LUCENE-10158: Add a new interface Unwrappable to the utils package to ease migration to new MMAPDirectory and its testing (#369)

lucene/CHANGES.txt

@@ -157,6 +157,10 @@ API Changes
 
 * LUCENE-9325: Sort is now final, and the `setSort()` method has been removed (Alan Woodward)
 
+* LUCENE-10158: Add a new interface Unwrappable to the utils package to allow code to
+  unwrap wrappers/delegators that are added by Lucene's testing framework. This will allow
+  testing new MMapDirectory implementation based on JDK Project Panama. (Uwe Schindler)
+
 Improvements
 
 * LUCENE-10139: ExternalRefSorter returns a covariant with a subtype of BytesRefIterator

lucene/core/src/java/org/apache/lucene/util/Unwrappable.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.util;
+
+/**
+ * An object with this interface is a wrapper around another object (e.g., a filter with a
+ * delegate). The method {@link #unwrap()} can be called to get the wrapped object
+ *
+ * @lucene.internal
+ */
+public interface Unwrappable<T> {
+
+  /** Unwraps this instance */
+  T unwrap();
+
+  /** Unwraps all {@code Unwrappable}s around the given object. */
+  @SuppressWarnings("unchecked")
+  public static <T> T unwrapAll(T o) {
+    while (o instanceof Unwrappable) {
+      o = ((Unwrappable<T>) o).unwrap();
+    }
+    return o;
+  }
+}

lucene/test-framework/src/java/org/apache/lucene/mockfile/FilterPath.java

@@ -29,12 +29,13 @@ import java.nio.file.WatchKey;
 import java.nio.file.WatchService;
 import java.util.Iterator;
 import org.apache.lucene.util.SuppressForbidden;
+import org.apache.lucene.util.Unwrappable;
 
 /**
  * A {@code FilterPath} contains another {@code Path}, which it uses as its basic source of data,
  * possibly transforming the data along the way or providing additional functionality.
  */
-public class FilterPath implements Path {
+public class FilterPath implements Path, Unwrappable<Path> {
 
   /** The underlying {@code Path} instance. */
   protected final Path delegate;
@@ -63,6 +64,11 @@ public class FilterPath implements Path {
     return delegate;
   }
 
+  @Override
+  public Path unwrap() {
+    return delegate;
+  }
+
   @Override
   public FileSystem getFileSystem() {
     return fileSystem;
@@ -261,10 +267,7 @@ public class FilterPath implements Path {
    * @return innermost Path instance
    */
   public static Path unwrap(Path path) {
-    while (path instanceof FilterPath) {
+    return Unwrappable.unwrapAll(path);
-      path = ((FilterPath) path).delegate;
-    }
-    return path;
   }
 
   /** Override this to customize the return wrapped path from various operations */