/*
 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.web.socket.handler;

import java.io.IOException;
import java.util.Queue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.web.socket.CloseStatus;
import org.springframework.web.socket.WebSocketMessage;
import org.springframework.web.socket.WebSocketSession;

/**
 * Wrap a {@link org.springframework.web.socket.WebSocketSession WebSocketSession}
 * to guarantee only one thread can send messages at a time.
 *
 * <p>If a send is slow, subsequent attempts to send more messages from other threads
 * will not be able to acquire the flush lock and messages will be buffered instead.
 * At that time, the specified buffer-size limit and send-time limit will be checked
 * and the session will be closed if the limits are exceeded.
 *
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 4.0.3
 */
public class ConcurrentWebSocketSessionDecorator extends WebSocketSessionDecorator {

        private static final Log logger = LogFactory.getLog(ConcurrentWebSocketSessionDecorator.class);


        private final int sendTimeLimit;

        private final int bufferSizeLimit;

        private final Queue<WebSocketMessage<?>> buffer = new LinkedBlockingQueue<WebSocketMessage<?>>();

        private final AtomicInteger bufferSize = new AtomicInteger();

        private volatile long sendStartTime;

        private volatile boolean limitExceeded;

        private volatile boolean closeInProgress;

        private final Lock flushLock = new ReentrantLock();

        private final Lock closeLock = new ReentrantLock();


        /**
         * Create a new {@code ConcurrentWebSocketSessionDecorator}.
         * @param delegate the {@code WebSocketSession} to delegate to
         * @param sendTimeLimit the send-time limit (milliseconds)
         * @param bufferSizeLimit the buffer-size limit (number of bytes)
         */
        public ConcurrentWebSocketSessionDecorator(WebSocketSession delegate, int sendTimeLimit, int bufferSizeLimit) {
                super(delegate);
                this.sendTimeLimit = sendTimeLimit;
                this.bufferSizeLimit = bufferSizeLimit;
        }


        /**
         * Return the configured send-time limit (milliseconds).
         * @since 4.3.13
         */
        public int getSendTimeLimit() {
                return this.sendTimeLimit;
        }

        /**
         * Return the configured buffer-size limit (number of bytes).
         * @since 4.3.13
         */
        public int getBufferSizeLimit() {
                return this.bufferSizeLimit;
        }

        /**
         * Return the current buffer size (number of bytes).
         */
        public int getBufferSize() {
                return this.bufferSize.get();
        }

        /**
         * Return the time (milliseconds) since the current send started,
         * or 0 if no send is currently in progress.
         */
        public long getTimeSinceSendStarted() {
                long start = this.sendStartTime;
                return (start > 0 ? (System.currentTimeMillis() - start) : 0);
        }


        @Override
        public void sendMessage(WebSocketMessage<?> message) throws IOException {
                if (shouldNotSend()) {
                        return;
                }

                this.buffer.add(message);
                this.bufferSize.addAndGet(message.getPayloadLength());

                do {
                        if (!tryFlushMessageBuffer()) {
                                if (logger.isTraceEnabled()) {
                                        logger.trace(String.format("Another send already in progress: " +
                                                        "session id '%s':, \"in-progress\" send time %d (ms), buffer size %d bytes",
                                                        getId(), getTimeSinceSendStarted(), getBufferSize()));
                                }
                                checkSessionLimits();
                                break;
                        }
                }
                while (!this.buffer.isEmpty() && !shouldNotSend());
        }

        private boolean shouldNotSend() {
                return (this.limitExceeded || this.closeInProgress);
        }

        private boolean tryFlushMessageBuffer() throws IOException {
                if (this.flushLock.tryLock()) {
                        try {
                                while (true) {
                                        WebSocketMessage<?> message = this.buffer.poll();
                                        if (message == null || shouldNotSend()) {
                                                break;
                                        }
                                        this.bufferSize.addAndGet(message.getPayloadLength() * -1);
                                        this.sendStartTime = System.currentTimeMillis();
                                        getDelegate().sendMessage(message);
                                        this.sendStartTime = 0;
                                }
                        }
                        finally {
                                this.sendStartTime = 0;
                                this.flushLock.unlock();
                        }
                        return true;
                }
                return false;
        }

        private void checkSessionLimits() {
                if (!shouldNotSend() && this.closeLock.tryLock()) {
                        try {
                                if (getTimeSinceSendStarted() > getSendTimeLimit()) {
                                        String format = "Message send time %d (ms) for session '%s' exceeded the allowed limit %d";
                                        String reason = String.format(format, getTimeSinceSendStarted(), getId(), getSendTimeLimit());
                                        limitExceeded(reason);
                                }
                                else if (getBufferSize() > getBufferSizeLimit()) {
                                        String format = "The send buffer size %d bytes for session '%s' exceeded the allowed limit %d";
                                        String reason = String.format(format, getBufferSize(), getId(), getBufferSizeLimit());
                                        limitExceeded(reason);
                                }
                        }
                        finally {
                                this.closeLock.unlock();
                        }
                }
        }

        private void limitExceeded(String reason) {
                this.limitExceeded = true;
                throw new SessionLimitExceededException(reason, CloseStatus.SESSION_NOT_RELIABLE);
        }

        @Override
        public void close(CloseStatus status) throws IOException {
                this.closeLock.lock();
                try {
                        if (this.closeInProgress) {
                                return;
                        }
                        if (!CloseStatus.SESSION_NOT_RELIABLE.equals(status)) {
                                try {
                                        checkSessionLimits();
                                }
                                catch (SessionLimitExceededException ex) {
                                        // Ignore
                                }
                                if (this.limitExceeded) {
                                        if (logger.isDebugEnabled()) {
                                                logger.debug("Changing close status " + status + " to SESSION_NOT_RELIABLE.");
                                        }
                                        status = CloseStatus.SESSION_NOT_RELIABLE;
                                }
                        }
                        this.closeInProgress = true;
                        super.close(status);
                }
                finally {
                        this.closeLock.unlock();
                }
        }


        @Override
        public String toString() {
                return getDelegate().toString();
        }

}