/*
 * Copyright (C) 2018, 2019 Thomas Wolf <thomas.wolf@paranor.ch> and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */
package org.eclipse.jgit.internal.transport.sshd;

import static java.text.MessageFormat.format;

import java.io.IOException;
import java.io.StreamCorruptedException;
import java.net.SocketAddress;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.PublicKey;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;

import org.apache.sshd.client.ClientFactoryManager;
import org.apache.sshd.client.config.hosts.HostConfigEntry;
import org.apache.sshd.client.keyverifier.ServerKeyVerifier;
import org.apache.sshd.client.session.ClientSessionImpl;
import org.apache.sshd.common.FactoryManager;
import org.apache.sshd.common.PropertyResolverUtils;
import org.apache.sshd.common.SshException;
import org.apache.sshd.common.config.keys.KeyUtils;
import org.apache.sshd.common.io.IoSession;
import org.apache.sshd.common.io.IoWriteFuture;
import org.apache.sshd.common.util.Readable;
import org.apache.sshd.common.util.buffer.Buffer;
import org.eclipse.jgit.errors.InvalidPatternException;
import org.eclipse.jgit.fnmatch.FileNameMatcher;
import org.eclipse.jgit.internal.transport.sshd.proxy.StatefulProxyConnector;
import org.eclipse.jgit.transport.CredentialsProvider;
import org.eclipse.jgit.transport.SshConstants;

/**
 * A {@link org.apache.sshd.client.session.ClientSession ClientSession} that can
 * be associated with the {@link HostConfigEntry} the session was created for.
 * The {@link JGitSshClient} creates such sessions and sets this association.
 * <p>
 * Also provides for associating a JGit {@link CredentialsProvider} with a
 * session.
 * </p>
 */
public class JGitClientSession extends ClientSessionImpl {

	/**
	 * Default setting for the maximum number of bytes to read in the initial
	 * protocol version exchange. 64kb is what OpenSSH < 8.0 read; OpenSSH 8.0
	 * changed it to 8Mb, but that seems excessive for the purpose stated in RFC
	 * 4253. The Apache MINA sshd default in
	 * {@link FactoryManager#DEFAULT_MAX_IDENTIFICATION_SIZE} is 16kb.
	 */
	private static final int DEFAULT_MAX_IDENTIFICATION_SIZE = 64 * 1024;

	private HostConfigEntry hostConfig;

	private CredentialsProvider credentialsProvider;

	private volatile StatefulProxyConnector proxyHandler;

	/**
	 * @param manager
	 * @param session
	 * @throws Exception
	 */
	public JGitClientSession(ClientFactoryManager manager, IoSession session)
			throws Exception {
		super(manager, session);
	}

	/**
	 * Retrieves the {@link HostConfigEntry} this session was created for.
	 *
	 * @return the {@link HostConfigEntry}, or {@code null} if none set
	 */
	public HostConfigEntry getHostConfigEntry() {
		return hostConfig;
	}

	/**
	 * Sets the {@link HostConfigEntry} this session was created for.
	 *
	 * @param hostConfig
	 *            the {@link HostConfigEntry}
	 */
	public void setHostConfigEntry(HostConfigEntry hostConfig) {
		this.hostConfig = hostConfig;
	}

	/**
	 * Sets the {@link CredentialsProvider} for this session.
	 *
	 * @param provider
	 *            to set
	 */
	public void setCredentialsProvider(CredentialsProvider provider) {
		credentialsProvider = provider;
	}

	/**
	 * Retrieves the {@link CredentialsProvider} set for this session.
	 *
	 * @return the provider, or {@code null} if none is set.
	 */
	public CredentialsProvider getCredentialsProvider() {
		return credentialsProvider;
	}

	/**
	 * Sets a {@link StatefulProxyConnector} to handle proxy connection
	 * protocols.
	 *
	 * @param handler
	 *            to set
	 */
	public void setProxyHandler(StatefulProxyConnector handler) {
		proxyHandler = handler;
	}

	@Override
	protected IoWriteFuture sendIdentification(String ident)
			throws IOException {
		StatefulProxyConnector proxy = proxyHandler;
		if (proxy != null) {
			try {
				// We must not block here; the framework starts reading messages
				// from the peer only once the initial sendKexInit() following
				// this call to sendIdentification() has returned!
				proxy.runWhenDone(() -> {
					JGitClientSession.super.sendIdentification(ident);
					return null;
				});
				// Called only from the ClientSessionImpl constructor, where the
				// return value is ignored.
				return null;
			} catch (IOException e) {
				throw e;
			} catch (Exception other) {
				throw new IOException(other.getLocalizedMessage(), other);
			}
		}
		return super.sendIdentification(ident);
	}

	@Override
	protected byte[] sendKexInit()
			throws IOException, GeneralSecurityException {
		StatefulProxyConnector proxy = proxyHandler;
		if (proxy != null) {
			try {
				// We must not block here; the framework starts reading messages
				// from the peer only once the initial sendKexInit() has
				// returned!
				proxy.runWhenDone(() -> {
					JGitClientSession.super.sendKexInit();
					return null;
				});
				// This is called only from the ClientSessionImpl
				// constructor, where the return value is ignored.
				return null;
			} catch (IOException | GeneralSecurityException e) {
				throw e;
			} catch (Exception other) {
				throw new IOException(other.getLocalizedMessage(), other);
			}
		}
		return super.sendKexInit();
	}

	/**
	 * {@inheritDoc}
	 *
	 * As long as we're still setting up the proxy connection, diverts messages
	 * to the {@link StatefulProxyConnector}.
	 */
	@Override
	public void messageReceived(Readable buffer) throws Exception {
		StatefulProxyConnector proxy = proxyHandler;
		if (proxy != null) {
			proxy.messageReceived(getIoSession(), buffer);
		} else {
			super.messageReceived(buffer);
		}
	}

	@Override
	protected void checkKeys() throws SshException {
		ServerKeyVerifier serverKeyVerifier = getServerKeyVerifier();
		// The super implementation always uses
		// getIoSession().getRemoteAddress(). In case of a proxy connection,
		// that would be the address of the proxy!
		SocketAddress remoteAddress = getConnectAddress();
		PublicKey serverKey = getKex().getServerKey();
		if (!serverKeyVerifier.verifyServerKey(this, remoteAddress,
				serverKey)) {
			throw new SshException(
					org.apache.sshd.common.SshConstants.SSH2_DISCONNECT_HOST_KEY_NOT_VERIFIABLE,
					SshdText.get().kexServerKeyInvalid);
		}
	}

	@Override
	protected String resolveAvailableSignaturesProposal(
			FactoryManager manager) {
		Set<String> defaultSignatures = new LinkedHashSet<>();
		defaultSignatures.addAll(getSignatureFactoriesNames());
		HostConfigEntry config = resolveAttribute(
				JGitSshClient.HOST_CONFIG_ENTRY);
		String hostKeyAlgorithms = config
				.getProperty(SshConstants.HOST_KEY_ALGORITHMS);
		if (hostKeyAlgorithms != null && !hostKeyAlgorithms.isEmpty()) {
			char first = hostKeyAlgorithms.charAt(0);
			switch (first) {
			case '+':
				// Additions make not much sense -- it's either in
				// defaultSignatures already, or we have no implementation for
				// it. No point in proposing it.
				return String.join(",", defaultSignatures); //$NON-NLS-1$
			case '-':
				// This takes wildcard patterns!
				removeFromList(defaultSignatures,
						SshConstants.HOST_KEY_ALGORITHMS,
						hostKeyAlgorithms.substring(1));
				if (defaultSignatures.isEmpty()) {
					// Too bad: user config error. Warn here, and then fail
					// later.
					log.warn(format(
							SshdText.get().configNoRemainingHostKeyAlgorithms,
							hostKeyAlgorithms));
				}
				return String.join(",", defaultSignatures); //$NON-NLS-1$
			default:
				// Default is overridden -- only accept the ones for which we do
				// have an implementation.
				List<String> newNames = filteredList(defaultSignatures,
						hostKeyAlgorithms);
				if (newNames.isEmpty()) {
					log.warn(format(
							SshdText.get().configNoKnownHostKeyAlgorithms,
							hostKeyAlgorithms));
					// Use the default instead.
				} else {
					return String.join(",", newNames); //$NON-NLS-1$
				}
				break;
			}
		}
		// No HostKeyAlgorithms; using default -- change order to put existing
		// keys first.
		ServerKeyVerifier verifier = getServerKeyVerifier();
		if (verifier instanceof ServerKeyLookup) {
			SocketAddress remoteAddress = resolvePeerAddress(
					resolveAttribute(JGitSshClient.ORIGINAL_REMOTE_ADDRESS));
			List<PublicKey> allKnownKeys = ((ServerKeyLookup) verifier)
					.lookup(this, remoteAddress);
			Set<String> reordered = new LinkedHashSet<>();
			for (PublicKey key : allKnownKeys) {
				if (key != null) {
					String keyType = KeyUtils.getKeyType(key);
					if (keyType != null) {
						reordered.add(keyType);
					}
				}
			}
			reordered.addAll(defaultSignatures);
			return String.join(",", reordered); //$NON-NLS-1$
		}
		return String.join(",", defaultSignatures); //$NON-NLS-1$
	}

	private void removeFromList(Set<String> current, String key,
			String patterns) {
		for (String toRemove : patterns.split("\\s*,\\s*")) { //$NON-NLS-1$
			if (toRemove.indexOf('*') < 0 && toRemove.indexOf('?') < 0) {
				current.remove(toRemove);
				continue;
			}
			try {
				FileNameMatcher matcher = new FileNameMatcher(toRemove, null);
				for (Iterator<String> i = current.iterator(); i.hasNext();) {
					matcher.reset();
					matcher.append(i.next());
					if (matcher.isMatch()) {
						i.remove();
					}
				}
			} catch (InvalidPatternException e) {
				log.warn(format(SshdText.get().configInvalidPattern, key,
						toRemove));
			}
		}
	}

	private List<String> filteredList(Set<String> known, String values) {
		List<String> newNames = new ArrayList<>();
		for (String newValue : values.split("\\s*,\\s*")) { //$NON-NLS-1$
			if (known.contains(newValue)) {
				newNames.add(newValue);
			}
		}
		return newNames;
	}

	/**
	 * Reads the RFC 4253, section 4.2 protocol version identification. The
	 * Apache MINA sshd default implementation checks for NUL bytes also in any
	 * preceding lines, whereas RFC 4253 requires such a check only for the
	 * actual identification string starting with "SSH-". Likewise, the 255
	 * character limit exists only for the identification string, not for the
	 * preceding lines. CR-LF handling is also relaxed.
	 *
	 * @param buffer
	 *            to read from
	 * @param server
	 *            whether we're an SSH server (should always be {@code false})
	 * @return the lines read, with the server identification line last, or
	 *         {@code null} if no identification line was found and more bytes
	 *         are needed
	 * @throws StreamCorruptedException
	 *             if the identification is malformed
	 * @see <a href="https://tools.ietf.org/html/rfc4253#section-4.2">RFC 4253,
	 *      section 4.2</a>
	 */
	@Override
	protected List<String> doReadIdentification(Buffer buffer, boolean server)
			throws StreamCorruptedException {
		if (server) {
			// Should never happen. No translation; internal bug.
			throw new IllegalStateException(
					"doReadIdentification of client called with server=true"); //$NON-NLS-1$
		}
		int maxIdentSize = PropertyResolverUtils.getIntProperty(this,
				FactoryManager.MAX_IDENTIFICATION_SIZE,
				DEFAULT_MAX_IDENTIFICATION_SIZE);
		int current = buffer.rpos();
		int end = current + buffer.available();
		if (current >= end) {
			return null;
		}
		byte[] raw = buffer.array();
		List<String> ident = new ArrayList<>();
		int start = current;
		boolean hasNul = false;
		for (int i = current; i < end; i++) {
			switch (raw[i]) {
			case 0:
				hasNul = true;
				break;
			case '\n':
				int eol = 1;
				if (i > start && raw[i - 1] == '\r') {
					eol++;
				}
				String line = new String(raw, start, i + 1 - eol - start,
						StandardCharsets.UTF_8);
				start = i + 1;
				if (log.isDebugEnabled()) {
					log.debug(format("doReadIdentification({0}) line: ", this) + //$NON-NLS-1$
							escapeControls(line));
				}
				ident.add(line);
				if (line.startsWith("SSH-")) { //$NON-NLS-1$
					if (hasNul) {
						throw new StreamCorruptedException(
								format(SshdText.get().serverIdWithNul,
										escapeControls(line)));
					}
					if (line.length() + eol > 255) {
						throw new StreamCorruptedException(
								format(SshdText.get().serverIdTooLong,
										escapeControls(line)));
					}
					buffer.rpos(start);
					return ident;
				}
				// If this were a server, we could throw an exception here: a
				// client is not supposed to send any extra lines before its
				// identification string.
				hasNul = false;
				break;
			default:
				break;
			}
			if (i - current + 1 >= maxIdentSize) {
				String msg = format(SshdText.get().serverIdNotReceived,
						Integer.toString(maxIdentSize));
				if (log.isDebugEnabled()) {
					log.debug(msg);
					log.debug(buffer.toHex());
				}
				throw new StreamCorruptedException(msg);
			}
		}
		// Need more data
		return null;
	}

	private static String escapeControls(String s) {
		StringBuilder b = new StringBuilder();
		int l = s.length();
		for (int i = 0; i < l; i++) {
			char ch = s.charAt(i);
			if (Character.isISOControl(ch)) {
				b.append(ch <= 0xF ? "\\u000" : "\\u00") //$NON-NLS-1$ //$NON-NLS-2$
						.append(Integer.toHexString(ch));
			} else {
				b.append(ch);
			}
		}
		return b.toString();
	}

}