/*
 * 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.client.session;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.SocketAddress;
import java.net.SocketTimeoutException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.rmi.RemoteException;
import java.rmi.ServerException;
import java.security.GeneralSecurityException;
import java.security.KeyPair;
import java.security.PublicKey;
import java.time.Duration;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;

import org.apache.sshd.client.ClientAuthenticationManager;
import org.apache.sshd.client.ClientFactoryManager;
import org.apache.sshd.client.auth.password.PasswordIdentityProvider;
import org.apache.sshd.client.channel.ChannelDirectTcpip;
import org.apache.sshd.client.channel.ChannelExec;
import org.apache.sshd.client.channel.ChannelShell;
import org.apache.sshd.client.channel.ChannelSubsystem;
import org.apache.sshd.client.channel.ClientChannel;
import org.apache.sshd.client.channel.ClientChannelEvent;
import org.apache.sshd.client.config.hosts.HostConfigEntry;
import org.apache.sshd.client.future.AuthFuture;
import org.apache.sshd.client.keyverifier.ServerKeyVerifier;
import org.apache.sshd.client.session.forward.DynamicPortForwardingTracker;
import org.apache.sshd.client.session.forward.ExplicitPortForwardingTracker;
import org.apache.sshd.common.AttributeRepository;
import org.apache.sshd.common.channel.PtyChannelConfigurationHolder;
import org.apache.sshd.common.forward.PortForwardingManager;
import org.apache.sshd.common.future.KeyExchangeFuture;
import org.apache.sshd.common.keyprovider.KeyIdentityProvider;
import org.apache.sshd.common.session.Session;
import org.apache.sshd.common.util.io.output.NoCloseOutputStream;
import org.apache.sshd.common.util.io.output.NullOutputStream;
import org.apache.sshd.common.util.net.SshdSocketAddress;

/**
 * <P>
 * An authenticated session to a given SSH server.
 * </P>
 *
 * <P>
 * A client session is established using the {@link org.apache.sshd.client.SshClient}. Once the session has been
 * created, the user has to authenticate using either {@link #addPasswordIdentity(String)} or
 * {@link #addPublicKeyIdentity(java.security.KeyPair)} followed by a call to {@link #auth()}.
 * </P>
 *
 * <P>
 * From this session, channels can be created using the {@link #createChannel(String)} method. Multiple channels can be
 * created on a given session concurrently.
 * </P>
 *
 * <P>
 * When using the client in an interactive mode, the {@link #waitFor(Collection, long)} method can be used to listen to
 * specific events such as the session being established, authenticated or closed.
 * </P>
 *
 * When a given session is no longer used, it must be closed using the {@link #close(boolean)} method.
 *
 * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
 */
public interface ClientSession
        extends Session,
        ClientAuthenticationManager, PortForwardingManager {
    enum ClientSessionEvent {
        TIMEOUT,
        CLOSED,
        WAIT_AUTH,
        AUTHED
    }

    Set<ClientChannelEvent> REMOTE_COMMAND_WAIT_EVENTS = Collections.unmodifiableSet(EnumSet.of(ClientChannelEvent.CLOSED));

    /**
     * Returns the original address (after having been translated through host configuration entries if any) that was
     * request to connect. It contains the original host or address string that was used. <B>Note:</B> this may be
     * different than the result of the {@link #getIoSession()} report of the remote peer
     *
     * @return The original requested address
     */
    SocketAddress getConnectAddress();

    /**
     * Retrieves the {@link HostConfigEntry} that was used to create this session.
     *
     * @return the {@link HostConfigEntry}; or {@code null} if none
     */
    HostConfigEntry getHostConfigEntry();

    /**
     * @return The &quot;context&quot; data provided when session connection was established - {@code null} if none.
     */
    AttributeRepository getConnectionContext();

    /**
     * Starts the authentication process. User identities will be tried until the server successfully authenticate the
     * user. User identities must be provided before calling this method using {@link #addPasswordIdentity(String)} or
     * {@link #addPublicKeyIdentity(java.security.KeyPair)}.
     *
     * @return             the authentication future
     * @throws IOException if failed to generate the future
     * @see                #addPasswordIdentity(String)
     * @see                #addPublicKeyIdentity(java.security.KeyPair)
     */
    AuthFuture auth() throws IOException;

    /**
     * Retrieves the server's host key: the one it presented in the last key exchange.
     *
     * @return the server's {@link PublicKey}; may be {@code null} if no key exchange has been completed yet
     */
    PublicKey getServerKey();

    /**
     * Explicitly registers a known host key for the session. Intended to be used if the server announces additional
     * host keys as part of the OpenSSH "hostkeys-00@openssh.com" extension, but it can also be used to register any
     * host key at any other time.
     * <p>
     * If the server presents a host key that matches any of the ones registered in the session, the key will be
     * accepted implicitly without consulting the {@link ServerKeyVerifier}.
     * </p>
     * <p>
     * If the {@code hostKey} is a host certificate, the CA key will be registered. When the server presents a
     * certificate, the certificate will be checked in any case and will be rejected if it is invalid or expired. If it
     * is valid and not expired, it will be accepted implicitly if its CA key is already registered.
     * </p>
     *
     * @param  hostKey                  the host key to register
     * @throws IllegalArgumentException if {@code hostKey} is a user certificate
     */
    void registerHostKey(PublicKey hostKey);

    /**
     * Obtains a read-only collection of all currently registered server host keys is OpenSSH format, i.e. key type as a
     * string, followed by the base64-encoded key, prefixed with "@cert-authority" if it's a CA key.
     * <p>
     * For a normal key: "key-type AAAA..."; for a host certificate CA key "@cert-authority key-type AAAA...".
     * </p>
     * <p>
     * Note that this is not <em>quite</em> the format used in a OpenSSH {@code known_hosts} file as there are no host
     * patterns.
     * </p>
     *
     * @return all currently registered server host keys; never {@code null} but possibly empty, and never containing
     *         {@code null}
     */
    Collection<String> getRegisteredHostKeys();

    /**
     * Create a channel of the given type. Same as calling <code>createChannel(type, null)</code>.
     *
     * @param  type        The channel type
     * @return             The created {@link ClientChannel}
     * @throws IOException If failed to create the requested channel
     */
    ClientChannel createChannel(String type) throws IOException;

    /**
     * Create a channel of the given type and sub-type.
     *
     * @param  type        The channel type
     * @param  subType     The channel sub-type
     * @return             The created {@link ClientChannel}
     * @throws IOException If failed to create the requested channel
     */
    ClientChannel createChannel(String type, String subType) throws IOException;

    /**
     * Create a channel to start a shell using default PTY settings and environment.
     *
     * @return             The created {@link ChannelShell}
     * @throws IOException If failed to create the requested channel
     */
    default ChannelShell createShellChannel() throws IOException {
        return createShellChannel(null, Collections.emptyMap());
    }

    /**
     * Create a channel to start a shell using specific PTY settings and/or environment.
     *
     * @param  ptyConfig   The PTY configuration to use - if {@code null} then internal defaults are used
     * @param  env         Extra environment configuration to be transmitted to the server - ignored if
     *                     {@code null}/empty.
     * @return             The created {@link ChannelShell}
     * @throws IOException If failed to create the requested channel
     */
    ChannelShell createShellChannel(
            PtyChannelConfigurationHolder ptyConfig, Map<String, ?> env)
            throws IOException;

    /**
     * Create a channel to execute a command using default PTY settings and environment.
     *
     * @param  command     The command to execute
     * @return             The created {@link ChannelExec}
     * @throws IOException If failed to create the requested channel
     */
    default ChannelExec createExecChannel(String command) throws IOException {
        return createExecChannel(command, null, Collections.emptyMap());
    }

    /**
     * Create a channel to execute a command using specific PTY settings and/or environment.
     *
     * @param  command     The command to execute
     * @param  ptyConfig   The PTY configuration to use - if {@code null} then internal defaults are used
     * @param  env         Extra environment configuration to be transmitted to the server - ignored if
     *                     {@code null}/empty.
     * @return             The created {@link ChannelExec}
     * @throws IOException If failed to create the requested channel
     */
    default ChannelExec createExecChannel(
            String command, PtyChannelConfigurationHolder ptyConfig, Map<String, ?> env)
            throws IOException {
        return createExecChannel(command, StandardCharsets.UTF_8, ptyConfig, env);
    }

    /**
     * Create a channel to execute a command using specific PTY settings and/or environment.
     *
     * @param  command     The command to execute
     * @param  charset     The {@link Charset} to use for sending the command string
     * @param  ptyConfig   The PTY configuration to use - if {@code null} then internal defaults are used
     * @param  env         Extra environment configuration to be transmitted to the server - ignored if
     *                     {@code null}/empty.
     * @return             The created {@link ChannelExec}
     * @throws IOException If failed to create the requested channel
     */
    ChannelExec createExecChannel(String command, Charset charset, PtyChannelConfigurationHolder ptyConfig, Map<String, ?> env)
            throws IOException;

    /**
     * Create a channel to execute a command using specific PTY settings and/or environment.
     *
     * @param  command     The command to execute
     * @param  ptyConfig   The PTY configuration to use - if {@code null} then internal defaults are used
     * @param  env         Extra environment configuration to be transmitted to the server - ignored if
     *                     {@code null}/empty.
     * @return             The created {@link ChannelExec}
     * @throws IOException If failed to create the requested channel
     */
    ChannelExec createExecChannel(byte[] command, PtyChannelConfigurationHolder ptyConfig, Map<String, ?> env)
            throws IOException;

    /**
     * Execute a command that requires no input and returns its output
     *
     * @param  command     The command to execute
     * @return             The command's standard output result (assumed to be in US-ASCII)
     * @throws IOException If failed to execute the command - including if <U>anything</U> was written to the standard
     *                     error or a non-zero exit status was received. If this happens, then a {@link RemoteException}
     *                     is thrown with a cause of {@link ServerException} containing the remote captured standard
     *                     error - including CR/LF(s)
     * @see                #executeRemoteCommand(String, Duration)
     */
    default String executeRemoteCommand(String command) throws IOException {
        return executeRemoteCommand(command, Duration.ZERO);
    }

    /**
     * Execute a command that requires no input and returns its output
     *
     * @param  command     The command to execute
     * @param  timeout     Timeout for the remote command execution. Applies to both channel opening and result waiting.
     *                     A zero or negative value means no timeout.
     * @return             The command's standard output result
     * @return             The command's standard output result (assumed to be in US-ASCII)
     * @throws IOException If failed to execute the command - including if <U>anything</U> was written to the standard
     *                     error or a non-zero exit status was received. If this happens, then a {@link RemoteException}
     *                     is thrown with a cause of {@link ServerException} containing the remote captured standard
     *                     error - including CR/LF(s)
     * @see                #executeRemoteCommand(String, OutputStream, Charset)
     * @see                #executeRemoteCommand(String, OutputStream, Charset, Duration)
     */
    default String executeRemoteCommand(String command, Duration timeout) throws IOException {
        try (ByteArrayOutputStream stderr = new ByteArrayOutputStream()) {
            try {
                return executeRemoteCommand(command, stderr, StandardCharsets.US_ASCII, timeout);
            } finally {
                if (stderr.size() > 0) {
                    String errorMessage = stderr.toString(StandardCharsets.US_ASCII.name());
                    throw new RemoteException("Error reported from remote command=" + command,
                            new ServerException(errorMessage));
                }
            }
        }
    }

    /**
     * Execute a command that requires no input and returns its output
     *
     * @param  command     The command to execute - without a terminating LF
     * @param  stderr      Standard error output stream - if {@code null} then error stream data is ignored.
     *                     <B>Note:</B> if the stream is not {@code null} then it will be left <U>open</U> when this
     *                     method returns or exception is thrown
     * @param  charset     The command {@link Charset} for input/output/error - if {@code null} then US_ASCII is assumed
     * @return             The command's standard output result
     * @throws IOException If failed to manage the command channel - <B>Note:</B> the code does not check if anything
     *                     was output to the standard error stream, but does check the reported exit status (if any) for
     *                     non-zero value. If non-zero exit status received then a {@link RemoteException} is thrown
     *                     with' a {@link ServerException} cause containing the exits value
     * @see                #executeRemoteCommand(String, OutputStream, OutputStream, Charset, Duration)
     */
    default String executeRemoteCommand(String command, OutputStream stderr, Charset charset) throws IOException {
        return executeRemoteCommand(command, stderr, charset, Duration.ZERO);
    }

    /**
     * Execute a command that requires no input and returns its output
     *
     * @param  command     The command to execute - without a terminating LF
     * @param  stderr      Standard error output stream - if {@code null} then error stream data is ignored.
     *                     <B>Note:</B> if the stream is not {@code null} then it will be left <U>open</U> when this
     *                     method returns or exception is thrown
     * @param  charset     The command {@link Charset} for input/output/error - if {@code null} then US_ASCII is assumed
     * @param  timeout     Timeout for the remote command execution. Applies to both channel opening and result waiting.
     *                     A zero or negative value means no timeout.
     * @return             The command's standard output result
     * @throws IOException If failed to manage the command channel - <B>Note:</B> the code does not check if anything
     *                     was output to the standard error stream, but does check the reported exit status (if any) for
     *                     non-zero value. If non-zero exit status received then a {@link RemoteException} is thrown
     *                     with' a {@link ServerException} cause containing the exits value
     * @see                #executeRemoteCommand(String, OutputStream, OutputStream, Charset, Duration)
     */
    default String executeRemoteCommand(String command, OutputStream stderr, Charset charset, Duration timeout)
            throws IOException {
        if (charset == null) {
            charset = StandardCharsets.US_ASCII;
        }

        try (ByteArrayOutputStream stdout = new ByteArrayOutputStream(Byte.MAX_VALUE)) {
            executeRemoteCommand(command, stdout, stderr, charset, timeout);
            byte[] outBytes = stdout.toByteArray();
            return new String(outBytes, charset);
        }
    }

    /**
     * Execute a command that requires no input and redirects its STDOUT/STDERR streams to the user-provided ones
     *
     * @param  command     The command to execute - without a terminating LF
     * @param  stdout      Standard output stream - if {@code null} then stream data is ignored. <B>Note:</B> if the
     *                     stream is not {@code null} then it will be left <U>open</U> when this method returns or
     *                     exception is thrown
     * @param  stderr      Error output stream - if {@code null} then stream data is ignored. <B>Note:</B> if the stream
     *                     is not {@code null} then it will be left <U>open</U> when this method returns or exception is
     *                     thrown
     * @param  charset     The command {@link Charset} for output/error - if {@code null} then US_ASCII is assumed
     * @throws IOException If failed to execute the command or got a non-zero exit status
     * @see                #executeRemoteCommand(String, OutputStream, OutputStream, Charset, Duration)
     */
    default void executeRemoteCommand(
            String command, OutputStream stdout, OutputStream stderr, Charset charset)
            throws IOException {
        executeRemoteCommand(command, stdout, stderr, charset, Duration.ZERO);
    }

    /**
     * Execute a command that requires no input and redirects its STDOUT/STDERR streams to the user-provided ones
     *
     * @param  command     The command to execute - without a terminating LF.
     * @param  stdout      Standard output stream - if {@code null} then stream data is ignored. <b>Note:</b> if the
     *                     stream is not {@code null}, it will be left <u>open</u> when this method returns or an
     *                     exception is thrown.
     * @param  stderr      Error output stream - if {@code null} then error stream data is ignored. <b>Note:</b> if the
     *                     stream is not {@code null}, it will be left <u>open</u> when this method returns or an
     *                     exception is thrown.
     * @param  charset     The charset to use for encoding the command and decoding the output/error streams. If
     *                     {@code null}, US-ASCII is assumed.
     * @param  timeout     Timeout for the remote command execution. Applies to both channel opening and result waiting.
     *                     A zero or negative value means no timeout.
     * @throws IOException If the command execution fails, times out, or returns a non-zero exit code. A
     *                     {@link RemoteException} may be thrown if the remote side reports an error.
     * @see                ClientChannel#open()#verify(long, java.util.concurrent.TimeUnit)
     * @see                ClientChannel#waitFor(Collection, long)
     * @see                ClientChannel#validateCommandExitStatusCode(String, Integer) validateCommandExitStatusCode
     */
    default void executeRemoteCommand(
            String command, OutputStream stdout, OutputStream stderr, Charset charset, Duration timeout)
            throws IOException {

        if (charset == null) {
            charset = StandardCharsets.US_ASCII;
        }

        if (timeout != null && timeout.isNegative()) {
            throw new IllegalArgumentException("Timeout must be non-negative");
        }

        try (OutputStream channelErr = (stderr == null) ? new NullOutputStream() : new NoCloseOutputStream(stderr);
             OutputStream channelOut = (stdout == null) ? new NullOutputStream() : new NoCloseOutputStream(stdout);
             ClientChannel channel = createExecChannel(command, charset, null, Collections.emptyMap())) {

            channel.setOut(channelOut);
            channel.setErr(channelErr);

            long waitTimeoutMillis;
            if (timeout != null && !timeout.isZero()) {
                long timeoutMillis = timeout.toMillis();
                long startTime = System.currentTimeMillis();
                channel.open().verify(timeoutMillis, TimeUnit.MILLISECONDS);

                long elapsed = System.currentTimeMillis() - startTime;
                waitTimeoutMillis = Math.max(1, timeoutMillis - elapsed);
            } else {
                channel.open().verify(); // wait indefinitely
                waitTimeoutMillis = 0L;
            }

            Collection<ClientChannelEvent> waitMask = channel.waitFor(REMOTE_COMMAND_WAIT_EVENTS, waitTimeoutMillis);
            if (waitMask.contains(ClientChannelEvent.TIMEOUT)) {
                throw new SocketTimeoutException(String.format(
                        "Failed to retrieve command '%s' result within timeout of %s ms", command, timeout));
            }

            Integer exitStatus = channel.getExitStatus();
            ClientChannel.validateCommandExitStatusCode(command, exitStatus);
        }
    }

    /**
     * Create a subsystem channel.
     *
     * @param  subsystem   The subsystem name
     * @return             The created {@link ChannelSubsystem}
     * @throws IOException If failed to create the requested channel
     */
    ChannelSubsystem createSubsystemChannel(String subsystem) throws IOException;

    /**
     * Create a direct tcp-ip channel which can be used to stream data to a remote port from the server.
     *
     * @param  local       The local address
     * @param  remote      The remote address
     * @return             The created {@link ChannelDirectTcpip}
     * @throws IOException If failed to create the requested channel
     */
    ChannelDirectTcpip createDirectTcpipChannel(
            SshdSocketAddress local, SshdSocketAddress remote)
            throws IOException;

    /**
     * Starts a local port forwarding and returns a tracker that stops the forwarding when the {@code close()} method is
     * called. This tracker can be used in a {@code try-with-resource} block to ensure cleanup of the set up forwarding.
     *
     * @param  localPort   The local port - if zero one is allocated
     * @param  remote      The remote address
     * @return             The tracker instance
     * @throws IOException If failed to set up the requested forwarding
     * @see                #startLocalPortForwarding(SshdSocketAddress, SshdSocketAddress)
     */
    default ExplicitPortForwardingTracker createLocalPortForwardingTracker(
            int localPort, SshdSocketAddress remote)
            throws IOException {
        return createLocalPortForwardingTracker(new SshdSocketAddress(localPort), remote);
    }

    /**
     * Starts a local port forwarding and returns a tracker that stops the forwarding when the {@code close()} method is
     * called. This tracker can be used in a {@code try-with-resource} block to ensure cleanup of the set up forwarding.
     *
     * @param  local       The local address
     * @param  remote      The remote address
     * @return             The tracker instance
     * @throws IOException If failed to set up the requested forwarding
     * @see                #startLocalPortForwarding(SshdSocketAddress, SshdSocketAddress)
     */
    default ExplicitPortForwardingTracker createLocalPortForwardingTracker(
            SshdSocketAddress local, SshdSocketAddress remote)
            throws IOException {
        return new ExplicitPortForwardingTracker(
                this, true, local, remote, startLocalPortForwarding(local, remote));
    }

    /**
     * Starts a remote port forwarding and returns a tracker that stops the forwarding when the {@code close()} method
     * is called. This tracker can be used in a {@code try-with-resource} block to ensure cleanup of the set up
     * forwarding.
     *
     * @param  remote      The remote address
     * @param  local       The local address
     * @return             The tracker instance
     * @throws IOException If failed to set up the requested forwarding
     * @see                #startRemotePortForwarding(SshdSocketAddress, SshdSocketAddress)
     */
    default ExplicitPortForwardingTracker createRemotePortForwardingTracker(
            SshdSocketAddress remote, SshdSocketAddress local)
            throws IOException {
        return new ExplicitPortForwardingTracker(
                this, false, local, remote, startRemotePortForwarding(remote, local));
    }

    /**
     * Starts a dynamic port forwarding and returns a tracker that stops the forwarding when the {@code close()} method
     * is called. This tracker can be used in a {@code try-with-resource} block to ensure cleanup of the set up
     * forwarding.
     *
     * @param  local       The local address
     * @return             The tracker instance
     * @throws IOException If failed to set up the requested forwarding
     * @see                #startDynamicPortForwarding(SshdSocketAddress)
     */
    default DynamicPortForwardingTracker createDynamicPortForwardingTracker(SshdSocketAddress local) throws IOException {
        return new DynamicPortForwardingTracker(this, local, startDynamicPortForwarding(local));
    }

    /**
     * @return A snapshot of the current session state
     * @see    #waitFor(Collection, long)
     */
    Set<ClientSessionEvent> getSessionState();

    /**
     * Wait for any one of a specific state to be signaled.
     *
     * @param  mask    The request {@link ClientSessionEvent}s mask
     * @param  timeout Wait time in milliseconds - non-positive means forever
     * @return         The actual state that was detected either due to the mask yielding one of the states or due to
     *                 timeout (in which case the {@link ClientSessionEvent#TIMEOUT} value is set)
     */
    Set<ClientSessionEvent> waitFor(Collection<ClientSessionEvent> mask, long timeout);

    /**
     * Wait for any one of a specific state to be signaled.
     *
     * @param  mask    The request {@link ClientSessionEvent}s mask
     * @param  timeout Wait time - null means forever
     * @return         The actual state that was detected either due to the mask yielding one of the states or due to
     *                 timeout (in which case the {@link ClientSessionEvent#TIMEOUT} value is set)
     */
    default Set<ClientSessionEvent> waitFor(Collection<ClientSessionEvent> mask, Duration timeout) {
        return waitFor(mask, timeout != null ? timeout.toMillis() : -1);
    }

    /**
     * Access to the metadata.
     *
     * @return The metadata {@link Map} - <B>Note:</B> access to the map is not {@code synchronized} in any way - up to
     *         the user to take care of mutual exclusion if necessary
     */
    Map<Object, Object> getMetadataMap();

    /**
     * @return The ClientFactoryManager for this session.
     */
    @Override
    ClientFactoryManager getFactoryManager();

    /**
     * <P>
     * Switch to a none cipher for performance.
     * </P>
     *
     * <P>
     * This should be done after the authentication phase has been performed. After such a switch, interactive channels
     * are not allowed anymore. Both client and server must have been configured to support the none cipher. If that's
     * not the case, the returned future will be set with an exception.
     * </P>
     *
     * @return             an {@link KeyExchangeFuture} that can be used to wait for the exchange to be finished
     * @throws IOException if a key exchange is already running
     */
    KeyExchangeFuture switchToNoneCipher() throws IOException;

    /**
     * Creates a &quot;unified&quot; {@link KeyIdentityProvider} of key pairs out of the registered {@link KeyPair}
     * identities and the extra available ones as a single iterator of key pairs
     *
     *
     * @param  session The {@link ClientSession} - ignored if {@code null} (i.e., empty iterator returned)
     * @return         The wrapping KeyIdentityProvider
     * @see            ClientSession#getRegisteredIdentities()
     * @see            ClientSession#getKeyIdentityProvider()
     */
    static KeyIdentityProvider providerOf(ClientSession session) {
        return (session == null)
                ? KeyIdentityProvider.EMPTY_KEYS_PROVIDER
                : KeyIdentityProvider.resolveKeyIdentityProvider(
                        session.getRegisteredIdentities(), session.getKeyIdentityProvider());
    }

    /**
     * Creates a &quot;unified&quot; {@link Iterator} of passwords out of the registered passwords and the extra
     * available ones as a single iterator of passwords
     *
     * @param  session                  The {@link ClientSession} - ignored if {@code null} (i.e., empty iterator
     *                                  returned)
     * @return                          The wrapping iterator
     * @throws IOException              If failed to load the passwords
     * @throws GeneralSecurityException If some security issue with the passwords
     * @see                             ClientSession#getRegisteredIdentities()
     * @see                             ClientSession#getPasswordIdentityProvider()
     */
    static Iterator<String> passwordIteratorOf(ClientSession session)
            throws IOException, GeneralSecurityException {
        return (session == null)
                ? Collections.<String> emptyIterator()
                : PasswordIdentityProvider.iteratorOf(
                        session, session.getRegisteredIdentities(), session.getPasswordIdentityProvider());
    }
}
