HDFS-13693. Remove unnecessary search in INodeDirectory.addChild during image loading. Contributed by Lisheng Sun.

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

@@ -269,7 +269,7 @@ private void addToParent(INodeDirectory parent, INode child) {
             + "name before upgrading to this release.");
       }
       // NOTE: This does not update space counts for parents
-      if (!parent.addChild(child)) {
+      if (!parent.addChildAtLoading(child)) {
         return;
       }
       dir.cacheName(child);
@@ -551,6 +551,8 @@ void serializeINodeDirectorySection(OutputStream out) throws IOException {
               ++numImageErrors;
             }
             if (!inode.isReference()) {
+              // Serialization must ensure that children are in order, related
+              // to HDFS-13693
               b.addChildren(inode.getId());
             } else {
               refList.add(inode.asReference());

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

@@ -572,6 +572,22 @@ public boolean addChild(INode node) {
     return true;
   }
 
+  /**
+   * During image loading, the search is unnecessary since the insert position
+   * should always be at the end of the map given the sequence they are
+   * serialized on disk.
+   */
+  public boolean addChildAtLoading(INode node) {
+    int pos;
+    if (!node.isReference()) {
+      pos = (children == null) ? (-1) : (-children.size() - 1);
+      addChild(node, pos);
+      return true;
+    } else {
+      return addChild(node);
+    }
+  }
+
   /**
    * Add the node to the children list at the given insertion point.
    * The basic add method which actually calls children.add(..).