Fix JSON parser handling for certain objects (#7162)

This PR also adds some comments and simplifies
the code. Primary handling is done to ensure
that we make sure to honor cached buffer.

Added unit tests as well

Fixes #7141

pkg/s3select/json/reader_test.go

@@ -0,0 +1,49 @@
+/*
+ * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package json
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestNewReader(t *testing.T) {
+	files, err := ioutil.ReadDir("data")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		f, err := os.Open(filepath.Join("data", file.Name()))
+		if err != nil {
+			t.Fatal(err)
+		}
+		r := NewReader(f, &ReaderArgs{})
+		for {
+			_, err = r.Read()
+			if err != nil {
+				break
+			}
+		}
+		r.Close()
+		if err != io.EOF {
+			t.Fatalf("Reading failed with %s, %s", err, file.Name())
+		}
+	}
+}