Support negative limits for find commands

GODRIVER-867

Change-Id: Ibaad9e1a081969499a91ca426f38c54bede7fad7

Author: Divjot Arora

mongo/collection_internal_test.go

@@ -1747,6 +1747,21 @@ func TestCollection_Find_Error(t *testing.T) {
 	})
 }
 
+func TestCollection_Find_NegativeLimit(t *testing.T) {
+	coll := createTestCollection(t, nil, nil)
+	initCollection(t, coll)
+
+	c, err := coll.Find(ctx, bson.D{}, options.Find().SetLimit(-2))
+	require.NoError(t, err)
+	require.Equal(t, int64(0), c.ID()) // single batch returned so cursor should not have valid ID
+
+	var numDocs int
+	for c.Next(ctx) {
+		numDocs++
+	}
+	require.Equal(t, 2, numDocs)
+}
+
 func TestCollection_FindOne_found(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping integration test in short mode")

x/mongo/driver/find.go

@@ -113,7 +113,13 @@ func Find(
 		cmd.Opts = append(cmd.Opts, hintElem)
 	}
 	if fo.Limit != nil {
-		cmd.Opts = append(cmd.Opts, bsonx.Elem{"limit", bsonx.Int64(*fo.Limit)})
+		limit := *fo.Limit
+		if limit < 0 {
+			cmd.Opts = append(cmd.Opts, bsonx.Elem{"singleBatch", bsonx.Boolean(true)})
+			limit = -1 * limit
+		}
+
+		cmd.Opts = append(cmd.Opts, bsonx.Elem{"limit", bsonx.Int64(limit)})
 	}
 	if fo.Max != nil {
 		maxElem, err := interfaceToElement("max", fo.Max, registry)