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/*
 * 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.sshd.common.session;

import java.util.List;
import java.util.Map;

import org.apache.sshd.common.io.IoWriteFuture;
import org.apache.sshd.common.kex.KexProposalOption;
import org.apache.sshd.common.util.SshdEventListener;
import org.apache.sshd.common.util.buffer.Buffer;

/**
 * Provides a way to listen and handle the {@code SSH_MSG_IGNORE} and {@code SSH_MSG_DEBUG} messages that are received
 * by a session, as well as proprietary and/or extension messages and behavior.
 *
 * @author Apache MINA SSHD Project
 */
public interface ReservedSessionMessagesHandler extends SshdEventListener {
    /**
     * Send the initial version exchange identification in and independent manner
     *
     * @param  session    The {@code Session} through which the version is exchange is being managed
     * @param  version    The version line that was resolved - Note: since this string is part of the KEX and is
     *                    cached in the calling session, any changes to it require updating the session's cached
     *                    value.
     * @param  extraLines Extra lines to be sent - valid only for server sessions. Note:/B> the handler may modify
     *                    these lines and return {@code null} thus signaling the session to proceed with sending the
     *                    identification
     * @return            A {@link IoWriteFuture} that can be used to wait for the data to be sent successfully. If
     *                    {@code null} then the session will send the identification, otherwise it is assumed that the
     *                    handler has sent it.
     * @throws Exception  if failed to handle the callback
     * @see               RFC 4253 - section 4.2 - Protocol
     *                    Version Exchange
     */
    default IoWriteFuture sendIdentification(
            Session session, String version, List extraLines)
            throws Exception {
        return null;
    }

    /**
     * Invoked before sending the {@code SSH_MSG_KEXINIT} packet
     *
     * @param  session   The {@code Session} through which the key exchange is being managed
     * @param  proposal  The KEX proposal that was used to build the packet
     * @param  packet    The packet containing the fully encoded message - Caveat: this packet later serves as
     *                   part of the key generation, so care must be taken if manipulating it.
     * @return           A non-{@code null} {@link IoWriteFuture} to signal that handler took care of the KEX packet
     *                   delivery.
     * @throws Exception if failed to handle the callback
     */
    default IoWriteFuture sendKexInitRequest(
            Session session, Map proposal, Buffer packet)
            throws Exception {
        return null;
    }

    /**
     * Invoked when an {@code SSH_MSG_IGNORE} packet is received
     *
     * @param  session   The {@code Session} through which the message was received
     * @param  buffer    The {@code Buffer} containing the data
     * @throws Exception If failed to handle the message
     * @see              RFC 4253 - section 11.2
     */
    default void handleIgnoreMessage(Session session, Buffer buffer) throws Exception {
        // ignored
    }

    /**
     * Invoked when an {@code SSH_MSG_DEBUG} packet is received
     *
     * @param  session   The {@code Session} through which the message was received
     * @param  buffer    The {@code Buffer} containing the data
     * @throws Exception If failed to handle the message
     * @see              RFC 4253 - section 11.3
     */
    default void handleDebugMessage(Session session, Buffer buffer) throws Exception {
        // ignored
    }

    /**
     * Invoked when a packet with an un-implemented message is received - including {@code SSH_MSG_UNIMPLEMENTED} itself
     *
     * @param  session   The {@code Session} through which the message was received
     * @param  cmd       The received (un-implemented) command
     * @param  buffer    The {@code Buffer} containing the data - positioned just beyond the command
     * @return           {@code true} if message handled internally, {@code false} if should return a
     *                   {@code SSH_MSG_UNIMPLEMENTED} reply (default behavior)
     * @throws Exception If failed to handle the message
     * @see              RFC 4253 - section 11.4
     */
    default boolean handleUnimplementedMessage(Session session, int cmd, Buffer buffer) throws Exception {
        return false;
    }

    /**
     * Invoked if the user configured usage of a proprietary heartbeat mechanism. Note: by default throws
     * {@code UnsupportedOperationException} so users who configure a proprietary heartbeat mechanism option must
     * provide an implementation for this method.
     *
     * @param  service   The {@link ConnectionService} through which the heartbeat is being executed.
     * @return           {@code true} whether heartbeat actually sent - Note: used mainly for debugging purposes.
     * @throws Exception If failed to send the heartbeat - Note: causes associated session termination.
     */
    default boolean sendReservedHeartbeat(ConnectionService service) throws Exception {
        throw new UnsupportedOperationException("Reserved heartbeat not implemented for " + service);
    }
}