HDFS-16923. [SBN read] getlisting RPC to observer will throw NPE if path does not exist (#5400)

Signed-off-by: Erik Krogen <xkrogen@apache.org>

(cherry picked from commit 6bd24448154fcd3ab9099d7783cc7f7f76c61e08)
(cherry picked from commit a7c1fad0c9a675195579c971962ddd32e5d9fc51)

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

@@ -4087,7 +4087,7 @@ public class FSNamesystem implements Namesystem, FSNamesystemMBean,
       logAuditEvent(false, operationName, src);
       throw e;
     }
-    if (needLocation && isObserver()) {
+    if (dl != null && needLocation && isObserver()) {
       for (HdfsFileStatus fs : dl.getPartialListing()) {
         if (fs instanceof HdfsLocatedFileStatus) {
           LocatedBlocks lbs = ((HdfsLocatedFileStatus) fs).getLocatedBlocks();

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

@@ -64,6 +64,7 @@ import org.apache.hadoop.hdfs.server.namenode.NameNodeAdapter;
 import org.apache.hadoop.hdfs.server.namenode.TestFsck;
 import org.apache.hadoop.hdfs.tools.GetGroups;
 import org.apache.hadoop.ipc.ObserverRetryOnActiveException;
+import org.apache.hadoop.test.LambdaTestUtils;
 import org.apache.hadoop.util.Time;
 import org.apache.hadoop.util.concurrent.HadoopExecutors;
 import org.junit.After;
@@ -608,6 +609,17 @@ public class TestObserverNode {
     }
   }
 
+  @Test
+  public void testGetListingForDeletedDir() throws Exception {
+    Path path = new Path("/dir1/dir2/testFile");
+    dfs.create(path).close();
+
+    assertTrue(dfs.delete(new Path("/dir1/dir2"), true));
+
+    LambdaTestUtils.intercept(FileNotFoundException.class,
+        () -> dfs.listLocatedStatus(new Path("/dir1/dir2")));
+  }
+
   @Test
   public void testSimpleReadEmptyDirOrFile() throws IOException {
     // read empty dir