Run ProcessQueuedAcks asynchronously (#4696)

Robust.Server/GameStates/PvsSystem.Ack.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Threading;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Player;
 using Robust.Shared.Threading;
@@ -29,10 +30,12 @@ internal sealed partial class PvsSystem
     /// <summary>
     ///     Processes queued client acks in parallel
     /// </summary>
-    internal void ProcessQueuedAcks()
+    internal WaitHandle ProcessQueuedAcks()
     {
         if (PendingAcks.Count == 0)
-            return;
+        {
+            return ParallelManager.DummyResetEvent.WaitHandle;
+        }
 
         _toAck.Clear();
 
@@ -41,8 +44,8 @@ internal sealed partial class PvsSystem
             _toAck.Add(session);
         }
 
-        _parallelManager.ProcessNow(_ackJob, _toAck.Count);
         PendingAcks.Clear();
+        return _parallelManager.Process(_ackJob, _toAck.Count);
     }
 
     private record struct PvsAckJob : IParallelRobustJob

Robust.Server/GameStates/ServerGameStateManager.cs

@@ -3,6 +3,7 @@ using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics.Tracing;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using JetBrains.Annotations;
 using Robust.Server.GameObjects;
@@ -173,6 +174,10 @@ Oldest acked clients: {string.Join(", ", players)}
         {
             var players = _playerManager.Sessions.Where(o => o.Status == SessionStatus.InGame).ToArray();
 
+            // Update client acks, which is used to figure out what data needs to be sent to clients
+            // This only needs SessionData which isn't touched during GetPVSData or ProcessCollections.
+            var ackJob = _pvs.ProcessQueuedAcks();
+
             // Update entity positions in PVS chunks/collections
             // TODO disable processing if culling is disabled? Need to check if toggling PVS breaks anything.
             // TODO parallelize?
@@ -189,11 +194,7 @@ Oldest acked clients: {string.Join(", ", players)}
                 pvsData = GetPVSData(players);
             }
 
-            // Update client acks, which is used to figure out what data needs to be sent to clients
-            using (_usageHistogram.WithLabels("Process Acks").NewTimer())
-            {
-                _pvs.ProcessQueuedAcks();
-            }
+            ackJob.WaitOne();
 
             // Construct & send the game state to each player.
             GameTick oldestAck;

Robust.Shared/Threading/ParallelManager.cs

@@ -51,6 +51,8 @@ internal sealed class ParallelManager : IParallelManagerInternal
     public event Action? ParallelCountChanged;
     public int ParallelProcessCount { get; private set; }
 
+    public static readonly ManualResetEventSlim DummyResetEvent = new(true);
+
     // Without pooling it's hard to keep task allocations down for classes
     // This lets us avoid re-allocating the ManualResetEventSlims constantly when we just need a way to signal job completion.