CSHARP-1461: fixing issue with calling Dispose on a null reference.

Author: craiggwilson

src/MongoDB.Driver.Core.Tests/Core/Connections/BinaryConnection_CommandEventTests.cs

@@ -675,6 +675,45 @@ public void Should_process_a_query_with_the_explain_modifier()
             commandSucceededEvent.RequestId.Should().Be(commandStartedEvent.RequestId);
         }
 
+        [Test]
+        public void Should_process_a_failed_query()
+        {
+            var expectedCommand = new BsonDocument
+            {
+                { "find", MessageHelper.DefaultCollectionNamespace.CollectionName },
+                { "filter", new BsonDocument("x", 1) },
+            };
+            var queryFailureDocument = BsonDocument.Parse("{ $err: \"Can't canonicalize query: BadValue $or needs an array\", code: 17287 }");
+
+            var requestMessage = MessageHelper.BuildQuery(
+                (BsonDocument)expectedCommand["filter"],
+                requestId: 10);
+            SendMessages(requestMessage);
+
+
+            var replyMessage = MessageHelper.BuildQueryFailedReply<BsonDocument>(
+                queryFailureDocument,
+                requestMessage.RequestId);
+            ReceiveMessages(replyMessage);
+
+            var commandStartedEvent = (CommandStartedEvent)_capturedEvents.Next();
+            var commandFailedEvent = (CommandFailedEvent)_capturedEvents.Next();
+
+            commandStartedEvent.CommandName.Should().Be(expectedCommand.GetElement(0).Name);
+            commandStartedEvent.Command.Should().Be(expectedCommand);
+            commandStartedEvent.ConnectionId.Should().Be(_subject.ConnectionId);
+            commandStartedEvent.DatabaseNamespace.Should().Be(MessageHelper.DefaultDatabaseNamespace);
+            commandStartedEvent.OperationId.Should().Be(EventContext.OperationId);
+            commandStartedEvent.RequestId.Should().Be(requestMessage.RequestId);
+
+            commandFailedEvent.CommandName.Should().Be(commandStartedEvent.CommandName);
+            commandFailedEvent.ConnectionId.Should().Be(commandStartedEvent.ConnectionId);
+            commandFailedEvent.Duration.Should().BeGreaterThan(TimeSpan.Zero);
+            commandFailedEvent.OperationId.Should().Be(commandStartedEvent.OperationId);
+            ((MongoCommandException)commandFailedEvent.Failure).Result.Should().Be(queryFailureDocument);
+            commandFailedEvent.RequestId.Should().Be(commandStartedEvent.RequestId);
+        }
+
         [Test]
         public void Should_process_an_update_without_gle()
         {

src/MongoDB.Driver.Core.Tests/Core/Helpers/MessageHelper.cs

@@ -155,6 +155,24 @@ public static InsertMessage<T> BuildInsert<T>(
                 false);
         }
 
+        public static ReplyMessage<T> BuildQueryFailedReply<T>(
+            BsonDocument queryFailureDocument,
+            int responseTo = 0)
+        {
+            return new ReplyMessage<T>(
+                false,
+                0,
+                false,
+                null,
+                1,
+                true,
+                queryFailureDocument,
+                0,
+                responseTo,
+                BsonSerializer.SerializerRegistry.GetSerializer<T>(),
+                0);
+        }
+
         public static ReplyMessage<T> BuildReply<T>(
             T document,
             IBsonSerializer<T> serializer = null,

src/MongoDB.Driver.Core/Core/Connections/CommandEventHelper.cs

@@ -599,7 +599,7 @@ private void ProcessReplyMessage(CommandState state, ResponseMessage message, IB
             }
             finally
             {
-                if (disposeOfDocuments)
+                if (disposeOfDocuments && replyMessage.Documents != null)
                 {
                     replyMessage.Documents.ForEach(d => d.Dispose());
                 }