SERVER-20853 eliminate copies in find and getMore path

4 files changed, 42 insertions, 2 deletions

diff --git a/src/mongo/bson/bsonmisc.cpp b/src/mongo/bson/bsonmisc.cpp
index b106121b590..28e88690b29 100644
--- a/src/mongo/bson/bsonmisc.cpp
+++ b/src/mongo/bson/bsonmisc.cpp
@@ -72,6 +72,11 @@ BSONObjBuilderValueStream::BSONObjBuilderValueStream(BSONObjBuilder* builder) {
     _builder = builder;
 }
 
+void BSONObjBuilderValueStream::reset() {
+    _fieldName = StringData();
+    _subobj.reset();
+}
+
 BSONObjBuilder& BSONObjBuilderValueStream::operator<<(const BSONElement& e) {
     _builder->appendAs(e, _fieldName);
     _fieldName = StringData();
diff --git a/src/mongo/bson/bsonmisc.h b/src/mongo/bson/bsonmisc.h
index d1c1894190a..f32319965f1 100644
--- a/src/mongo/bson/bsonmisc.h
+++ b/src/mongo/bson/bsonmisc.h
@@ -253,6 +253,11 @@ public:
         return *_builder;
     }
 
+    /**
+     * Restores this object to its empty state.
+     */
+    void reset();
+
 private:
     StringData _fieldName;
     BSONObjBuilder* _builder;
diff --git a/src/mongo/bson/bsonobjbuilder.h b/src/mongo/bson/bsonobjbuilder.h
index 5b744a62120..ed21c7b63e1 100644
--- a/src/mongo/bson/bsonobjbuilder.h
+++ b/src/mongo/bson/bsonobjbuilder.h
@@ -601,6 +601,20 @@ public:
     BSONObjBuilder& append(StringData fieldName, const std::map<K, T>& vals);
 
     /**
+     * Resets this BSONObjBulder to an empty state. All previously added fields are lost.  If this
+     * BSONObjBuilder is using an externally provided BufBuilder, this method does not affect the
+     * bytes before the start of this object.
+     *
+     * Invalid to call if done() has already been called in order to finalize the BSONObj.
+     */
+    void resetToEmpty() {
+        invariant(!_doneCalled);
+        _s.reset();
+        // Reset the position the next write will go to right after our size reservation.
+        _b.setlen(_offset + sizeof(int));
+    }
+
+    /**
      * destructive
      * The returned BSONObj will free the buffer when it is finished.
      * @return owned BSONObj
@@ -887,10 +901,9 @@ private:
 
 template <class T>
 inline BSONObjBuilder& BSONObjBuilder::append(StringData fieldName, const std::vector<T>& vals) {
-    BSONObjBuilder arrBuilder;
+    BSONObjBuilder arrBuilder(subarrayStart(fieldName));
     for (unsigned int i = 0; i < vals.size(); ++i)
         arrBuilder.append(numStr(i), vals[i]);
-    appendArray(fieldName, arrBuilder.done());
     return *this;
 }
 
diff --git a/src/mongo/bson/bsonobjbuilder_test.cpp b/src/mongo/bson/bsonobjbuilder_test.cpp
index d92d48d6098..fdee41b8ba1 100644
--- a/src/mongo/bson/bsonobjbuilder_test.cpp
+++ b/src/mongo/bson/bsonobjbuilder_test.cpp
@@ -303,4 +303,21 @@ TEST(BSONObjBuilderTest, ResumeBuildingWithNesting) {
                                             << "dd")) << "a" << BSON("c" << 3)));
 }
 
+TEST(BSONObjBuilderTest, ResetToEmptyResultsInEmptyObj) {
+    BSONObjBuilder bob;
+    bob.append("a", 3);
+    bob.resetToEmpty();
+    ASSERT_EQ(BSONObj(), bob.obj());
+}
+
+TEST(BSONObjBuilderTest, ResetToEmptyForNestedBuilderOnlyResetsInnerObj) {
+    BSONObjBuilder bob;
+    bob.append("a", 3);
+    BSONObjBuilder innerObj(bob.subobjStart("nestedObj"));
+    innerObj.append("b", 4);
+    innerObj.resetToEmpty();
+    innerObj.done();
+    ASSERT_EQ(BSON("a" << 3 << "nestedObj" << BSONObj()), bob.obj());
+}
+
 }  // unnamed namespace