[GitHub] [incubator-doris] xy720 commented on a change in pull request #6207: [New Feature][Meta][Image] Add file header and footer for image

[GitBox <gi...@apache.org>]

xy720 commented on a change in pull request #6207:
URL: https://github.com/apache/incubator-doris/pull/6207#discussion_r670971608



##########
File path: fe/fe-core/src/main/java/org/apache/doris/common/MetaFooter.java
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@@ -0,0 +1,86 @@
+// 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.doris.common;
+
+import com.google.common.collect.Lists;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.util.Arrays;
+import java.util.List;
+
+public class MetaFooter {
+    private static final Logger LOG = LogManager.getLogger(MetaFooter.class);
+
+    private static final long FOOTER_LENGTH_SIZE = 8L;
+    public static final MetaFooter EMPTY_FOOTER = new MetaFooter(null, 0L);
+
+    // length of footer
+    private long length;
+    // meta indices
+    public List<MetaIndex> metaIndices;
+
+    public static MetaFooter read(File imageFile) throws IOException {
+        try(RandomAccessFile raf = new RandomAccessFile(imageFile, "r")) {
+            long fileLength = raf.length();
+            long footerLengthIndex = fileLength - FOOTER_LENGTH_SIZE - MetaMagicNumber.MAGIC_STR.length();
+            raf.seek(footerLengthIndex);
+            long footerLength = raf.readLong();
+            MetaMagicNumber magicNumber = MetaMagicNumber.read(raf);
+            if (!Arrays.equals(MetaMagicNumber.MAGIC, magicNumber.getBytes())) {
+                LOG.warn("Image file {} format mismatch. Expected magic number is {}, actual is {}",
+                        imageFile.getPath(), Arrays.toString(MetaMagicNumber.MAGIC), Arrays.toString(magicNumber.getBytes()));
+                return EMPTY_FOOTER;

Review comment:
       When we read a old version image, the magic number is not exist, if we throw a exception here, it will fail to read the image. So my idea is, if we can't read the correct magic number here, then read this file as an old version of the image, and the old version of the image has no header or footer, so an empty header or footer is returned here.




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