CSHARP-1510: Fix GetMore throws InvalidOperationException under certain conditions.

Author: rstam

src/MongoDB.Driver.Core.Tests/Core/Operations/FindOpcodeOperationTests.cs

@@ -230,6 +230,30 @@ public async Task ExecuteAsync_should_find_all_the_documents_matching_the_query(
             result.Should().HaveCount(5);
         }
 
+        [Test]
+        [RequiresServer(VersionLessThan = "3.2.0")]
+        public async Task ExecuteAsync_should_find_all_the_documents_matching_the_query_when_limit_is_used(
+            [Values(1, 5, 6, 12)] int limit)
+        {
+            var collectionNamespace = CoreTestConfiguration.GetCollectionNamespaceForTestMethod();
+            for (var id = 1; id <= limit + 1; id++)
+            {
+                var document = new BsonDocument { { "id", id }, { "filler", new string('x', 1000000) } }; // about 1MB big
+                var requests = new[] { new InsertRequest(document) };
+                var insertOperation = new BulkMixedWriteOperation(collectionNamespace, requests, new MessageEncoderSettings());
+                ExecuteOperation(insertOperation);
+            }
+            var subject = new FindOpcodeOperation<BsonDocument>(collectionNamespace, BsonDocumentSerializer.Instance, _messageEncoderSettings)
+            {
+                Limit = limit
+            };
+
+            var cursor = await ExecuteOperationAsync(subject);
+            var result = await ReadCursorToEndAsync(cursor);
+
+            result.Should().HaveCount(limit);
+        }
+
         [Test]
         [RequiresServer("EnsureTestData")]
         public async Task ExecuteAsync_should_find_all_the_documents_matching_the_query_when_split_across_batches()

src/MongoDB.Driver.Core/Core/Operations/AsyncCursor.cs

@@ -128,10 +128,10 @@ private int CalculateGetMoreProtocolNumberToReturn()
             var numberToReturn = _batchSize ?? 0;
             if (_limit > 0)
             {
-                numberToReturn = _limit.Value - _count;
-                if (_batchSize != 0 && numberToReturn > _batchSize.Value)
+                var remaining = _limit.Value - _count;
+                if (numberToReturn == 0 || numberToReturn > remaining)
                 {
-                    numberToReturn = _batchSize.Value;
+                    numberToReturn = remaining;
                 }
             }
             return numberToReturn;