server/hosts/servo_host.py

# Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
#
# Expects to be run in an environment with sudo and no interactive password
# prompt, such as within the Chromium OS development chroot.


"""This file provides core logic for servo verify/repair process."""


import httplib
import logging
import socket
import xmlrpclib
import os

from autotest_lib.client.bin import utils
from autotest_lib.client.common_lib import error
from autotest_lib.client.common_lib import global_config
from autotest_lib.client.common_lib import hosts
from autotest_lib.client.common_lib import lsbrelease_utils
from autotest_lib.client.common_lib.cros import dev_server
from autotest_lib.client.common_lib.cros import retry
from autotest_lib.client.common_lib.cros.network import ping_runner
from autotest_lib.client.cros import constants as client_constants
from autotest_lib.server import afe_utils
from autotest_lib.server import site_utils as server_utils
from autotest_lib.server.cros import autoupdater
from autotest_lib.server.cros.dynamic_suite import frontend_wrappers
from autotest_lib.server.cros.servo import servo
from autotest_lib.server.hosts import servo_repair
from autotest_lib.server.hosts import ssh_host
from autotest_lib.site_utils.rpm_control_system import rpm_client

try:
    from chromite.lib import metrics
except ImportError:
    metrics = utils.metrics_mock


# Names of the host attributes in the database that represent the values for
# the servo_host and servo_port for a servo connected to the DUT.
SERVO_HOST_ATTR = 'servo_host'
SERVO_PORT_ATTR = 'servo_port'
SERVO_BOARD_ATTR = 'servo_board'
# Model is inferred from host labels.
SERVO_MODEL_ATTR = 'servo_model'
SERVO_SERIAL_ATTR = 'servo_serial'
SERVO_ATTR_KEYS = (
        SERVO_BOARD_ATTR,
        SERVO_HOST_ATTR,
        SERVO_PORT_ATTR,
        SERVO_SERIAL_ATTR,
)

_CONFIG = global_config.global_config
ENABLE_SSH_TUNNEL_FOR_SERVO = _CONFIG.get_config_value(
        'CROS', 'enable_ssh_tunnel_for_servo', type=bool, default=False)

AUTOTEST_BASE = _CONFIG.get_config_value(
        'SCHEDULER', 'drone_installation_directory',
        default='/usr/local/autotest')


class ServoHost(ssh_host.SSHHost):
    """Host class for a host that controls a servo, e.g. beaglebone."""

    DEFAULT_PORT = int(os.getenv('SERVOD_PORT', '9999'))

    # Timeout for initializing servo signals.
    INITIALIZE_SERVO_TIMEOUT_SECS = 60

    # Ready test function
    SERVO_READY_METHOD = 'get_version'

    REBOOT_CMD = 'sleep 1; reboot & sleep 10; reboot -f'


    def _initialize(self, servo_host='localhost',
                    servo_port=DEFAULT_PORT, servo_board=None,
                    servo_model=None, servo_serial=None, is_in_lab=None,
                    *args, **dargs):
        """Initialize a ServoHost instance.

        A ServoHost instance represents a host that controls a servo.

        @param servo_host: Name of the host where the servod process
                           is running.
        @param servo_port: Port the servod process is listening on. Defaults
                           to the SERVOD_PORT environment variable if set,
                           otherwise 9999.
        @param servo_board: Board that the servo is connected to.
        @param servo_model: Model that the servo is connected to.
        @param is_in_lab: True if the servo host is in Cros Lab. Default is set
                          to None, for which utils.host_is_in_lab_zone will be
                          called to check if the servo host is in Cros lab.

        """
        super(ServoHost, self)._initialize(hostname=servo_host,
                                           *args, **dargs)
        self.servo_port = int(servo_port)
        self.servo_board = servo_board
        self.servo_model = servo_model
        self.servo_serial = servo_serial
        self._servo = None
        self._repair_strategy = (
                servo_repair.create_servo_repair_strategy())
        self._is_localhost = (self.hostname == 'localhost')
        if self._is_localhost:
            self._is_in_lab = False
        elif is_in_lab is None:
            self._is_in_lab = utils.host_is_in_lab_zone(self.hostname)
        else:
            self._is_in_lab = is_in_lab

        # Commands on the servo host must be run by the superuser.
        # Our account on a remote host is root, but if our target is
        # localhost then we might be running unprivileged.  If so,
        # `sudo` will have to be added to the commands.
        if self._is_localhost:
            self._sudo_required = utils.system_output('id -u') != '0'
        else:
            self._sudo_required = False


    def connect_servo(self):
        """Establish a connection to the servod server on this host.

        Initializes `self._servo` and then verifies that all network
        connections are working.  This will create an ssh tunnel if
        it's required.

        As a side effect of testing the connection, all signals on the
        target servo are reset to default values, and the USB stick is
        set to the neutral (off) position.
        """
        servo_obj = servo.Servo(servo_host=self, servo_serial=self.servo_serial)
        timeout, _ = retry.timeout(
                servo_obj.initialize_dut,
                timeout_sec=self.INITIALIZE_SERVO_TIMEOUT_SECS)
        if timeout:
            raise hosts.AutoservVerifyError(
                    'Servo initialize timed out.')
        self._servo = servo_obj


    def disconnect_servo(self):
        """Disconnect our servo if it exists.

        If we've previously successfully connected to our servo,
        disconnect any established ssh tunnel, and set `self._servo`
        back to `None`.
        """
        if self._servo:
            # N.B. This call is safe even without a tunnel:
            # rpc_server_tracker.disconnect() silently ignores
            # unknown ports.
            self.rpc_server_tracker.disconnect(self.servo_port)
            self._servo = None


    def is_in_lab(self):
        """Check whether the servo host is a lab device.

        @returns: True if the servo host is in Cros Lab, otherwise False.

        """
        return self._is_in_lab


    def is_localhost(self):
        """Checks whether the servo host points to localhost.

        @returns: True if it points to localhost, otherwise False.

        """
        return self._is_localhost


    def get_servod_server_proxy(self):
        """Return a proxy that can be used to communicate with servod server.

        @returns: An xmlrpclib.ServerProxy that is connected to the servod
                  server on the host.
        """
        if ENABLE_SSH_TUNNEL_FOR_SERVO and not self.is_localhost():
            return self.rpc_server_tracker.xmlrpc_connect(
                    None, self.servo_port,
                    ready_test_name=self.SERVO_READY_METHOD,
                    timeout_seconds=60,
                    request_timeout_seconds=3600)
        else:
            remote = 'http://%s:%s' % (self.hostname, self.servo_port)
            return xmlrpclib.ServerProxy(remote)


    def is_cros_host(self):
        """Check if a servo host is running chromeos.

        @return: True if the servo host is running chromeos.
            False if it isn't, or we don't have enough information.
        """
        try:
            result = self.run('grep -q CHROMEOS /etc/lsb-release',
                              ignore_status=True, timeout=10)
        except (error.AutoservRunError, error.AutoservSSHTimeout):
            return False
        return result.exit_status == 0


    def make_ssh_command(self, user='root', port=22, opts='', hosts_file=None,
                         connect_timeout=None, alive_interval=None,
                         alive_count_max=None, connection_attempts=None):
        """Override default make_ssh_command to use tuned options.

        Tuning changes:
          - ConnectTimeout=30; maximum of 30 seconds allowed for an SSH
          connection failure. Consistency with remote_access.py.

          - ServerAliveInterval=180; which causes SSH to ping connection every
          180 seconds. In conjunction with ServerAliveCountMax ensures
          that if the connection dies, Autotest will bail out quickly.

          - ServerAliveCountMax=3; consistency with remote_access.py.

          - ConnectAttempts=4; reduce flakiness in connection errors;
          consistency with remote_access.py.

          - UserKnownHostsFile=/dev/null; we don't care about the keys.

          - SSH protocol forced to 2; needed for ServerAliveInterval.

        @param user User name to use for the ssh connection.
        @param port Port on the target host to use for ssh connection.
        @param opts Additional options to the ssh command.
        @param hosts_file Ignored.
        @param connect_timeout Ignored.
        @param alive_interval Ignored.
        @param alive_count_max Ignored.
        @param connection_attempts Ignored.

        @returns: An ssh command with the requested settings.

        """
        options = ' '.join([opts, '-o Protocol=2'])
        return super(ServoHost, self).make_ssh_command(
            user=user, port=port, opts=options, hosts_file='/dev/null',
            connect_timeout=30, alive_interval=180, alive_count_max=3,
            connection_attempts=4)


    def _make_scp_cmd(self, sources, dest):
        """Format scp command.

        Given a list of source paths and a destination path, produces the
        appropriate scp command for encoding it. Remote paths must be
        pre-encoded. Overrides _make_scp_cmd in AbstractSSHHost
        to allow additional ssh options.

        @param sources: A list of source paths to copy from.
        @param dest: Destination path to copy to.

        @returns: An scp command that copies |sources| on local machine to
                  |dest| on the remote servo host.

        """
        command = ('scp -rq %s -o BatchMode=yes -o StrictHostKeyChecking=no '
                   '-o UserKnownHostsFile=/dev/null -P %d %s "%s"')
        return command % (self.master_ssh_option,
                          self.port, ' '.join(sources), dest)


    def run(self, command, timeout=3600, ignore_status=False,
            stdout_tee=utils.TEE_TO_LOGS, stderr_tee=utils.TEE_TO_LOGS,
            connect_timeout=30, ssh_failure_retry_ok=False,
            options='', stdin=None, verbose=True, args=()):
        """Run a command on the servo host.

        Extends method `run` in SSHHost. If the servo host is a remote device,
        it will call `run` in SSHost without changing anything.
        If the servo host is 'localhost', it will call utils.system_output.

        @param command: The command line string.
        @param timeout: Time limit in seconds before attempting to
                        kill the running process. The run() function
                        will take a few seconds longer than 'timeout'
                        to complete if it has to kill the process.
        @param ignore_status: Do not raise an exception, no matter
                              what the exit code of the command is.
        @param stdout_tee/stderr_tee: Where to tee the stdout/stderr.
        @param connect_timeout: SSH connection timeout (in seconds)
                                Ignored if host is 'localhost'.
        @param options: String with additional ssh command options
                        Ignored if host is 'localhost'.
        @param ssh_failure_retry_ok: when True and ssh connection failure is
                                     suspected, OK to retry command (but not
                                     compulsory, and likely not needed here)
        @param stdin: Stdin to pass (a string) to the executed command.
        @param verbose: Log the commands.
        @param args: Sequence of strings to pass as arguments to command by
                     quoting them in " and escaping their contents if necessary.

        @returns: A utils.CmdResult object.

        @raises AutoservRunError if the command failed.
        @raises AutoservSSHTimeout SSH connection has timed out. Only applies
                when servo host is not 'localhost'.

        """
        run_args = {'command': command, 'timeout': timeout,
                    'ignore_status': ignore_status, 'stdout_tee': stdout_tee,
                    'stderr_tee': stderr_tee, 'stdin': stdin,
                    'verbose': verbose, 'args': args}
        if self.is_localhost():
            if self._sudo_required:
                run_args['command'] = 'sudo -n sh -c "%s"' % utils.sh_escape(
                        command)
            try:
                return utils.run(**run_args)
            except error.CmdError as e:
                logging.error(e)
                raise error.AutoservRunError('command execution error',
                                             e.result_obj)
        else:
            run_args['connect_timeout'] = connect_timeout
            run_args['options'] = options
            return super(ServoHost, self).run(**run_args)


    def _get_release_version(self):
        """Get the value of attribute CHROMEOS_RELEASE_VERSION from lsb-release.

        @returns The version string in lsb-release, under attribute
                 CHROMEOS_RELEASE_VERSION.
        """
        lsb_release_content = self.run(
                    'cat "%s"' % client_constants.LSB_RELEASE).stdout.strip()
        return lsbrelease_utils.get_chromeos_release_version(
                    lsb_release_content=lsb_release_content)


    def get_attached_duts(self, afe):
        """Gather a list of duts that use this servo host.

        @param afe: afe instance.

        @returns list of duts.
        """
        return afe.get_hosts_by_attribute(
                attribute=SERVO_HOST_ATTR, value=self.hostname)


    def get_board(self):
        """Determine the board for this servo host.

        @returns a string representing this servo host's board.
        """
        return lsbrelease_utils.get_current_board(
                lsb_release_content=self.run('cat /etc/lsb-release').stdout)


    def reboot(self, *args, **dargs):
        """Reboot using special servo host reboot command."""
        super(ServoHost, self).reboot(reboot_cmd=self.REBOOT_CMD,
                                      *args, **dargs)


    def _maybe_reboot_post_upgrade(self, updater):
        """Reboot this servo host if an upgrade is waiting.

        If the host has successfully downloaded and finalized a new
        build, reboot.

        @param updater: a ChromiumOSUpdater instance for checking
            whether reboot is needed.
        """
        if updater.check_update_status() != autoupdater.UPDATER_NEED_REBOOT:
            return

        if self._needs_synchronized_reboot():
            logging.info('Servohost requies synchronized reboot, which is no'
                         ' longer supported. Manually reboot servohost instead.'
                         ' See crbug/848528')
            return

        self._reboot_post_upgrade()


    def _needs_synchronized_reboot(self):
        """Does this servohost need synchronized reboot across multiple DUTs"""
        # TODO(pprabhu) Use HostInfo in this check instead of hitting AFE.
        afe = frontend_wrappers.RetryingAFE(
                timeout_min=5, delay_sec=10,
                server=server_utils.get_global_afe_hostname())
        dut_list = self.get_attached_duts(afe)
        return len(dut_list) > 1


    def _reboot_post_upgrade(self):
        """Reboot this servo host because an upgrade is waiting."""
        logging.info('Rebooting servo host %s from build %s', self.hostname,
                     self._get_release_version())
        # Tell the reboot() call not to wait for completion.
        # Otherwise, the call will log reboot failure if servo does
        # not come back.  The logged reboot failure will lead to
        # test job failure.  If the test does not require servo, we
        # don't want servo failure to fail the test with error:
        # `Host did not return from reboot` in status.log.
        self.reboot(fastsync=True, wait=False)

        # We told the reboot() call not to wait, but we need to wait
        # for the reboot before we continue.  Alas.  The code from
        # here below is basically a copy of Host.wait_for_restart(),
        # with the logging bits ripped out, so that they can't cause
        # the failure logging problem described above.
        #
        # The black stain that this has left on my soul can never be
        # erased.
        old_boot_id = self.get_boot_id()
        if not self.wait_down(timeout=self.WAIT_DOWN_REBOOT_TIMEOUT,
                                warning_timer=self.WAIT_DOWN_REBOOT_WARNING,
                                old_boot_id=old_boot_id):
            raise error.AutoservHostError(
                    'servo host %s failed to shut down.' %
                    self.hostname)
        if self.wait_up(timeout=120):
            logging.info('servo host %s back from reboot, with build %s',
                            self.hostname, self._get_release_version())
        else:
            raise error.AutoservHostError(
                    'servo host %s failed to come back from reboot.' %
                    self.hostname)


    def update_image(self, wait_for_update=False):
        """Update the image on the servo host, if needed.

        This method recognizes the following cases:
          * If the Host is not running Chrome OS, do nothing.
          * If a previously triggered update is now complete, reboot
            to the new version.
          * If the host is processing a previously triggered update,
            do nothing.
          * If the host is running a version of Chrome OS different
            from the default for servo Hosts, trigger an update, but
            don't wait for it to complete.

        @param wait_for_update If an update needs to be applied and
            this is true, then don't return until the update is
            downloaded and finalized, and the host rebooted.
        @raises dev_server.DevServerException: If all the devservers are down.
        @raises site_utils.ParseBuildNameException: If the devserver returns
            an invalid build name.
        @raises AutoservRunError: If the update_engine_client isn't present on
            the host, and the host is a cros_host.

        """
        # servod could be running in a Ubuntu workstation.
        if not self.is_cros_host():
            logging.info('Not attempting an update, either %s is not running '
                         'chromeos or we cannot find enough information about '
                         'the host.', self.hostname)
            return

        if lsbrelease_utils.is_moblab():
            logging.info('Not attempting an update, %s is running moblab.',
                         self.hostname)
            return

        target_build = afe_utils.get_stable_cros_image_name(self.get_board())
        target_build_number = server_utils.ParseBuildName(
                target_build)[3]
        # For servo image staging, we want it as more widely distributed as
        # possible, so that devservers' load can be evenly distributed. So use
        # hostname instead of target_build as hash.
        ds = dev_server.ImageServer.resolve(self.hostname,
                                            hostname=self.hostname)
        url = ds.get_update_url(target_build)

        updater = autoupdater.ChromiumOSUpdater(update_url=url, host=self)
        self._maybe_reboot_post_upgrade(updater)
        current_build_number = self._get_release_version()
        status = updater.check_update_status()
        update_pending = True
        if status in autoupdater.UPDATER_PROCESSING_UPDATE:
            logging.info('servo host %s already processing an update, update '
                         'engine client status=%s', self.hostname, status)
        elif status == autoupdater.UPDATER_NEED_REBOOT:
            return
        elif current_build_number != target_build_number:
            logging.info('Using devserver url: %s to trigger update on '
                         'servo host %s, from %s to %s', url, self.hostname,
                         current_build_number, target_build_number)
            try:
                ds.stage_artifacts(target_build,
                                   artifacts=['full_payload'])
            except Exception as e:
                logging.error('Staging artifacts failed: %s', str(e))
                logging.error('Abandoning update for this cycle.')
            else:
                try:
                    updater.trigger_update()
                except autoupdater.RootFSUpdateError as e:
                    trigger_download_status = 'failed with %s' % str(e)
                    metrics.Counter('chromeos/autotest/servo/'
                                    'rootfs_update_failed').increment()
                else:
                    trigger_download_status = 'passed'
                logging.info('Triggered download and update %s for %s, '
                             'update engine currently in status %s',
                             trigger_download_status, self.hostname,
                             updater.check_update_status())
        else:
            logging.info('servo host %s does not require an update.',
                         self.hostname)
            update_pending = False

        if update_pending and wait_for_update:
            logging.info('Waiting for servo update to complete.')
            self.run('update_engine_client --follow', ignore_status=True)


    def verify(self, silent=False):
        """Update the servo host and verify it's in a good state.

        @param silent   If true, suppress logging in `status.log`.
        """
        message = 'Beginning verify for servo host %s port %s serial %s'
        message %= (self.hostname, self.servo_port, self.servo_serial)
        self.record('INFO', None, None, message)
        try:
            self._repair_strategy.verify(self, silent)
        except:
            self.disconnect_servo()
            raise


    def repair(self, silent=False):
        """Attempt to repair servo host.

        @param silent   If true, suppress logging in `status.log`.
        """
        message = 'Beginning repair for servo host %s port %s serial %s'
        message %= (self.hostname, self.servo_port, self.servo_serial)
        self.record('INFO', None, None, message)
        try:
            self._repair_strategy.repair(self, silent)
        except:
            self.disconnect_servo()
            raise


    def has_power(self):
        """Return whether or not the servo host is powered by PoE."""
        # TODO(fdeng): See crbug.com/302791
        # For now, assume all servo hosts in the lab have power.
        return self.is_in_lab()


    def power_cycle(self):
        """Cycle power to this host via PoE if it is a lab device.

        @raises AutoservRepairError if it fails to power cycle the
                servo host.

        """
        if self.has_power():
            try:
                rpm_client.set_power(self, 'CYCLE')
            except (socket.error, xmlrpclib.Error,
                    httplib.BadStatusLine,
                    rpm_client.RemotePowerException) as e:
                raise hosts.AutoservRepairError(
                        'Power cycling %s failed: %s' % (self.hostname, e),
                        'power_cycle_via_rpm_failed'
                )
        else:
            logging.info('Skipping power cycling, not a lab device.')


    def get_servo(self):
        """Get the cached servo.Servo object.

        @return: a servo.Servo object.
        """
        return self._servo


    def close(self):
        """Stop UART logging and close the host object."""
        if self._servo:
            # In some cases when we run as lab-tools, the job object is None.
            if self.job:
                self._servo.dump_uart_streams(self.job.resultdir)
            self._servo.close()

        super(ServoHost, self).close()


def make_servo_hostname(dut_hostname):
    """Given a DUT's hostname, return the hostname of its servo.

    @param dut_hostname: hostname of a DUT.

    @return hostname of the DUT's servo.

    """
    host_parts = dut_hostname.split('.')
    host_parts[0] = host_parts[0] + '-servo'
    return '.'.join(host_parts)


def servo_host_is_up(servo_hostname):
    """Given a servo host name, return if it's up or not.

    @param servo_hostname: hostname of the servo host.

    @return True if it's up, False otherwise
    """
    # Technically, this duplicates the SSH ping done early in the servo
    # proxy initialization code.  However, this ping ends in a couple
    # seconds when if fails, rather than the 60 seconds it takes to decide
    # that an SSH ping has timed out.  Specifically, that timeout happens
    # when our servo DNS name resolves, but there is no host at that IP.
    logging.info('Pinging servo host at %s', servo_hostname)
    ping_config = ping_runner.PingConfig(
            servo_hostname, count=3,
            ignore_result=True, ignore_status=True)
    return ping_runner.PingRunner().ping(ping_config).received > 0


def _map_afe_board_to_servo_board(afe_board):
    """Map a board we get from the AFE to a servo appropriate value.

    Many boards are identical to other boards for servo's purposes.
    This function makes that mapping.

    @param afe_board string board name received from AFE.
    @return board we expect servo to have.

    """
    KNOWN_SUFFIXES = ['-freon', '_freon', '_moblab', '-cheets']
    BOARD_MAP = {'gizmo': 'panther'}
    mapped_board = afe_board
    if afe_board in BOARD_MAP:
        mapped_board = BOARD_MAP[afe_board]
    else:
        for suffix in KNOWN_SUFFIXES:
            if afe_board.endswith(suffix):
                mapped_board = afe_board[0:-len(suffix)]
                break
    if mapped_board != afe_board:
        logging.info('Mapping AFE board=%s to %s', afe_board, mapped_board)
    return mapped_board


def get_servo_args_for_host(dut_host):
    """Return servo data associated with a given DUT.

    @param dut_host   Instance of `Host` on which to find the servo
                      attributes.
    @return `servo_args` dict with host and an optional port.
    """
    info = dut_host.host_info_store.get()
    servo_args = {k: v for k, v in info.attributes.iteritems()
                  if k in SERVO_ATTR_KEYS}

    if SERVO_PORT_ATTR in servo_args:
        try:
            servo_args[SERVO_PORT_ATTR] = int(servo_args[SERVO_PORT_ATTR])
        except ValueError:
            logging.error('servo port is not an int: %s',
                          servo_args[SERVO_PORT_ATTR])
            # Reset servo_args because we don't want to use an invalid port.
            servo_args.pop(SERVO_HOST_ATTR, None)

    if info.board:
        servo_args[SERVO_BOARD_ATTR] = _map_afe_board_to_servo_board(info.board)
    if info.model:
        servo_args[SERVO_MODEL_ATTR] = info.model
    return servo_args if SERVO_HOST_ATTR in servo_args else None


def _tweak_args_for_ssp_moblab(servo_args):
    if servo_args[SERVO_HOST_ATTR] in ['localhost', '127.0.0.1']:
        servo_args[SERVO_HOST_ATTR] = _CONFIG.get_config_value(
                'SSP', 'host_container_ip', type=str, default=None)


def create_servo_host(dut, servo_args, try_lab_servo=False,
                      try_servo_repair=False):
    """Create a ServoHost object for a given DUT, if appropriate.

    This function attempts to create and verify or repair a `ServoHost`
    object for a servo connected to the given `dut`, subject to various
    constraints imposed by the parameters:
      * When the `servo_args` parameter is not `None`, a servo
        host must be created, and must be checked with `repair()`.
      * Otherwise, if a servo exists in the lab and `try_lab_servo` is
        true:
          * If `try_servo_repair` is true, then create a servo host and
            check it with `repair()`.
          * Otherwise, if the servo responds to `ping` then create a
            servo host and check it with `verify()`.

    In cases where `servo_args` was not `None`, repair failure
    exceptions are passed back to the caller; otherwise, exceptions
    are logged and then discarded.  Note that this only happens in cases
    where we're called from a test (not special task) control file that
    has an explicit dependency on servo.  In that case, we require that
    repair not write to `status.log`, so as to avoid polluting test
    results.

    TODO(jrbarnette):  The special handling for servo in test control
    files is a thorn in my flesh; I dearly hope to see it cut out before
    my retirement.

    Parameters for a servo host consist of a host name, port number, and
    DUT board, and are determined from one of these sources, in order of
    priority:
      * Servo attributes from the `dut` parameter take precedence over
        all other sources of information.
      * If a DNS entry for the servo based on the DUT hostname exists in
        the CrOS lab network, that hostname is used with the default
        port and the DUT's board.
      * If no other options are found, the parameters will be taken
        from the `servo_args` dict passed in from the caller.

    @param dut            An instance of `Host` from which to take
                          servo parameters (if available).
    @param servo_args     A dictionary with servo parameters to use if
                          they can't be found from `dut`.  If this
                          argument is supplied, unrepaired exceptions
                          from `verify()` will be passed back to the
                          caller.
    @param try_lab_servo  If not true, servo host creation will be
                          skipped unless otherwise required by the
                          caller.
    @param try_servo_repair  If true, check a servo host with
                          `repair()` instead of `verify()`.

    @returns: A ServoHost object or None. See comments above.

    """
    servo_dependency = servo_args is not None
    if dut is not None and (try_lab_servo or servo_dependency):
        servo_args_override = get_servo_args_for_host(dut)
        if servo_args_override is not None:
            if utils.in_moblab_ssp():
                _tweak_args_for_ssp_moblab(servo_args_override)
            logging.debug(
                    'Overriding provided servo_args (%s) with arguments'
                    ' determined from the host (%s)',
                    servo_args,
                    servo_args_override,
            )
            servo_args = servo_args_override

    if servo_args is None:
        logging.debug('No servo_args provided, and failed to find overrides.')
        return None
    if SERVO_HOST_ATTR not in servo_args:
        logging.debug('%s attribute missing from servo_args: %s',
                      SERVO_HOST_ATTR, servo_args)
        return None
    if (not servo_dependency and not try_servo_repair and
            not servo_host_is_up(servo_args[SERVO_HOST_ATTR])):
        logging.debug('ServoHost is not up.')
        return None

    newhost = ServoHost(
            is_in_lab=(servo_args
                       and server_utils.host_in_lab(
                               servo_args[SERVO_HOST_ATTR])),
            **servo_args
    )
    # Note that the logic of repair() includes everything done
    # by verify().  It's sufficient to call one or the other;
    # we don't need both.
    if servo_dependency:
        newhost.repair(silent=True)
        return newhost

    if try_servo_repair:
        try:
            newhost.repair()
        except Exception:
            logging.exception('servo repair failed for %s', newhost.hostname)
    else:
        try:
            newhost.verify()
        except Exception:
            logging.exception('servo verify failed for %s', newhost.hostname)
    return newhost