Github user rdblue commented on a diff in the pull request:
https://github.com/apache/spark/pull/19136#discussion_r138681562
--- Diff:
sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/DataReader.java
---
@@ -0,0 +1,36 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.spark.sql.sources.v2.reader;
+
+import java.io.Closeable;
+
+/**
+ * A data reader returned by a read task and is responsible for outputting
data for a RDD partition.
+ */
+public interface DataReader<T> extends Closeable {
--- End diff --
The initialization is done when creating this `DataReader` from a
`ReadTask`. That ensures that the initialization happens (easy to forget
`open()`) and simplifies the checks that need to be done because `DataReader`
can't exist otherwise.
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