HADOOP-15516. Add test cases to cover FileUtil#readLink. Contributed by Giovanni Matteo Fumarola.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileUtil.java

@@ -198,6 +198,12 @@ public static String readLink(File f) {
      * use getCanonicalPath in File to get the target of the symlink but that
      * does not indicate if the given path refers to a symlink.
      */
+
+    if (f == null) {
+      LOG.warn("Can not read a null symLink");
+      return "";
+    }
+
     try {
       return Shell.execCommand(
           Shell.getReadlinkCommand(f.toString())).trim();

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/TestFileUtil.java

@@ -1441,4 +1441,56 @@ private void putEntriesInTar(TarArchiveOutputStream tos, File f)
     }
   }
 
+  /**
+   * This test validates the correctness of {@link FileUtil#readLink(File)} in
+   * case of null pointer inputs.
+   */
+  @Test
+  public void testReadSymlinkWithNullInput() {
+    String result = FileUtil.readLink(null);
+    Assert.assertEquals("", result);
+  }
+
+  /**
+   * This test validates the correctness of {@link FileUtil#readLink(File)}.
+   *
+   * @throws IOException
+   */
+  @Test
+  public void testReadSymlink() throws IOException {
+    Assert.assertFalse(del.exists());
+    del.mkdirs();
+
+    File file = new File(del, FILE);
+    File link = new File(del, "_link");
+
+    // Create a symbolic link
+    FileUtil.symLink(file.getAbsolutePath(), link.getAbsolutePath());
+
+    String result = FileUtil.readLink(link);
+    Assert.assertEquals(file.getAbsolutePath(), result);
+
+    file.delete();
+    link.delete();
+  }
+
+  /**
+   * This test validates the correctness of {@link FileUtil#readLink(File)} when
+   * it gets a file in input.
+   *
+   * @throws IOException
+   */
+  @Test
+  public void testReadSymlinkWithAFileAsInput() throws IOException {
+    Assert.assertFalse(del.exists());
+    del.mkdirs();
+
+    File file = new File(del, FILE);
+
+    String result = FileUtil.readLink(file);
+    Assert.assertEquals("", result);
+
+    file.delete();
+  }
+
 }