HDFS-9696. Garbage snapshot records linger forever. Contributed by Kihwal Lee

(cherry picked from commit 83e57e083f)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestSaveNamespace.java

(cherry picked from commit 4766a3153d)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormatProtobuf.java

@@ -455,7 +455,11 @@ public final class FSImageFormatProtobuf {
           this, summary, context, context.getSourceNamesystem());
 
       snapshotSaver.serializeSnapshotSection(sectionOutputStream);
-      snapshotSaver.serializeSnapshotDiffSection(sectionOutputStream);
+      // Skip snapshot-related sections when there is no snapshot.
+      if (context.getSourceNamesystem().getSnapshotManager()
+          .getNumSnapshots() > 0) {
+        snapshotSaver.serializeSnapshotDiffSection(sectionOutputStream);
+      }
       snapshotSaver.serializeINodeReferenceSection(sectionOutputStream);
     }
 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestSaveNamespace.java

@@ -625,6 +625,47 @@ public class TestSaveNamespace {
     }
   }
 
+  @Test
+  public void testSkipSnapshotSection() throws Exception {
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(new Configuration())
+        .numDataNodes(1).build();
+    cluster.waitActive();
+    DistributedFileSystem fs = cluster.getFileSystem();
+    OutputStream out = null;
+    try {
+      String path = "/skipSnapshot";
+      out = fs.create(new Path(path));
+      out.close();
+
+      // add a bogus filediff
+      FSDirectory dir = cluster.getNamesystem().getFSDirectory();
+      INodeFile file = dir.getINode(path).asFile();
+      file.addSnapshotFeature(null).getDiffs()
+          .saveSelf2Snapshot(-1, file, null, false);
+
+      // make sure it has a diff
+      assertTrue("Snapshot fileDiff is missing.",
+          file.getFileWithSnapshotFeature().getDiffs() != null);
+
+      // saveNamespace
+      fs.setSafeMode(SafeModeAction.SAFEMODE_ENTER);
+      cluster.getNameNodeRpc().saveNamespace();
+      fs.setSafeMode(SafeModeAction.SAFEMODE_LEAVE);
+
+      // restart namenode
+      cluster.restartNameNode(true);
+      dir = cluster.getNamesystem().getFSDirectory();
+      file = dir.getINode(path).asFile();
+
+      // there should be no snapshot feature for the inode, when there is
+      // no snapshot.
+      assertTrue("There should be no snapshot feature for this INode.",
+          file.getFileWithSnapshotFeature() == null);
+    } finally {
+      cluster.shutdown();
+    }
+  }
+
   private void doAnEdit(FSNamesystem fsn, int id) throws IOException {
     // Make an edit
     fsn.mkdirs(