//
// ========================================================================
// Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under
// the terms of the Eclipse Public License 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0
//
// This Source Code may also be made available under the following
// Secondary Licenses when the conditions for such availability set
// forth in the Eclipse Public License, v. 2.0 are satisfied:
// the Apache License v2.0 which is available at
// https://www.apache.org/licenses/LICENSE-2.0
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//

package org.eclipse.jetty.server.handler;

import java.nio.file.Files;
import java.nio.file.Path;

import org.eclipse.jetty.server.handler.ContextHandler.AliasCheck;
import org.eclipse.jetty.util.resource.PathResource;
import org.eclipse.jetty.util.resource.Resource;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

Symbolic Link AliasChecker.
An instance of this class can be registered with ContextHandler.addAliasCheck(AliasCheck) to check resources that are aliased to other locations. The checker uses the Java Files.readSymbolicLink(Path) and Path.toRealPath(LinkOption...) APIs to check if a file is aliased with symbolic links.
/**
 * Symbolic Link AliasChecker.
 * <p>An instance of this class can be registered with {@link ContextHandler#addAliasCheck(AliasCheck)}
 * to check resources that are aliased to other locations.   The checker uses the
 * Java {@link Files#readSymbolicLink(Path)} and {@link Path#toRealPath(java.nio.file.LinkOption...)}
 * APIs to check if a file is aliased with symbolic links.</p>
 */
public class AllowSymLinkAliasChecker implements AliasCheck
{
    private static final Logger LOG = LoggerFactory.getLogger(AllowSymLinkAliasChecker.class);

    @Override
    public boolean check(String uri, Resource resource)
    {
        // Only support PathResource alias checking
        if (!(resource instanceof PathResource))
            return false;

        PathResource pathResource = (PathResource)resource;

        try
        {
            Path path = pathResource.getPath();
            Path alias = pathResource.getAliasPath();

            if (PathResource.isSameName(alias, path))
                return false; // Unknown why this is an alias

            if (hasSymbolicLink(path) && Files.isSameFile(path, alias))
            {
                if (LOG.isDebugEnabled())
                    LOG.debug("Allow symlink {} --> {}", resource, pathResource.getAliasPath());
                return true;
            }
        }
        catch (Exception e)
        {
            LOG.trace("IGNORED", e);
        }

        return false;
    }

    private boolean hasSymbolicLink(Path path)
    {
        // Is file itself a symlink?
        if (Files.isSymbolicLink(path))
        {
            return true;
        }

        // Lets try each path segment
        Path base = path.getRoot();
        for (Path segment : path)
        {
            base = base.resolve(segment);
            if (Files.isSymbolicLink(base))
            {
                return true;
            }
        }

        return false;
    }
}