Add support for blocking pop instead of polling

osu.Server.QueueProcessor/QueueConfiguration.cs

@@ -34,5 +34,14 @@ public class QueueConfiguration
         /// Setting above 1 will allow processing in batches (see <see cref="QueueProcessor{T}.ProcessResults"/>).
         /// </summary>
         public int BatchSize { get; set; } = 1;
+
+        /// <summary>
+        /// When enabled, uses <c>BRPOP</c> to wait for items instead of polling with <c>RPOP</c>.
+        /// </summary>
+        /// <remarks>
+        /// <see cref="BatchSize"/> is ignored when this is enabled.
+        /// <see cref="TimeBetweenPolls"/> is still used as a delay for when processor is overloaded.
+        /// </remarks>
+        public bool UseBlockingPop { get; set; } = false;
     }
 }

osu.Server.QueueProcessor/QueueProcessor.cs

@@ -85,6 +85,9 @@
         /// <param name="cancellation">An optional cancellation token.</param>
         public void Run(CancellationToken cancellation = default)
         {
+            // dedicated redis connection for the BRPOP path, workaround for StackExchange.Redis not offering blocking operations
+            var blockingRedis = new Lazy<IDatabase>(() => RedisAccess.GetConnection().GetDatabase());
+
             using (SentrySdk.Init(setupSentry))
             using (new Timer(_ => outputStats(), null, TimeSpan.Zero, TimeSpan.FromSeconds(5)))
             using (var cts = new GracefulShutdownSource(cancellation))
@@ -100,20 +103,38 @@
 
                         try
                         {
-                            if (totalInFlight >= config.MaxInFlightItems || consecutiveErrors > config.ErrorThreshold)
+                            // avoid processing too many items at once
+                            if (totalInFlight >= config.MaxInFlightItems)
                             {
                                 Thread.Sleep(config.TimeBetweenPolls);
                                 continue;
                             }
 
-                            var redisItems = Redis.ListRightPop(QueueName, config.BatchSize);
+                            RedisValue[] redisItems;
 
-                            // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract (https://github.com/StackExchange/StackExchange.Redis/issues/2697)
-                            // queue doesn't exist.
-                            if (redisItems == null)
+                            if (config.UseBlockingPop)
                             {
-                                Thread.Sleep(config.TimeBetweenPolls);
-                                continue;
+                                // timeout in seconds, can't be higher than StackExchange.Redis timeout (default is 5 seconds)
+                                const string timeout = "1";
+
+                                RedisResult redisResult = blockingRedis.Value.Execute("BRPOP", QueueName, timeout);
+
+                                if (redisResult.IsNull)
+                                    continue;
+
+                                redisItems = [(RedisValue)redisResult[1]];
+                            }
+                            else
+                            {
+                                redisItems = Redis.ListRightPop(QueueName, config.BatchSize);
+
+                                // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract (https://github.com/StackExchange/StackExchange.Redis/issues/2697)
+                                // queue doesn't exist.
+                                if (redisItems == null)
+                                {
+                                    Thread.Sleep(config.TimeBetweenPolls);
+                                    continue;
+                                }
                             }
 
                             List<T> items = new List<T>();
@@ -260,7 +281,7 @@
         public void PublishMessage<TMessage>(string channelName, TMessage message)
         {
             Redis.Publish(new RedisChannel(channelName, RedisChannel.PatternMode.Auto), JsonConvert.SerializeObject(message));
             DogStatsd.Increment("messages_published", tags: new[] { $"channel:{channelName}", $"type:{typeof(TMessage).FullName}" });
         }
 
         /// <summary>