocked.CompareExchange(ref stormLogSnapshot, null, null);
-            return result;
+            return Volatile.Read(ref stormLogSnapshot);
         }
         /// <summary>
         /// Resets the log of unusual busy patterns

StackExchange.Redis/PhysicalBridge.cs

@@ -541,7 +541,7 @@ internal bool TryEnqueue(List<Message> messages, bool isSlave)
 
         private readonly object SingleWriterLock = new object();
 
-        private int reentrantCount = 0;
+        private Message _activeMesssage;
         /// <summary>
         /// This writes a message to the output stream
         /// </summary>
@@ -565,9 +565,11 @@ internal WriteResult WriteMessageTakingWriteLock(PhysicalConnection physical, Me
                     return WriteResult.TimeoutBeforeWrite;
                 }
 
-                if(reentrantCount++ != 0)
+                var existingMessage = Interlocked.CompareExchange(ref _activeMesssage, message, null);
+                if (existingMessage != null)
                 {
-                    Multiplexer?.OnInfoMessage("reentrant call to WriteMessageTakingWriteLock for " + message.CommandAndKey);
+                    Multiplexer?.OnInfoMessage($"reentrant call to WriteMessageTakingWriteLock for {message.CommandAndKey}, {existingMessage.CommandAndKey} is still active");
+                    return WriteResult.NoConnectionAvailable;
                 }
                 physical.SetWriting();
                 var messageIsSent = false;
@@ -608,7 +610,7 @@ internal WriteResult WriteMessageTakingWriteLock(PhysicalConnection physical, Me
             {
                 if (haveLock)
                 {
-                    reentrantCount--;
+                    Interlocked.CompareExchange(ref _activeMesssage, null, message); // remove if it is us
                     Monitor.Exit(SingleWriterLock);
                 }
             }
@@ -834,5 +836,7 @@ internal void SimulateConnectionFailure()
             }
             physical?.RecordConnectionFailed(ConnectionFailureType.SocketFailure);
         }
+
+        internal RedisCommand? GetActiveMessage() => Volatile.Read(ref _activeMesssage)?.Command;
     }
 }

StackExchange.Redis/PhysicalConnection.cs

@@ -56,7 +56,7 @@ private static readonly Message
         private IDuplexPipe _ioPipe;
 
         private Socket _socket;
-        private Socket VolatileSocket => Interlocked.CompareExchange(ref _socket, null, null);
+        private Socket VolatileSocket => Volatile.Read(ref _socket);
 
         public PhysicalConnection(PhysicalBridge bridge)
         {
@@ -1345,7 +1345,7 @@ private void OnDebugAbort()
                 // this CEX is just a hardcore "seriously, read the actual value" - there's no
                 // convenient "Thread.VolatileRead<T>(ref T field) where T : class", and I don't
                 // want to make the field volatile just for this one place that needs it
-                if (isReading && Interlocked.CompareExchange(ref _ioPipe, null, null) == null)
+                if (isReading && Volatile.Read(ref _ioPipe) == null)
                 {
                     // yeah, that's fine... don't worry about it
                 }

StackExchange.Redis/RedisSubscriber.cs

@@ -216,7 +216,7 @@ public Task UnsubscribeFromServer(RedisChannel channel, CommandFlags flags, obje
                 return oldOwner.WriteDirectAsync(msg, ResultProcessor.TrackSubscriptions, asyncState);
             }
 
-            internal ServerEndPoint GetOwner() => Interlocked.CompareExchange(ref owner, null, null);
+            internal ServerEndPoint GetOwner() => Volatile.Read(ref owner);
 
             internal void Resubscribe(RedisChannel channel, ServerEndPoint server)
             {
@@ -232,7 +232,7 @@ internal void Resubscribe(RedisChannel channel, ServerEndPoint server)
             internal bool Validate(ConnectionMultiplexer multiplexer, RedisChannel channel)
             {
                 bool changed = false;
-                var oldOwner = Interlocked.CompareExchange(ref owner, null, null);
+                var oldOwner = Volatile.Read(ref owner);
                 if (oldOwner != null && !oldOwner.IsSelectable(RedisCommand.PSUBSCRIBE))
                 {
                     if (UnsubscribeFromServer(channel, CommandFlags.FireAndForget, null, true) != null)

StackExchange.Redis/SocketManager.cs

@@ -42,7 +42,7 @@ internal static SocketManager Shared
                         shared = null;
                 }
                 finally { shared?.Dispose(); }
-                return Interlocked.CompareExchange(ref _shared, null, null);
+                return Volatile.Read(ref _shared);
             }
         }