Make changing concurrency threadsafe (#1536)

Author: WilliamBZA

src/NServiceBus.Transport.SqlServer/Receiving/MessageReceiver.cs

@@ -95,11 +95,17 @@ public Task StartReceive(CancellationToken cancellationToken = default)
 
         public async Task ChangeConcurrency(PushRuntimeSettings newLimitations, CancellationToken cancellationToken = default)
         {
-            var oldLimiter = concurrencyLimiter;
-            var oldMaxConcurrency = maxConcurrency;
-            concurrencyLimiter = new SemaphoreSlim(newLimitations.MaxConcurrency);
-            limitations = newLimitations;
-            maxConcurrency = limitations.MaxConcurrency;
+            SemaphoreSlim oldLimiter;
+            int oldMaxConcurrency;
+
+            lock (lockObject)
+            {
+                oldLimiter = concurrencyLimiter;
+                oldMaxConcurrency = maxConcurrency;
+                concurrencyLimiter = new SemaphoreSlim(newLimitations.MaxConcurrency);
+                limitations = newLimitations;
+                maxConcurrency = limitations.MaxConcurrency;
+            }
 
             try
             {
@@ -279,5 +285,7 @@ async Task PurgeExpiredMessages(CancellationToken cancellationToken)
         public ISubscriptionManager Subscriptions { get; }
         public string Id { get; }
         public string ReceiveAddress { get; }
+
+        public static object lockObject = new();
     }
 }