/*
    * 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.transport.sshd;
  
  import java.io.Closeable;
  import java.io.File;
  import java.io.IOException;
  import java.nio.file.Files;
  import java.nio.file.Path;
  import java.security.KeyPair;
  import java.time.Duration;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.stream.Collectors;
  
  import org.apache.sshd.client.ClientBuilder;
  import org.apache.sshd.client.SshClient;
  import org.apache.sshd.client.auth.UserAuthFactory;
  import org.apache.sshd.client.auth.keyboard.UserAuthKeyboardInteractiveFactory;
  import org.apache.sshd.client.auth.pubkey.UserAuthPublicKeyFactory;
  import org.apache.sshd.client.config.hosts.HostConfigEntryResolver;
  import org.apache.sshd.common.compression.BuiltinCompressions;
  import org.apache.sshd.common.config.keys.FilePasswordProvider;
  import org.apache.sshd.common.config.keys.loader.openssh.kdf.BCryptKdfOptions;
  import org.apache.sshd.common.keyprovider.KeyIdentityProvider;
  import org.eclipse.jgit.annotations.NonNull;
  import org.eclipse.jgit.errors.TransportException;
  import org.eclipse.jgit.internal.transport.ssh.OpenSshConfigFile;
  import org.eclipse.jgit.internal.transport.sshd.CachingKeyPairProvider;
  import org.eclipse.jgit.internal.transport.sshd.GssApiWithMicAuthFactory;
  import org.eclipse.jgit.internal.transport.sshd.JGitPasswordAuthFactory;
  import org.eclipse.jgit.internal.transport.sshd.JGitServerKeyVerifier;
  import org.eclipse.jgit.internal.transport.sshd.JGitSshClient;
  import org.eclipse.jgit.internal.transport.sshd.JGitSshConfig;
  import org.eclipse.jgit.internal.transport.sshd.JGitUserInteraction;
  import org.eclipse.jgit.internal.transport.sshd.OpenSshServerKeyDatabase;
  import org.eclipse.jgit.internal.transport.sshd.PasswordProviderWrapper;
  import org.eclipse.jgit.internal.transport.sshd.SshdText;
  import org.eclipse.jgit.transport.CredentialsProvider;
  import org.eclipse.jgit.transport.SshConfigStore;
  import org.eclipse.jgit.transport.SshConstants;
  import org.eclipse.jgit.transport.SshSessionFactory;
  import org.eclipse.jgit.transport.URIish;
  import org.eclipse.jgit.util.FS;
  
  /**
   * A {@link SshSessionFactory} that uses Apache MINA sshd. Classes from Apache
   * MINA sshd are kept private to avoid API evolution problems when Apache MINA
   * sshd interfaces change.
   *
   * @since 5.2
   */
  public class SshdSessionFactory extends SshSessionFactory implements Closeable {
  
  	private static final String MINA_SSHD = "mina-sshd"; //$NON-NLS-1$
  
  	private final AtomicBoolean closing = new AtomicBoolean();
  
  	private final Set<SshdSession> sessions = new HashSet<>();
  
  	private final Map<Tuple, HostConfigEntryResolver> defaultHostConfigEntryResolver = new ConcurrentHashMap<>();
  
  	private final Map<Tuple, ServerKeyDatabase> defaultServerKeyDatabase = new ConcurrentHashMap<>();
  
  	private final Map<Tuple, Iterable<KeyPair>> defaultKeys = new ConcurrentHashMap<>();
  
  	private final KeyCache keyCache;
  
  	private final ProxyDataFactory proxies;
  
  	private File sshDirectory;
  
  	private File homeDirectory;
  
  	/**
  	 * Creates a new {@link SshdSessionFactory} without key cache and a
  	 * {@link DefaultProxyDataFactory}.
  	 */
  	public SshdSessionFactory() {
  		this(null, new DefaultProxyDataFactory());
  	}
  
  	/**
  	 * Creates a new {@link SshdSessionFactory} using the given {@link KeyCache}
  	 * and {@link ProxyDataFactory}. The {@code keyCache} is used for all sessions
 	 * created through this session factory; cached keys are destroyed when the
 	 * session factory is {@link #close() closed}.
 	 * <p>
 	 * Caching ssh keys in memory for an extended period of time is generally
 	 * considered bad practice, but there may be circumstances where using a
 	 * {@link KeyCache} is still the right choice, for instance to avoid that a
 	 * user gets prompted several times for the same password for the same key.
 	 * In general, however, it is preferable <em>not</em> to use a key cache but
 	 * to use a {@link #createKeyPasswordProvider(CredentialsProvider)
 	 * KeyPasswordProvider} that has access to some secure storage and can save
 	 * and retrieve passwords from there without user interaction. Another
 	 * approach is to use an ssh agent.
 	 * </p>
 	 * <p>
 	 * Note that the underlying ssh library (Apache MINA sshd) may or may not
 	 * keep ssh keys in memory for unspecified periods of time irrespective of
 	 * the use of a {@link KeyCache}.
 	 * </p>
 	 *
 	 * @param keyCache
 	 *            {@link KeyCache} to use for caching ssh keys, or {@code null}
 	 *            to not use a key cache
 	 * @param proxies
 	 *            {@link ProxyDataFactory} to use, or {@code null} to not use a
 	 *            proxy database (in which case connections through proxies will
 	 *            not be possible)
 	 */
 	public SshdSessionFactory(KeyCache keyCache, ProxyDataFactory proxies) {
 		super();
 		this.keyCache = keyCache;
 		this.proxies = proxies;
 		// sshd limits the number of BCrypt KDF rounds to 255 by default.
 		// Decrypting such a key takes about two seconds on my machine.
 		// I consider this limit too low. The time increases linearly with the
 		// number of rounds.
 		BCryptKdfOptions.setMaxAllowedRounds(16384);
 	}
 
 	@Override
 	public String getType() {
 		return MINA_SSHD;
 	}
 
 	/** A simple general map key. */
 	private static final class Tuple {
 		private Object[] objects;
 
 		public Tuple(Object[] objects) {
 			this.objects = objects;
 		}
 
 		@Override
 		public boolean equals(Object obj) {
 			if (obj == this) {
 				return true;
 			}
 			if (obj != null && obj.getClass() == Tuple.class) {
 				Tuple other = (Tuple) obj;
 				return Arrays.equals(objects, other.objects);
 			}
 			return false;
 		}
 
 		@Override
 		public int hashCode() {
 			return Arrays.hashCode(objects);
 		}
 	}
 
 	// We can't really use a single client. Clients need to be stopped
 	// properly, and we don't really know when to do that. Instead we use
 	// a dedicated SshClient instance per session. We need a bit of caching to
 	// avoid re-loading the ssh config and keys repeatedly.
 
 	@Override
 	public SshdSession getSession(URIish uri,
 			CredentialsProvider credentialsProvider, FS fs, int tms)
 			throws TransportException {
 		SshdSession session = null;
 		try {
 			session = new SshdSession(uri, () -> {
 				File home = getHomeDirectory();
 				if (home == null) {
 					// Always use the detected filesystem for the user home!
 					// It makes no sense to have different "user home"
 					// directories depending on what file system a repository
 					// is.
 					home = FS.DETECTED.userHome();
 				}
 				File sshDir = getSshDirectory();
 				if (sshDir == null) {
 					sshDir = new File(home, SshConstants.SSH_DIR);
 				}
 				HostConfigEntryResolver configFile = getHostConfigEntryResolver(
 						home, sshDir);
 				KeyIdentityProvider defaultKeysProvider = toKeyIdentityProvider(
 						getDefaultKeys(sshDir));
 				KeyPasswordProvider passphrases = createKeyPasswordProvider(
 						credentialsProvider);
 				SshClient client = ClientBuilder.builder()
 						.factory(JGitSshClient::new)
 						.filePasswordProvider(
 								createFilePasswordProvider(passphrases))
 						.hostConfigEntryResolver(configFile)
 						.serverKeyVerifier(new JGitServerKeyVerifier(
 								getServerKeyDatabase(home, sshDir)))
 						.compressionFactories(
 								new ArrayList<>(BuiltinCompressions.VALUES))
 						.build();
 				client.setUserInteraction(
 						new JGitUserInteraction(credentialsProvider));
 				client.setUserAuthFactories(getUserAuthFactories());
 				client.setKeyIdentityProvider(defaultKeysProvider);
 				// JGit-specific things:
 				JGitSshClient jgitClient = (JGitSshClient) client;
 				jgitClient.setKeyCache(getKeyCache());
 				jgitClient.setCredentialsProvider(credentialsProvider);
 				jgitClient.setProxyDatabase(proxies);
 				String defaultAuths = getDefaultPreferredAuthentications();
 				if (defaultAuths != null) {
 					jgitClient.setAttribute(
 							JGitSshClient.PREFERRED_AUTHENTICATIONS,
 							defaultAuths);
 				}
 				// Other things?
 				return client;
 			});
 			session.addCloseListener(s -> unregister(s));
 			register(session);
 			session.connect(Duration.ofMillis(tms));
 			return session;
 		} catch (Exception e) {
 			unregister(session);
 			throw new TransportException(uri, e.getMessage(), e);
 		}
 	}
 
 	@Override
 	public void close() {
 		closing.set(true);
 		boolean cleanKeys = false;
 		synchronized (this) {
 			cleanKeys = sessions.isEmpty();
 		}
 		if (cleanKeys) {
 			KeyCache cache = getKeyCache();
 			if (cache != null) {
 				cache.close();
 			}
 		}
 	}
 
 	private void register(SshdSession newSession) throws IOException {
 		if (newSession == null) {
 			return;
 		}
 		if (closing.get()) {
 			throw new IOException(SshdText.get().sshClosingDown);
 		}
 		synchronized (this) {
 			sessions.add(newSession);
 		}
 	}
 
 	private void unregister(SshdSession oldSession) {
 		boolean cleanKeys = false;
 		synchronized (this) {
 			sessions.remove(oldSession);
 			cleanKeys = closing.get() && sessions.isEmpty();
 		}
 		if (cleanKeys) {
 			KeyCache cache = getKeyCache();
 			if (cache != null) {
 				cache.close();
 			}
 		}
 	}
 
 	/**
 	 * Set a global directory to use as the user's home directory
 	 *
 	 * @param homeDir
 	 *            to use
 	 */
 	public void setHomeDirectory(@NonNull File homeDir) {
 		if (homeDir.isAbsolute()) {
 			homeDirectory = homeDir;
 		} else {
 			homeDirectory = homeDir.getAbsoluteFile();
 		}
 	}
 
 	/**
 	 * Retrieves the global user home directory
 	 *
 	 * @return the directory, or {@code null} if not set
 	 */
 	public File getHomeDirectory() {
 		return homeDirectory;
 	}
 
 	/**
 	 * Set a global directory to use as the .ssh directory
 	 *
 	 * @param sshDir
 	 *            to use
 	 */
 	public void setSshDirectory(@NonNull File sshDir) {
 		if (sshDir.isAbsolute()) {
 			sshDirectory = sshDir;
 		} else {
 			sshDirectory = sshDir.getAbsoluteFile();
 		}
 	}
 
 	/**
 	 * Retrieves the global .ssh directory
 	 *
 	 * @return the directory, or {@code null} if not set
 	 */
 	public File getSshDirectory() {
 		return sshDirectory;
 	}
 
 	/**
 	 * Obtain a {@link HostConfigEntryResolver} to read the ssh config file and
 	 * to determine host entries for connections.
 	 *
 	 * @param homeDir
 	 *            home directory to use for ~ replacement
 	 * @param sshDir
 	 *            to use for looking for the config file
 	 * @return the resolver
 	 */
 	@NonNull
 	private HostConfigEntryResolver getHostConfigEntryResolver(
 			@NonNull File homeDir, @NonNull File sshDir) {
 		return defaultHostConfigEntryResolver.computeIfAbsent(
 				new Tuple(new Object[] { homeDir, sshDir }),
 				t -> new JGitSshConfig(createSshConfigStore(homeDir,
 						getSshConfig(sshDir), getLocalUserName())));
 	}
 
 	/**
 	 * Determines the ssh config file. The default implementation returns
 	 * ~/.ssh/config. If the file does not exist and is created later it will be
 	 * picked up. To not use a config file at all, return {@code null}.
 	 *
 	 * @param sshDir
 	 *            representing ~/.ssh/
 	 * @return the file (need not exist), or {@code null} if no config file
 	 *         shall be used
 	 * @since 5.5
 	 */
 	protected File getSshConfig(@NonNull File sshDir) {
 		return new File(sshDir, SshConstants.CONFIG);
 	}
 
 	/**
 	 * Obtains a {@link SshConfigStore}, or {@code null} if not SSH config is to
 	 * be used. The default implementation returns {@code null} if
 	 * {@code configFile == null} and otherwise an OpenSSH-compatible store
 	 * reading host entries from the given file.
 	 *
 	 * @param homeDir
 	 *            may be used for ~-replacements by the returned config store
 	 * @param configFile
 	 *            to use, or {@code null} if none
 	 * @param localUserName
 	 *            user name of the current user on the local OS
 	 * @return A {@link SshConfigStore}, or {@code null} if none is to be used
 	 *
 	 * @since 5.8
 	 */
 	protected SshConfigStore createSshConfigStore(@NonNull File homeDir,
 			File configFile, String localUserName) {
 		return configFile == null ? null
 				: new OpenSshConfigFile(homeDir, configFile, localUserName);
 	}
 
 	/**
 	 * Obtains a {@link ServerKeyDatabase} to verify server host keys. The
 	 * default implementation returns a {@link ServerKeyDatabase} that
 	 * recognizes the two openssh standard files {@code ~/.ssh/known_hosts} and
 	 * {@code ~/.ssh/known_hosts2} as well as any files configured via the
 	 * {@code UserKnownHostsFile} option in the ssh config file.
 	 *
 	 * @param homeDir
 	 *            home directory to use for ~ replacement
 	 * @param sshDir
 	 *            representing ~/.ssh/
 	 * @return the {@link ServerKeyDatabase}
 	 * @since 5.5
 	 */
 	@NonNull
 	protected ServerKeyDatabase getServerKeyDatabase(@NonNull File homeDir,
 			@NonNull File sshDir) {
 		return defaultServerKeyDatabase.computeIfAbsent(
 				new Tuple(new Object[] { homeDir, sshDir }),
 				t -> createServerKeyDatabase(homeDir, sshDir));
 
 	}
 
 	/**
 	 * Creates a {@link ServerKeyDatabase} to verify server host keys. The
 	 * default implementation returns a {@link ServerKeyDatabase} that
 	 * recognizes the two openssh standard files {@code ~/.ssh/known_hosts} and
 	 * {@code ~/.ssh/known_hosts2} as well as any files configured via the
 	 * {@code UserKnownHostsFile} option in the ssh config file.
 	 *
 	 * @param homeDir
 	 *            home directory to use for ~ replacement
 	 * @param sshDir
 	 *            representing ~/.ssh/
 	 * @return the {@link ServerKeyDatabase}
 	 * @since 5.8
 	 */
 	@NonNull
 	protected ServerKeyDatabase createServerKeyDatabase(@NonNull File homeDir,
 			@NonNull File sshDir) {
 		return new OpenSshServerKeyDatabase(true,
 				getDefaultKnownHostsFiles(sshDir));
 	}
 
 	/**
 	 * Gets the list of default user known hosts files. The default returns
 	 * ~/.ssh/known_hosts and ~/.ssh/known_hosts2. The ssh config
 	 * {@code UserKnownHostsFile} overrides this default.
 	 *
 	 * @param sshDir
 	 * @return the possibly empty list of default known host file paths.
 	 */
 	@NonNull
 	protected List<Path> getDefaultKnownHostsFiles(@NonNull File sshDir) {
 		return Arrays.asList(sshDir.toPath().resolve(SshConstants.KNOWN_HOSTS),
 				sshDir.toPath().resolve(SshConstants.KNOWN_HOSTS + '2'));
 	}
 
 	/**
 	 * Determines the default keys. The default implementation will lazy load
 	 * the {@link #getDefaultIdentities(File) default identity files}.
 	 * <p>
 	 * Subclasses may override and return an {@link Iterable} of whatever keys
 	 * are appropriate. If the returned iterable lazily loads keys, it should be
 	 * an instance of
 	 * {@link org.apache.sshd.common.keyprovider.AbstractResourceKeyPairProvider
 	 * AbstractResourceKeyPairProvider} so that the session can later pass it
 	 * the {@link #createKeyPasswordProvider(CredentialsProvider) password
 	 * provider} wrapped as a {@link FilePasswordProvider} via
 	 * {@link org.apache.sshd.common.keyprovider.AbstractResourceKeyPairProvider#setPasswordFinder(FilePasswordProvider)
 	 * AbstractResourceKeyPairProvider#setPasswordFinder(FilePasswordProvider)}
 	 * so that encrypted, password-protected keys can be loaded.
 	 * </p>
 	 * <p>
 	 * The default implementation uses exactly this mechanism; class
 	 * {@link CachingKeyPairProvider} may serve as a model for a customized
 	 * lazy-loading {@link Iterable} implementation
 	 * </p>
 	 * <p>
 	 * If the {@link Iterable} returned has the keys already pre-loaded or
 	 * otherwise doesn't need to decrypt encrypted keys, it can be any
 	 * {@link Iterable}, for instance a simple {@link java.util.List List}.
 	 * </p>
 	 *
 	 * @param sshDir
 	 *            to look in for keys
 	 * @return an {@link Iterable} over the default keys
 	 * @since 5.3
 	 */
 	@NonNull
 	protected Iterable<KeyPair> getDefaultKeys(@NonNull File sshDir) {
 		List<Path> defaultIdentities = getDefaultIdentities(sshDir);
 		return defaultKeys.computeIfAbsent(
 				new Tuple(defaultIdentities.toArray(new Path[0])),
 				t -> new CachingKeyPairProvider(defaultIdentities,
 						getKeyCache()));
 	}
 
 	/**
 	 * Converts an {@link Iterable} of {link KeyPair}s into a
 	 * {@link KeyIdentityProvider}.
 	 *
 	 * @param keys
 	 *            to provide via the returned {@link KeyIdentityProvider}
 	 * @return a {@link KeyIdentityProvider} that provides the given
 	 *         {@code keys}
 	 */
 	private KeyIdentityProvider toKeyIdentityProvider(Iterable<KeyPair> keys) {
 		if (keys instanceof KeyIdentityProvider) {
 			return (KeyIdentityProvider) keys;
 		}
 		return (session) -> keys;
 	}
 
 	/**
 	 * Gets a list of default identities, i.e., private key files that shall
 	 * always be tried for public key authentication. Typically those are
 	 * ~/.ssh/id_dsa, ~/.ssh/id_rsa, and so on. The default implementation
 	 * returns the files defined in {@link SshConstants#DEFAULT_IDENTITIES}.
 	 *
 	 * @param sshDir
 	 *            the directory that represents ~/.ssh/
 	 * @return a possibly empty list of paths containing default identities
 	 *         (private keys)
 	 */
 	@NonNull
 	protected List<Path> getDefaultIdentities(@NonNull File sshDir) {
 		return Arrays
 				.asList(SshConstants.DEFAULT_IDENTITIES).stream()
 				.map(s -> new File(sshDir, s).toPath()).filter(Files::exists)
 				.collect(Collectors.toList());
 	}
 
 	/**
 	 * Obtains the {@link KeyCache} to use to cache loaded keys.
 	 *
 	 * @return the {@link KeyCache}, or {@code null} if none.
 	 */
 	protected final KeyCache getKeyCache() {
 		return keyCache;
 	}
 
 	/**
 	 * Creates a {@link KeyPasswordProvider} for a new session.
 	 *
 	 * @param provider
 	 *            the {@link CredentialsProvider} to delegate to for user
 	 *            interactions
 	 * @return a new {@link KeyPasswordProvider}
 	 */
 	@NonNull
 	protected KeyPasswordProvider createKeyPasswordProvider(
 			CredentialsProvider provider) {
 		return new IdentityPasswordProvider(provider);
 	}
 
 	/**
 	 * Creates a {@link FilePasswordProvider} for a new session.
 	 *
 	 * @param provider
 	 *            the {@link KeyPasswordProvider} to delegate to
 	 * @return a new {@link FilePasswordProvider}
 	 */
 	@NonNull
 	private FilePasswordProvider createFilePasswordProvider(
 			KeyPasswordProvider provider) {
 		return new PasswordProviderWrapper(provider);
 	}
 
 	/**
 	 * Gets the user authentication mechanisms (or rather, factories for them).
 	 * By default this returns gssapi-with-mic, public-key, password, and
 	 * keyboard-interactive, in that order. The order is only significant if the
 	 * ssh config does <em>not</em> set {@code PreferredAuthentications}; if it
 	 * is set, the order defined there will be taken.
 	 *
 	 * @return the non-empty list of factories.
 	 */
 	@NonNull
 	private List<UserAuthFactory> getUserAuthFactories() {
 		// About the order of password and keyboard-interactive, see upstream
 		// bug https://issues.apache.org/jira/projects/SSHD/issues/SSHD-866 .
 		// Password auth doesn't have this problem.
 		return Collections.unmodifiableList(
 				Arrays.asList(GssApiWithMicAuthFactory.INSTANCE,
 						UserAuthPublicKeyFactory.INSTANCE,
 						JGitPasswordAuthFactory.INSTANCE,
 						UserAuthKeyboardInteractiveFactory.INSTANCE));
 	}
 
 	/**
 	 * Gets the list of default preferred authentication mechanisms. If
 	 * {@code null} is returned the openssh default list will be in effect. If
 	 * the ssh config defines {@code PreferredAuthentications} the value from
 	 * the ssh config takes precedence.
 	 *
 	 * @return a comma-separated list of mechanism names, or {@code null} if
 	 *         none
 	 */
 	protected String getDefaultPreferredAuthentications() {
 		return null;
 	}
 }