fix: accessing null object (cache) after shutdown

src/Azure/Orleans.Streaming.EventHubs/Providers/Streams/EventHub/EventHubAdapterReceiver.cs

@@ -107,7 +107,7 @@ private async Task Initialize()
             try
             {
                 this.checkpointer = await this.checkpointerFactory(this.settings.Partition);
-                if(this.cache != null)
+                if (this.cache is not null)
                 {
                     this.cache.Dispose();
                     this.cache = null;
@@ -178,17 +178,24 @@ public async Task<IList<IBatchContainer>> GetQueueMessagesAsync(int maxCount)
 
             this.monitor?.TrackMessagesReceived(messages.Count, oldestMessageEnqueueTime, newestMessageEnqueueTime);
 
-            List<StreamPosition> messageStreamPositions = this.cache.Add(messages, dequeueTimeUtc);
-            foreach (var streamPosition in messageStreamPositions)
+            // The this.cache field can be null if Shutdown() has been called
+            List<StreamPosition> messageStreamPositions = this.cache?.Add(messages, dequeueTimeUtc);
+
+            if (messageStreamPositions is { Count: > 0 })
             {
-                batches.Add(new StreamActivityNotificationBatch(streamPosition));
+                foreach (var pos in messageStreamPositions)
+                {
+                    batches.Add(new StreamActivityNotificationBatch(pos));
+                }
             }
+
             if (!this.checkpointer.CheckpointExists)
             {
                 this.checkpointer.Update(
                     messages[0].OffsetString,
                     DateTime.UtcNow);
             }
+
             return batches;
         }
 
@@ -201,10 +208,11 @@ public bool TryPurgeFromCache(out IList<IBatchContainer> purgedItems)
         {
             purgedItems = null;
 
-            //if not under pressure, signal the cache to do a time based purge
-            //if under pressure, which means consuming speed is less than producing speed, then shouldn't purge, and don't read more message into the cache
+            // if not under pressure, signal the cache to do a time based purge
+            // if under pressure, which means consuming speed is less than producing speed, then shouldn't purge, and don't read more message into the cache
             if (!this.IsUnderPressure())
-                this.cache.SignalPurge();
+                this.cache?.SignalPurge();
+
             return false;
         }
 
@@ -247,6 +255,7 @@ public async Task Shutdown(TimeSpan timeout)
                 {
                     closeTask = localReceiver.CloseAsync();
                 }
+
                 // dispose of cache
                 localCache?.Dispose();