Add method to determine size of queue (#315)

Author: Scott Bommarito

src/NuGet.Services.Storage/AzureStorageQueue.cs

@@ -64,5 +64,12 @@ public async Task RemoveAsync(StorageQueueMessage message, CancellationToken tok
 
             await (await _queueTask.Value).DeleteMessageAsync((message as AzureStorageQueueMessage).Message, token);
         }
+
+        public async Task<int?> GetMessageCount(CancellationToken token)
+        {
+            var queue = await _queueTask.Value;
+            await queue.FetchAttributesAsync(token);
+            return queue.ApproximateMessageCount;
+        }
     }
 }

src/NuGet.Services.Storage/IStorageQueue.cs

@@ -34,6 +34,13 @@ public interface IStorageQueue
         /// This method should throw if <paramref name="message"/> was not returned by <see cref="OnGetNext(CancellationToken)"/>.
         /// </remarks>
         Task RemoveAsync(StorageQueueMessage message, CancellationToken token);
+
+        /// <summary>
+        /// Fetches the number of messages in the queue (or a close approximation).
+        /// </summary>
+        /// <param name="token">A token to cancel the task with.</param>
+        /// <returns>An approximation of the number of messages in the queue or null if no approximation could be made.</returns>
+        Task<int?> GetMessageCount(CancellationToken token);
     }
 
     /// <summary>
@@ -68,5 +75,12 @@ public interface IStorageQueue<T>
         /// <paramref name="message"/> MUST have been previously returned by <see cref="GetNextAsync(CancellationToken)"/>.
         /// </remarks>
         Task RemoveAsync(StorageQueueMessage<T> message, CancellationToken token);
+
+        /// <summary>
+        /// Fetches the number of messages in the queue (or a close approximation).
+        /// </summary>
+        /// <param name="token">A token to cancel the task with.</param>
+        /// <returns>An approximation of the number of messages in the queue or null if no approximation could be made.</returns>
+        Task<int?> GetMessageCount(CancellationToken token);
     }
 }

src/NuGet.Services.Storage/StorageQueue.cs

@@ -42,6 +42,11 @@ public Task RemoveAsync(StorageQueueMessage<T> message, CancellationToken token)
             return _queue.RemoveAsync(SerializeMessage(message), token);
         }
 
+        public Task<int?> GetMessageCount(CancellationToken token)
+        {
+            return _queue.GetMessageCount(token);
+        }
+
         private StorageQueueMessage SerializeMessage(StorageQueueMessage<T> message)
         {
             if (message is DeserializedStorageQueueMessage<T> deserializedMessage)