/*
 * 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.store;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.file.FileAlreadyExistsException;
import java.nio.file.Files;
import java.nio.file.NoSuchFileException;
import java.nio.file.Path;
import java.security.AccessController;
import java.security.PrivilegedAction;

This directory wrapper overrides Directory.copyFrom(Directory, String, String, IOContext) in order to optionally use a hard-link instead of a full byte by byte file copy if applicable. Hard-links are only used if the underlying filesystem supports it and if the LinkPermission "hard" is granted. 
NOTE: Using hard-links changes the copy semantics of Directory.copyFrom(Directory, String, String, IOContext). When hard-links are used changes to the source file will be reflected in the target file and vice-versa. Within Lucene, files are write once and should not be modified after they have been written. This directory should not be used in situations where files change after they have been written. 
/**
 * This directory wrapper overrides {@link Directory#copyFrom(Directory, String, String, IOContext)} in order
 * to optionally use a hard-link instead of a full byte by byte file copy if applicable. Hard-links are only used if the
 * underlying filesystem supports it and if the {@link java.nio.file.LinkPermission} "hard" is granted.
 *
 * <p><b>NOTE:</b> Using hard-links changes the copy semantics of
 * {@link Directory#copyFrom(Directory, String, String, IOContext)}. When hard-links are used changes to the source file
 * will be reflected in the target file and vice-versa. Within Lucene, files are write once and should not be modified
 * after they have been written. This directory should not be used in situations where files change after they have
 * been written.
 * </p>
 */
public final class HardlinkCopyDirectoryWrapper extends FilterDirectory {
  
Creates a new HardlinkCopyDirectoryWrapper delegating to the given directory
/**
   * Creates a new HardlinkCopyDirectoryWrapper delegating to the given directory
   */
  public HardlinkCopyDirectoryWrapper(Directory in) {
    super(in);
  }

  @Override
  public void copyFrom(Directory from, String srcFile, String destFile, IOContext context) throws IOException {
    final Directory fromUnwrapped = FilterDirectory.unwrap(from);
    final Directory toUnwrapped = FilterDirectory.unwrap(this);
    // try to unwrap to FSDirectory - we might be able to just create hard-links of these files and save copying
    // the entire file.
    Exception suppressedException = null;
    boolean tryCopy = true;
    if (fromUnwrapped instanceof FSDirectory
        && toUnwrapped instanceof FSDirectory) {
      final Path fromPath = ((FSDirectory) fromUnwrapped).getDirectory();
      final Path toPath = ((FSDirectory) toUnwrapped).getDirectory();

      if (Files.isReadable(fromPath.resolve(srcFile)) && Files.isWritable(toPath)) {
        // only try hardlinks if we have permission to access the files
        // if not super.copyFrom() will give us the right exceptions
        suppressedException = AccessController.doPrivileged((PrivilegedAction<Exception>) () -> {
          try {
            Files.createLink(toPath.resolve(destFile), fromPath.resolve(srcFile));
          } catch (FileNotFoundException | NoSuchFileException | FileAlreadyExistsException ex) {
            return ex; // in these cases we bubble up since it's a true error condition.
          } catch (IOException
              | UnsupportedOperationException // if the FS doesn't support hard-links
              | SecurityException ex // we don't have permission to use hard-links just fall back to byte copy
          ) {
            // hard-links are not supported or the files are on different filesystems
            // we could go deeper and check if their filesstores are the same and opt
            // out earlier but for now we just fall back to normal file-copy
            return ex;
          }
          return null;
        });
        tryCopy = suppressedException != null;
      }
    }
    if (tryCopy) {
      try {
        super.copyFrom(from, srcFile, destFile, context);
      } catch (Exception ex) {
        if (suppressedException != null) {
          ex.addSuppressed(suppressedException);
        }
        throw ex;
      }
    }
  }
}