Eliminate unnecessary killCursors command when batchSize == limit (mongodb#1656)

JAVA-5667

Author: NathanQingyangXu

driver-core/src/main/com/mongodb/internal/operation/FindOperation.java

@@ -390,7 +390,12 @@ private BsonDocument getCommand(final OperationContext operationContext, final i
             if (batchSize < 0 && Math.abs(batchSize) < limit) {
                 commandDocument.put("limit", new BsonInt32(Math.abs(batchSize)));
             } else if (batchSize != 0) {
-                commandDocument.put("batchSize", new BsonInt32(Math.abs(batchSize)));
+                int effectiveBatchSize = Math.abs(batchSize);
+                if (effectiveBatchSize == limit) {
+                    // avoid an open cursor on server side when batchSize and limit are equal
+                    effectiveBatchSize++;
+                }
+                commandDocument.put("batchSize", new BsonInt32(effectiveBatchSize));
             }
         }
         if (limit < 0 || batchSize < 0) {

driver-core/src/test/resources/unified-test-format/crud/find.json

@@ -237,6 +237,68 @@
           ]
         }
       ]
+    },
+    {
+      "description": "Find with batchSize equal to limit",
+      "operations": [
+        {
+          "object": "collection0",
+          "name": "find",
+          "arguments": {
+            "filter": {
+              "_id": {
+                "$gt": 1
+              }
+            },
+            "sort": {
+              "_id": 1
+            },
+            "limit": 4,
+            "batchSize": 4
+          },
+          "expectResult": [
+            {
+              "_id": 2,
+              "x": 22
+            },
+            {
+              "_id": 3,
+              "x": 33
+            },
+            {
+              "_id": 4,
+              "x": 44
+            },
+            {
+              "_id": 5,
+              "x": 55
+            }
+          ]
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "find": "coll0",
+                  "filter": {
+                    "_id": {
+                      "$gt": 1
+                    }
+                  },
+                  "limit": 4,
+                  "batchSize": 5
+                },
+                "commandName": "find",
+                "databaseName": "find-tests"
+              }
+            }
+          ]
+        }
+      ]
     }
   ]
 }