/*
 * JBoss, Home of Professional Open Source.
 * Copyright 2014 Red Hat, Inc., and individual contributors
 * as indicated by the @author tags.
 *
 * Licensed 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 io.undertow.util;

import io.undertow.UndertowMessages;
import io.undertow.connector.PooledByteBuffer;

import java.nio.ByteBuffer;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;

A reference counted pooled implementation, that basically consists of a main buffer, that can be sliced off into smaller buffers,
and the underlying buffer will not be freed until all the slices and the main buffer itself have also been freed.
This also supports the notion of un-freeing the main buffer. Basically this allows the buffer be re-used, so if only a small slice of the
buffer was used for read operations the main buffer can potentially be re-used. This prevents buffer exhaustion attacks where content
is sent in many small packets, and you end up allocating a large number of buffers to hold a small amount of data.
Author:
Stuart Douglas/**
 * A reference counted pooled implementation, that basically consists of a main buffer, that can be sliced off into smaller buffers,
 * and the underlying buffer will not be freed until all the slices and the main buffer itself have also been freed.
 *
 * This also supports the notion of un-freeing the main buffer. Basically this allows the buffer be re-used, so if only a small slice of the
 * buffer was used for read operations the main buffer can potentially be re-used. This prevents buffer exhaustion attacks where content
 * is sent in many small packets, and you end up allocating a large number of buffers to hold a small amount of data.
 *
 * @author Stuart Douglas
 */
public class ReferenceCountedPooled implements PooledByteBuffer {

    private final PooledByteBuffer underlying;
    @SuppressWarnings("unused")
    private volatile int referenceCount;
    boolean mainFreed = false;
    private ByteBuffer slice = null;
    private final FreeNotifier freeNotifier;

    private static final AtomicIntegerFieldUpdater<ReferenceCountedPooled> referenceCountUpdater = AtomicIntegerFieldUpdater.newUpdater(ReferenceCountedPooled.class, "referenceCount");

    public ReferenceCountedPooled(PooledByteBuffer underlying, int referenceCount) {
        this(underlying, referenceCount, null);
    }

    public ReferenceCountedPooled(PooledByteBuffer underlying, int referenceCount, FreeNotifier freeNotifier) {
        this.underlying = underlying;
        this.referenceCount = referenceCount;
        this.freeNotifier = freeNotifier;
    }

    @Override
    public void close() {
        if(mainFreed) {
            return;
        }
        mainFreed = true;
        freeInternal();
    }

    @Override
    public boolean isOpen() {
        return !mainFreed;
    }

    public boolean isFreed() {
        return mainFreed;
    }

    public boolean tryUnfree() {
        int refs;
        do {
            refs  = referenceCountUpdater.get(this);
            if(refs <= 0) {
                 return false;
            }
        } while (!referenceCountUpdater.compareAndSet(this, refs, refs + 1));
        ByteBuffer resource = slice != null ? slice : underlying.getBuffer();
        resource.position(resource.limit());
        resource.limit(resource.capacity());
        slice = resource.slice();
        mainFreed = false;
        return true;
    }

    private void freeInternal() {
        if(referenceCountUpdater.decrementAndGet(this) == 0) {
            underlying.close();
            if(freeNotifier != null) {
                freeNotifier.freed();
            }
        }
    }

    @Override
    public ByteBuffer getBuffer() throws IllegalStateException {
        if(mainFreed) {
            throw UndertowMessages.MESSAGES.bufferAlreadyFreed();
        }
        if(slice != null) {
            return slice;
        }
        return underlying.getBuffer();
    }

    public PooledByteBuffer createView(final ByteBuffer newValue) {
        increaseReferenceCount();
        return new PooledByteBuffer() {

            boolean free = false;

            @Override
            public void close() {
                //make sure that a given view can only be freed once
                if(!free) {
                    free = true;
                    ReferenceCountedPooled.this.freeInternal();
                }
            }

            @Override
            public boolean isOpen() {
                return !free;
            }

            @Override
            public ByteBuffer getBuffer() throws IllegalStateException {
                if(free) {
                    throw UndertowMessages.MESSAGES.bufferAlreadyFreed();
                }
                return newValue;
            }

            @Override
            public String toString() {
                return "ReferenceCountedPooled$view{" +
                        "buffer=" + newValue +
                        "free=" + free +
                        "underlying=" + underlying +
                        ", referenceCount=" + referenceCount +
                        ", mainFreed=" + mainFreed +
                        ", slice=" + slice +
                        '}';
            }
        };
    }

    public void increaseReferenceCount() {
        int val;
        do {
            val = referenceCountUpdater.get(this);
            if(val == 0) {
                //should never happen, as this should only be called from
                //code that already has a reference
                throw UndertowMessages.MESSAGES.objectWasFreed();
            }
        } while (!referenceCountUpdater.compareAndSet(this, val, val + 1));
    }

    public interface FreeNotifier {
        void freed();
    }

    @Override
    public String toString() {
        return "ReferenceCountedPooled{" +
                "underlying=" + underlying +
                ", referenceCount=" + referenceCount +
                ", mainFreed=" + mainFreed +
                ", slice=" + slice +
                '}';
    }
}