Using a fifo semaphore to ensure that requests are granted access to …

…the API in fifo order.

ShopifySharp/Infrastructure/Policies/LeakyBucketPolicy/FifoSemaphore.cs

@@ -0,0 +1,37 @@
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ShopifySharp
+{
+    internal class FifoSemaphore
+    {
+        private SemaphoreSlim _semaphore;
+
+        private ConcurrentQueue<TaskCompletionSource<bool>> queue = new ();
+
+        public int QueueCount => queue.Count;
+
+        public FifoSemaphore(int initialCount, int maxConcurrency)
+        {
+            _semaphore = new SemaphoreSlim(initialCount, maxConcurrency);
+        }
+
+        public Task WaitAsync(CancellationToken cancellationToken = default)
+        {
+            var tcs = new TaskCompletionSource<bool>();
+            queue.Enqueue(tcs);
+            _semaphore.WaitAsync(cancellationToken).ContinueWith(t =>
+            {
+                if (queue.TryDequeue(out var popped))
+                    popped.SetResult(true);
+            });
+            return tcs.Task;
+        }
+
+        public void Release()
+        {
+            _semaphore.Release();
+        }
+    }
+}

ShopifySharp/Infrastructure/Policies/LeakyBucketPolicy/LeakyBucket.cs

@@ -9,7 +9,7 @@ internal class LeakyBucket
         private class Request
         {
             public int cost;
-            public SemaphoreSlim semaphore = new SemaphoreSlim(0, 1);
+            public FifoSemaphore fifoSemaphore = new FifoSemaphore(0, 1);
             public CancellationToken cancelToken;
 
             public Request(int cost, CancellationToken cancelToken)
@@ -109,7 +109,7 @@ public async Task WaitForAvailableAsync(int requestCost, CancellationToken cance
                 if (_waitingRequests.Count == 1)
                     ScheduleTryGrantNextPendingRequest(r);
             }
-            await r.semaphore.WaitAsync(cancellationToken);
+            await r.fifoSemaphore.WaitAsync(cancellationToken);
         }
 
         private void ScheduleTryGrantNextPendingRequest(Request r)
@@ -132,7 +132,7 @@ private void TryGrantNextPendingRequest()
                     var r = _waitingRequests.Dequeue();
                     if (!r.cancelToken.IsCancellationRequested)
                     {
-                        r.semaphore.Release();
+                        r.fifoSemaphore.Release();
                         ConsumeAvailable(r);
                     }
                 }