/*

 * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package jdk.vm.ci.hotspot;

import java.util.ArrayList;

import java.util.List;

import java.util.Objects;

import jdk.vm.ci.services.Services;

/**

 * A mechanism for limiting the lifetime of a foreign object reference encapsulated in a

 * {@link HotSpotObjectConstant}.

 *

 * A {@link HotSpotObjectConstant} allocated in a {@linkplain #openLocalScope local} scope will have

 * its reference to any foreign object cleared when the scope {@linkplain #close() closes}. This

 * allows the foreign memory manager to reclaim the foreign object (once there are no other strong

 * references to it).

 *

 * {@link HotSpotObjectConstantScope}s have no impact on {@link HotSpotObjectConstant}s that do not

 * encapsulate a foreign object reference.

 *

 * The object returned by {@link #enterGlobalScope()} or {@link #openLocalScope(Object)} should

 * always be used in a try-with-resources statement. Failure to close a scope will almost certainly

 * result in foreign objects being leaked.

 */

public final class HotSpotObjectConstantScope implements AutoCloseable {

    static final ThreadLocal<HotSpotObjectConstantScope> CURRENT = new ThreadLocal<>();

    private final HotSpotObjectConstantScope parent;

    private List<IndirectHotSpotObjectConstantImpl> foreignObjects;

    /**

     * An object whose {@link Object#toString()} value describes a non-global scope. This is

     * {@code null} iff this is a global scope.

     */

    final Object localScopeDescription;

    /**

     * Opens a local scope that upon closing, will release foreign object references encapsulated by

     * {@link HotSpotObjectConstant}s created in the scope.

     *

     * @param description an non-null object whose {@link Object#toString()} value describes the

     *            scope being opened

     * @return {@code null} if the current runtime does not support remote object references

     */

    public static HotSpotObjectConstantScope openLocalScope(Object description) {

        return Services.IS_IN_NATIVE_IMAGE ? new HotSpotObjectConstantScope(Objects.requireNonNull(description)) : null;

    }

    /**

     * Enters the global scope. This is useful to escape a local scope for execution that will

     * create foreign object references that need to outlive the local scope.

     *

     * Foreign object references encapsulated by {@link HotSpotObjectConstant}s created in the

     * global scope are only subject to reclamation once the {@link HotSpotObjectConstant} wrapper

     * dies.

     *

     * @return {@code null} if the current runtime does not support remote object references or if

     *         this thread is currently in the global scope

     */

    public static HotSpotObjectConstantScope enterGlobalScope() {

        return Services.IS_IN_NATIVE_IMAGE && CURRENT.get() != null ? new HotSpotObjectConstantScope(null) : null;

    }

    private HotSpotObjectConstantScope(Object localScopeDescription) {

        this.parent = CURRENT.get();

        CURRENT.set(this);

        this.localScopeDescription = localScopeDescription;

    }

    /**

     * Determines if this scope is global.

     */

    boolean isGlobal() {

        return localScopeDescription == null;

    }

    void add(IndirectHotSpotObjectConstantImpl obj) {

        assert !isGlobal();

        if (foreignObjects == null) {

            foreignObjects = new ArrayList<>();

        }

        foreignObjects.add(obj);

    }

    @VMEntryPoint

    @Override

    public void close() {

        if (CURRENT.get() != this) {

            throw new IllegalStateException("Cannot close non-active scope");

        }

        if (foreignObjects != null) {

            for (IndirectHotSpotObjectConstantImpl obj : foreignObjects) {

                obj.clear(localScopeDescription);

            }

            foreignObjects = null;

        }

        CURRENT.set(parent);

    }

}