8255576: (fs) Files.isHidden() throws ArrayIndexOutOfBoundsException (unix)

Reviewed-by: alanb

Author: sormuras

src/java.base/unix/classes/sun/nio/fs/UnixFileSystemProvider.java

@@ -352,7 +352,14 @@ public boolean isHidden(Path obj) {
         UnixPath name = file.getFileName();
         if (name == null)
             return false;
-        return (name.asByteArray()[0] == '.');
+
+        byte[] path;
+        if (name.isEmpty()) { // corner case for empty paths
+            path = name.getFileSystem().defaultDirectory();
+        } else {
+            path = name.asByteArray();
+        }
+        return path[0] == '.';
     }
 
     /**

src/java.base/unix/classes/sun/nio/fs/UnixPath.java

@@ -243,7 +243,7 @@ private void initOffsets() {
     }
 
     // returns {@code true} if this path is an empty path
-    private boolean isEmpty() {
+    boolean isEmpty() {
         return path.length == 0;
     }
 

test/jdk/java/nio/file/Files/Misc.java

@@ -22,7 +22,7 @@
  */
 
 /* @test
- * @bug 4313887 6838333 8005566 8032220 8215467
+ * @bug 4313887 6838333 8005566 8032220 8215467 8255576
  * @summary Unit test for miscellenous methods in java.nio.file.Files
  * @library ..
  */
@@ -87,6 +87,9 @@ static void testCreateDirectories(Path tmpdir) throws IOException {
      * Tests isHidden
      */
     static void testIsHidden(Path tmpdir) throws IOException {
+        // passing an empty path must not throw any runtime exception
+        assertTrue(!isHidden(Path.of("")));
+
         assertTrue(!isHidden(tmpdir));
 
         Path file = tmpdir.resolve(".foo");