Fix very rare deadlock

Fixes #1751

* Drain and log pending work when AsyncConsumerDispatcher loop ends.
* Make quiescing thread safe.
* Ensure that dequeued RPC continuations are always disposed. Found by @DenisMayorko ⭐

Author: lukebakken

projects/RabbitMQ.Client/ConsumerDispatching/AsyncConsumerDispatcher.cs

@@ -3,6 +3,7 @@
 using System.Threading.Tasks;
 using RabbitMQ.Client.Events;
 using RabbitMQ.Client.Impl;
+using RabbitMQ.Client.Logging;
 
 namespace RabbitMQ.Client.ConsumerDispatching
 {
@@ -71,6 +72,16 @@ await _channel.OnCallbackExceptionAsync(CallbackExceptionEventArgs.Build(e, work
                     throw;
                 }
             }
+            finally
+            {
+                while (_reader.TryRead(out WorkStruct work))
+                {
+                    using (work)
+                    {
+                        ESLog.Warn($"discarding consumer work: {work.WorkType}");
+                    }
+                }
+            }
         }
     }
 }

projects/RabbitMQ.Client/ConsumerDispatching/ConsumerDispatcherChannelBase.cs

@@ -44,7 +44,7 @@ internal abstract class ConsumerDispatcherChannelBase : ConsumerDispatcherBase,
         private readonly System.Threading.Channels.ChannelWriter<WorkStruct> _writer;
         private readonly Task _worker;
         private readonly ushort _concurrency;
-        private bool _quiesce = false;
+        private long _isQuiescing;
         private bool _disposed;
 
         internal ConsumerDispatcherChannelBase(Impl.Channel channel, ushort concurrency)
@@ -79,15 +79,15 @@ internal ConsumerDispatcherChannelBase(Impl.Channel channel, ushort concurrency)
             }
         }
 
-        public bool IsShutdown => _quiesce;
+        public bool IsShutdown => IsQuiescing;
 
         public ushort Concurrency => _concurrency;
 
         public async ValueTask HandleBasicConsumeOkAsync(IAsyncBasicConsumer consumer, string consumerTag, CancellationToken cancellationToken)
         {
             cancellationToken.ThrowIfCancellationRequested();
 
-            if (false == _disposed && false == _quiesce)
+            if (false == _disposed && false == IsQuiescing)
             {
                 try
                 {
@@ -110,7 +110,7 @@ public async ValueTask HandleBasicDeliverAsync(string consumerTag, ulong deliver
         {
             cancellationToken.ThrowIfCancellationRequested();
 
-            if (false == _disposed && false == _quiesce)
+            if (false == _disposed && false == IsQuiescing)
             {
                 IAsyncBasicConsumer consumer = GetConsumerOrDefault(consumerTag);
                 var work = WorkStruct.CreateDeliver(consumer, consumerTag, deliveryTag, redelivered, exchange, routingKey, basicProperties, body);
@@ -123,7 +123,7 @@ public async ValueTask HandleBasicCancelOkAsync(string consumerTag, Cancellation
         {
             cancellationToken.ThrowIfCancellationRequested();
 
-            if (false == _disposed && false == _quiesce)
+            if (false == _disposed && false == IsQuiescing)
             {
                 IAsyncBasicConsumer consumer = GetAndRemoveConsumer(consumerTag);
                 WorkStruct work = WorkStruct.CreateCancelOk(consumer, consumerTag);
@@ -136,7 +136,7 @@ public async ValueTask HandleBasicCancelAsync(string consumerTag, CancellationTo
         {
             cancellationToken.ThrowIfCancellationRequested();
 
-            if (false == _disposed && false == _quiesce)
+            if (false == _disposed && false == IsQuiescing)
             {
                 IAsyncBasicConsumer consumer = GetAndRemoveConsumer(consumerTag);
                 WorkStruct work = WorkStruct.CreateCancel(consumer, consumerTag);
@@ -147,7 +147,7 @@ await _writer.WriteAsync(work, cancellationToken)
 
         public void Quiesce()
         {
-            _quiesce = true;
+            Interlocked.Exchange(ref _isQuiescing, 1);
         }
 
         public async Task WaitForShutdownAsync()
@@ -157,7 +157,7 @@ public async Task WaitForShutdownAsync()
                 return;
             }
 
-            if (_quiesce)
+            if (IsQuiescing)
             {
                 try
                 {
@@ -193,6 +193,19 @@ await _worker
             }
         }
 
+        protected bool IsQuiescing
+        {
+            get
+            {
+                if (Interlocked.Read(ref _isQuiescing) == 1)
+                {
+                    return true;
+                }
+
+                return false;
+            }
+        }
+
         protected sealed override void ShutdownConsumer(IAsyncBasicConsumer consumer, ShutdownEventArgs reason)
         {
             _writer.TryWrite(WorkStruct.CreateShutdown(consumer, reason));

projects/RabbitMQ.Client/Impl/Channel.cs

@@ -744,11 +744,13 @@ protected async Task<bool> HandleChannelCloseOkAsync(IncomingCommand cmd, Cancel
             await FinishCloseAsync(cancellationToken)
                 .ConfigureAwait(false);
 
-            if (_continuationQueue.TryPeek<ChannelCloseAsyncRpcContinuation>(out ChannelCloseAsyncRpcContinuation? k))
+            if (_continuationQueue.TryPeek(out ChannelCloseAsyncRpcContinuation? k))
             {
-                _continuationQueue.Next();
-                await k.HandleCommandAsync(cmd)
-                    .ConfigureAwait(false);
+                using (IRpcContinuation c = _continuationQueue.Next())
+                {
+                    await k.HandleCommandAsync(cmd)
+                        .ConfigureAwait(false);
+                }
             }
 
             return true;
@@ -818,10 +820,12 @@ await ModelSendAsync(in replyMethod, cancellationToken)
 
         protected async Task<bool> HandleConnectionSecureAsync(IncomingCommand cmd, CancellationToken cancellationToken)
         {
-            var k = (ConnectionSecureOrTuneAsyncRpcContinuation)_continuationQueue.Next();
-            await k.HandleCommandAsync(new IncomingCommand())
-                .ConfigureAwait(false); // release the continuation.
-            return true;
+            using (var k = (ConnectionSecureOrTuneAsyncRpcContinuation)_continuationQueue.Next())
+            {
+                await k.HandleCommandAsync(new IncomingCommand())
+                    .ConfigureAwait(false); // release the continuation.
+                return true;
+            }
         }
 
         protected async Task<bool> HandleConnectionStartAsync(IncomingCommand cmd, CancellationToken cancellationToken)
@@ -848,12 +852,12 @@ await Session.Connection.CloseAsync(reason, false,
 
         protected async Task<bool> HandleConnectionTuneAsync(IncomingCommand cmd, CancellationToken cancellationToken)
         {
-            // Note: `using` here to ensure instance is disposed
-            using var k = (ConnectionSecureOrTuneAsyncRpcContinuation)_continuationQueue.Next();
-
-            // Note: releases the continuation and returns the buffers
-            await k.HandleCommandAsync(cmd)
-                .ConfigureAwait(false);
+            using (var k = (ConnectionSecureOrTuneAsyncRpcContinuation)_continuationQueue.Next())
+            {
+                // Note: releases the continuation and returns the buffers
+                await k.HandleCommandAsync(cmd)
+                    .ConfigureAwait(false);
+            }
 
             return true;
         }

projects/RabbitMQ.Client/Impl/RpcContinuationQueue.cs

@@ -97,7 +97,10 @@ public void Enqueue(IRpcContinuation k)
         ///</remarks>
         public void HandleChannelShutdown(ShutdownEventArgs reason)
         {
-            Next().HandleChannelShutdown(reason);
+            using (IRpcContinuation c = Next())
+            {
+                c.HandleChannelShutdown(reason);
+            }
         }
 
         ///<summary>Retrieve the next waiting continuation.</summary>