Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/5362#discussion_r165573730
--- Diff:
flink-core/src/main/java/org/apache/flink/core/io/PostVersionedIOReadableWritable.java
---
@@ -0,0 +1,78 @@
+/*
+ * 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.flink.core.io;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.core.memory.DataInputView;
+import org.apache.flink.core.memory.DataInputViewStreamWrapper;
+import org.apache.flink.core.memory.DataOutputView;
+import org.apache.flink.core.memory.InputStreamViewWrapper;
+
+import java.io.IOException;
+import java.io.PushbackInputStream;
+import java.util.Arrays;
+
+/**
+ * A {@link VersionedIOReadableWritable} which allows to differentiate
whether the previous
+ * data was versioned with a {@link VersionedIOReadableWritable}. This can
be used if previously
+ * written data was not versioned, and is to be migrated to a versioned
format.
+ */
+@Internal
+public abstract class PostVersionedIOReadableWritable extends
VersionedIOReadableWritable {
+
+ /** NOTE: CANNOT CHANGE! */
+ private static final byte[] VERSIONED_IDENTIFIER = new byte[] {-15,
-51, -123, -97};
+
+ /**
+ * Read from the provided {@link DataInputView in}. A flag {@code
wasVersioned} can be
+ * used to determine whether or not the data to read was previously
written
+ * by a {@link VersionedIOReadableWritable}.
+ */
+ protected abstract void read(DataInputView in, boolean wasVersioned)
throws IOException;
+
+ @Override
+ public void write(DataOutputView out) throws IOException {
+ out.write(VERSIONED_IDENTIFIER);
+ super.write(out);
+ }
+
+ /**
+ * This read attempts to first identify if the input view contains the
special
+ * {@link #VERSIONED_IDENTIFIER} by reading and buffering the first few
bytes.
+ * If identified to be versioned, the usual version resolution read path
+ * in {@link VersionedIOReadableWritable#read(DataInputView)} is
invoked.
+ * Otherwise, we "reset" the input view by pushing back the read
buffered bytes
+ * into the stream.
+ */
+ @Override
+ public final void read(DataInputView in) throws IOException {
+ PushbackInputStream stream = new PushbackInputStream(new
InputStreamViewWrapper(in), VERSIONED_IDENTIFIER.length);
+
+ byte[] tmp = new byte[VERSIONED_IDENTIFIER.length];
+ stream.read(tmp);
+
+ if (Arrays.equals(tmp, VERSIONED_IDENTIFIER)) {
+ super.read(in);
+ read(in, true);
+ } else {
+ stream.unread(tmp);
+ read(new DataInputViewStreamWrapper(stream), false);
+ }
--- End diff --
Not-so-nice things about this current implementation is:
1) it requires several layers of transforming back and forth between a
`DataInputView` and `InputStream`, and
2) it uses a separate `read(DataInputView, boolean)` method in order to
wrap a "reset" `DataInputView` for the remaining reads.
I think the implementation would have been much more elegant if
`DataInputView` has an `unread(byte[])` method, though I'm not sure how
non-trivial it is to support this across all subclasses.
Maybe a food for thought for the future ..
---
