Related to discussion #1793

* Expand throttling delay code, add comment.
* Add `ThrowsExceptionRateLimiter` and use it in `MaybeSomethingUpWithRateLimiter_GH1793`, which reproduces the issue.
* Reproduce issue by artifically never acquiring rate limiter leases.
* Fix issue by catching `SemaphoreFullException` and checking that the exception was thrown in a valid failure case.

projects/RabbitMQ.Client/Impl/Channel.PublisherConfirms.cs

@@ -344,7 +344,24 @@ private async Task MaybeEndPublisherConfirmationTracking(PublisherConfirmationIn
         {
             if (_publisherConfirmationsEnabled)
             {
-                _confirmSemaphore.Release();
+                try
+                {
+                    _confirmSemaphore.Release();
+                }
+                catch (SemaphoreFullException ex)
+                {
+                    /*
+                     * rabbitmq/rabbitmq-dotnet-client-1793
+                     * If MaybeStartPublisherConfirmationTracking throws an exception *prior* to acquiring
+                     * _confirmSemaphore, the above Release() call will throw SemaphoreFullException.
+                     * In "normal" cases, publisherConfirmationInfo will thus be null, but if not, throw
+                     * a "bug found" exception here.
+                     */
+                    if (publisherConfirmationInfo is not null)
+                    {
+                        throw new InvalidOperationException(InternalConstants.BugFound, ex);
+                    }
+                }
 
                 if (publisherConfirmationInfo is not null)
                 {

projects/RabbitMQ.Client/ThrottlingRateLimiter.cs

@@ -134,13 +134,24 @@ protected override void Dispose(bool disposing)
 
         private int CalculateDelay()
         {
-            long? availablePermits = _concurrencyLimiter.GetStatistics()?.CurrentAvailablePermits;
-            if (!(availablePermits < _throttlingThreshold))
+            RateLimiterStatistics? rateLimiterStatistics = _concurrencyLimiter.GetStatistics();
+            if (rateLimiterStatistics is null)
             {
                 return 0;
             }
 
-            return (int)((1.0 - availablePermits / (double)_maxConcurrency) * 1000);
+            long availablePermits = rateLimiterStatistics.CurrentAvailablePermits;
+            if (availablePermits >= _throttlingThreshold)
+            {
+                /*
+                 * Note: do NOT add a delay because available permits exceeeds the threshold
+                 * below which throttling begins
+                 */
+                return 0;
+            }
+
+            double percentageUsed = 1.0 - (availablePermits / (double)_maxConcurrency);
+            return (int)(percentageUsed * 1000);
         }
     }
 }

projects/Test/Integration/GH/NeverAcquiredRateLimiter.cs

@@ -0,0 +1,68 @@
+// This source code is dual-licensed under the Apache License, version
+// 2.0, and the Mozilla Public License, version 2.0.
+//
+// The APL v2.0:
+//
+//---------------------------------------------------------------------------
+//   Copyright (c) 2007-2024 Broadcom. All Rights Reserved.
+//
+//   Licensed under the Apache License, Version 2.0 (the "License");
+//   you may not use this file except in compliance with the License.
+//   You may obtain a copy of the License at
+//
+//       https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+//---------------------------------------------------------------------------
+//
+// The MPL v2.0:
+//
+//---------------------------------------------------------------------------
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+//
+//  Copyright (c) 2007-2024 Broadcom. All Rights Reserved.
+//---------------------------------------------------------------------------
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.RateLimiting;
+using System.Threading.Tasks;
+
+namespace Integration.GH
+{
+    public class NeverAcquiredRateLimiter : RateLimiter
+    {
+        public override TimeSpan? IdleDuration => throw new NotImplementedException();
+        public override RateLimiterStatistics GetStatistics() => throw new NotImplementedException();
+
+        protected override ValueTask<RateLimitLease> AcquireAsyncCore(int permitCount, CancellationToken cancellationToken)
+        {
+            return new ValueTask<RateLimitLease>(new NotAcquiredRateLimitLease());
+        }
+
+        protected override RateLimitLease AttemptAcquireCore(int permitCount)
+        {
+            return new NotAcquiredRateLimitLease();
+        }
+    }
+
+    public class NotAcquiredRateLimitLease : RateLimitLease
+    {
+        public override bool IsAcquired => false;
+
+        public override IEnumerable<string> MetadataNames => [];
+
+        public override bool TryGetMetadata(string metadataName, out object metadata)
+        {
+            metadata = string.Empty;
+            return true;
+        }
+    }
+}

projects/Test/Integration/GH/TestGitHubIssues.cs

@@ -34,6 +34,7 @@
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
+using Integration.GH;
 using RabbitMQ.Client;
 using RabbitMQ.Client.Events;
 using RabbitMQ.Client.Exceptions;
@@ -329,5 +330,58 @@ await ch.BasicPublishAsync(exchange: string.Empty, routingKey: queueName,
                 _output.WriteLine("saw {0} publishExceptions", publishExceptions.Count);
             }
         }
+
+        [Fact]
+        public async Task MaybeSomethingUpWithRateLimiter_GH1793()
+        {
+            const int messageCount = 16;
+
+            _connFactory = new ConnectionFactory
+            {
+                AutomaticRecoveryEnabled = true
+            };
+
+            _conn = await _connFactory.CreateConnectionAsync();
+
+            var channelOpts = new CreateChannelOptions(
+                publisherConfirmationsEnabled: true,
+                publisherConfirmationTrackingEnabled: true,
+                outstandingPublisherConfirmationsRateLimiter: new NeverAcquiredRateLimiter()
+            );
+
+            _channel = await _conn.CreateChannelAsync(channelOpts);
+
+            var properties = new BasicProperties
+            {
+                DeliveryMode = DeliveryModes.Persistent
+            };
+
+            for (int i = 0; i < messageCount; i++)
+            {
+                int retryCount = 0;
+                const int maxRetries = 3;
+                while (retryCount <= maxRetries)
+                {
+                    try
+                    {
+                        byte[] bytes = Encoding.UTF8.GetBytes("message");
+                        await Assert.ThrowsAnyAsync<InvalidOperationException>(async () =>
+                        {
+                            await _channel.BasicPublishAsync(string.Empty, string.Empty, true, properties, bytes);
+                        });
+                        break;
+                    }
+                    catch (SemaphoreFullException ex0)
+                    {
+                        _output.WriteLine("{0} ex: {1}", _testDisplayName, ex0);
+                        retryCount++;
+                    }
+                    catch (PublishException)
+                    {
+                        retryCount++;
+                    }
+                }
+            }
+        }
     }
 }