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 logging
import os
import re
import tarfile
import time
import traceback
import xmlrpclib

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.cros import retry
from autotest_lib.client.common_lib.cros.network import ping_runner
from autotest_lib.server.cros.servo import servo
from autotest_lib.server.hosts import servo_repair
from autotest_lib.server.hosts import base_servohost


# 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,
)

# Timeout value for stop/start servod process.
SERVOD_TEARDOWN_TIMEOUT = 3
SERVOD_QUICK_STARTUP_TIMEOUT = 20
SERVOD_STARTUP_TIMEOUT = 60

# pools that support dual v4. (go/cros-fw-lab-strategy)
POOLS_SUPPORT_DUAL_V4 = {'faft-cr50',
                         'faft-cr50-experimental',
                         'faft-cr50-tot',
                         'faft-cr50-debug',
                         'faft_cr50_debug'
                         'faft-pd-debug',
                         'faft_pd_debug'}

_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')

SERVO_STATE_LABEL_PREFIX = 'servo_state'
SERVO_STATE_WORKING = 'WORKING'
SERVO_STATE_BROKEN = 'BROKEN'


class ServoHost(base_servohost.BaseServoHost):
    """Host class for a servo host(e.g. beaglebone, labstation)
     that with a servo instance for a specific port.

     @type _servo: servo.Servo | None
     """

    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'

    # Directory prefix on the servo host where the servod logs are stored.
    SERVOD_LOG_PREFIX = '/var/log/servod'

    # Exit code to use when symlinks for servod logs are not found.
    NO_SYMLINKS_CODE = 9

    # Directory in the job's results directory to dump the logs into.
    LOG_DIR = 'servod'

    # Prefix for joint loglevel files in the logs.
    JOINT_LOG_PREFIX = 'log'

    # Regex group to extract timestamp from logfile name.
    TS_GROUP = 'ts'

    # This regex is used to extract the timestamp from servod logs.
             # files always start with log.
    TS_RE = (r'log.'
             # The timestamp is of format %Y-%m-%d--%H-%M-%S.MS
             r'(?P<%s>\d{4}(\-\d{2}){2}\-(-\d{2}){3}.\d{3})'
             # The loglevel is optional depending on labstation version.
             r'(.(INFO|DEBUG|WARNING))?' % TS_GROUP)
    TS_EXTRACTOR = re.compile(TS_RE)

    # Regex group to extract MCU name from logline in servod logs.
    MCU_GROUP = 'mcu'

    # Regex group to extract logline from MCU logline in servod logs.
    LINE_GROUP = 'line'

    # This regex is used to extract the mcu and the line content from an
    # MCU logline in servod logs. e.g. EC or servo_v4 console logs.
    # Here is an example log-line:
    #
    # 2020-01-23 13:15:12,223 - servo_v4 - EC3PO.Console - DEBUG -
    # console.py:219:LogConsoleOutput - /dev/pts/9 - cc polarity: cc1
    #
    # Here is conceptually how they are formatted:
    #
    #  <time> - <MCU> - EC3PO.Console - <LVL> - <file:line:func> - <pts> -
    #  <output>
    #
              # The log format starts with a timestamp
    MCU_RE = (r'[\d\-]+ [\d:,]+ '
              # The mcu that is logging this is next.
              r'- (?P<%s>\w+) - '
              # Next, we have more log outputs before the actual line.
              # Information about the file line, logging function etc.
              # Anchor on EC3PO Console, LogConsoleOutput and dev/pts.
              # NOTE: if the log format changes, this regex needs to be
              # adjusted.
              r'EC3PO\.Console[\s\-\w\d:.]+LogConsoleOutput - /dev/pts/\d+ - '
              # Lastly, we get the MCU's console line.
              r'(?P<%s>.+$)' % (MCU_GROUP, LINE_GROUP))
    MCU_EXTRACTOR = re.compile(MCU_RE)

    # Suffix to identify compressed logfiles.
    COMPRESSION_SUFFIX = '.tbz2'

    def _init_attributes(self):
        self._servo_state = None
        self.servo_port = None
        self.servo_board = None
        self.servo_model = None
        self.servo_serial = None
        self._servo = None
        self._servod_server_proxy = None
        # Flag to make sure that multiple calls to close do not result in the
        # logic executing multiple times.
        self._closed = False

    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,
                                           is_in_lab=is_in_lab, *args, **dargs)
        self._init_attributes()
        self.servo_port = int(servo_port)
        self.servo_board = servo_board
        self.servo_model = servo_model
        self.servo_serial = servo_serial

        # The location of the log files on the servo host for this instance.
        self.remote_log_dir = '%s_%s' % (self.SERVOD_LOG_PREFIX,
                                         self.servo_port)
        # Path of the servo host lock file.
        self._lock_file = (self.TEMP_FILE_DIR + str(self.servo_port)
                           + self.LOCK_FILE_POSTFIX)
        # File path to declare a reboot request.
        self._reboot_file = (self.TEMP_FILE_DIR + str(self.servo_port)
                             + self.REBOOT_FILE_POSTFIX)

        # Lock the servo host if it's an in-lab labstation to prevent other
        # task to reboot it until current task completes. We also wait and
        # make sure the labstation is up here, in the case of the labstation is
        # in the middle of reboot.
        self._is_locked = False
        if (self.wait_up(self.REBOOT_TIMEOUT) and self.is_in_lab()
            and self.is_labstation()):
            self._lock()

        self._repair_strategy = (
                servo_repair.create_servo_repair_strategy())

    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)
        self._servo = servo_obj
        timeout, _ = retry.timeout(
                servo_obj.initialize_dut,
                timeout_sec=self.INITIALIZE_SERVO_TIMEOUT_SECS)
        if timeout:
            raise hosts.AutoservVerifyError(
                    'Servo initialize timed out.')


    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 _create_servod_server_proxy(self):
        """Create 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 get_servod_server_proxy(self):
        """Return a cached proxy if exists; otherwise, create a new one.

        @returns: An xmlrpclib.ServerProxy that is connected to the servod
                  server on the host.
        """
        # Single-threaded execution, no race
        if self._servod_server_proxy is None:
            self._servod_server_proxy = self._create_servod_server_proxy()
        return self._servod_server_proxy


    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)
            self._servo_state = SERVO_STATE_WORKING
            self.record('INFO', None, None, 'ServoHost verify set servo_state as WORKING')
        except:
            self._servo_state = SERVO_STATE_BROKEN
            self.record('INFO', None, None, 'ServoHost verify set servo_state as BROKEN')
            self.disconnect_servo()
            self.stop_servod()
            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)
            self._servo_state = SERVO_STATE_WORKING
            self.record('INFO', None, None, 'ServoHost repair set servo_state as WORKING')
            # If target is a labstation then try to withdraw any existing
            # reboot request created by this servo because it passed repair.
            if self.is_labstation():
                self.withdraw_reboot_request()
        except:
            self._servo_state = SERVO_STATE_BROKEN
            self.record('INFO', None, None, 'ServoHost repair set servo_state as BROKEN')
            self.disconnect_servo()
            self.stop_servod()
            raise


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

        @return: a servo.Servo object.
        @rtype: autotest_lib.server.cros.servo.servo.Servo
        """
        return self._servo


    def request_reboot(self):
        """Request servohost to be rebooted when it's safe to by touch a file.
        """
        logging.debug('Request to reboot servohost %s has been created by '
                      'servo with port # %s', self.hostname, self.servo_port)
        self.run('touch %s' % self._reboot_file, ignore_status=True)


    def withdraw_reboot_request(self):
        """Withdraw a servohost reboot request if exists by remove the flag
        file.
        """
        logging.debug('Withdrawing request to reboot servohost %s that created'
                      ' by servo with port # %s if exists.',
                      self.hostname, self.servo_port)
        self.run('rm -f %s' % self._reboot_file, ignore_status=True)


    def start_servod(self, quick_startup=False):
        """Start the servod process on servohost.
        """
        # Skip if running on the localhost.(crbug.com/1038168)
        if self.is_localhost():
            logging.debug("Servohost is a localhost, skipping start servod.")
            return

        cmd = 'start servod'
        if self.servo_board:
            cmd += ' BOARD=%s' % self.servo_board
            if self.servo_model:
                cmd += ' MODEL=%s' % self.servo_model
        else:
            logging.warning('Board for DUT is unknown; starting servod'
                            ' assuming a pre-configured board.')

        cmd += ' PORT=%d' % self.servo_port
        if self.servo_serial:
            cmd += ' SERIAL=%s' % self.servo_serial

        # Start servod with dual_v4 if the DUT/servo from designated pools.
        dut_host_info = self.get_dut_host_info()
        if dut_host_info:
            if bool(dut_host_info.pools & POOLS_SUPPORT_DUAL_V4):
                logging.debug('The DUT is detected in following designated'
                              ' pools %s,starting servod with DUAL_V4 option.',
                              POOLS_SUPPORT_DUAL_V4)
                cmd += ' DUAL_V4=1'

        # Remove the symbolic links from the logs. This helps ensure that
        # a failed servod instantiation does not cause us to grab old logs
        # by mistake.
        self.remove_latest_log_symlinks()
        self.run(cmd, timeout=60)

        # There's a lag between when `start servod` completes and when
        # the _ServodConnectionVerifier trigger can actually succeed.
        # The call to time.sleep() below gives time to make sure that
        # the trigger won't fail after we return.

        # Normally servod on servo_v3 and labstation take ~10 seconds to ready,
        # But in the rare case all servo on a labstation are in heavy use they
        # may take ~30 seconds. So the timeout value will double these value,
        # and we'll try quick start up when first time initialize servohost,
        # and use standard start up timeout in repair.
        if quick_startup:
            timeout = SERVOD_QUICK_STARTUP_TIMEOUT
        else:
            timeout = SERVOD_STARTUP_TIMEOUT
        logging.debug('Wait %s seconds for servod process fully up.', timeout)
        time.sleep(timeout)


    def stop_servod(self):
        """Stop the servod process on servohost.
        """
        # Skip if running on the localhost.(crbug.com/1038168)
        if self.is_localhost():
            logging.debug("Servohost is a localhost, skipping stop servod.")
            return

        logging.debug('Stopping servod on port %s', self.servo_port)
        self.run('stop servod PORT=%d' % self.servo_port,
                 timeout=60, ignore_status=True)
        logging.debug('Wait %s seconds for servod process fully teardown.',
                      SERVOD_TEARDOWN_TIMEOUT)
        time.sleep(SERVOD_TEARDOWN_TIMEOUT)


    def restart_servod(self, quick_startup=False):
        """Restart the servod process on servohost.
        """
        self.stop_servod()
        self.start_servod(quick_startup)

    def _extract_compressed_logs(self, logdir, relevant_files):
        """Decompress servod logs in |logdir|.

        @param logdir: directory containing compressed servod logs.
        @param relevant_files: list of files in |logdir| to consider.

        @returns: tuple, (tarfiles, files) where
                  tarfiles: list of the compressed filenames that have been
                            extracted and deleted
                  files:  list of the uncompressed files that were generated
        """
        # For all tar-files, first extract them to the directory, and
        # then let the common flow handle them.
        tarfiles = [cf for cf in relevant_files if
                    cf.endswith(self.COMPRESSION_SUFFIX)]
        files = []
        for f in tarfiles:
            norm_name = os.path.basename(f)[:-len(self.COMPRESSION_SUFFIX)]
            with tarfile.open(f) as tf:
                # Each tarfile has only one member, as
                # that's the compressed log.
                member = tf.members[0]
                # Manipulate so that it only extracts the basename, and not
                # the directories etc.
                member.name = norm_name
                files.append(os.path.join(logdir, member.name))
                tf.extract(member, logdir)
            # File has been extracted: remove the compressed file.
            os.remove(f)
        return tarfiles, files

    def _extract_mcu_logs(self, log_subdir):
        """Extract MCU (EC, Cr50, etc) console output from servod debug logs.

        Using the MCU_EXTRACTOR regex (above) extract and split out MCU console
        lines from the logs to generate invidiual console logs e.g. after
        this method, you can find an ec.txt and servo_v4.txt in |log_dir| if
        those MCUs had any console input/output.

        @param log_subdir: directory with log.DEBUG.txt main servod debug logs.
        """
        # Extract the MCU for each one. The MCU logs are only in the .DEBUG
        # files
        mcu_lines_file = os.path.join(log_subdir, 'log.DEBUG.txt')
        if not os.path.exists(mcu_lines_file):
            logging.info('No DEBUG logs found to extract MCU logs from.')
            return
        mcu_files = {}
        mcu_file_template = '%s.txt'
        with open(mcu_lines_file, 'r') as f:
            for line in f:
                match = self.MCU_EXTRACTOR.match(line)
                if match:
                    mcu = match.group(self.MCU_GROUP).lower()
                    line = match.group(self.LINE_GROUP)
                    if mcu not in mcu_files:
                        mcu_file = os.path.join(log_subdir,
                                                mcu_file_template % mcu)
                        mcu_files[mcu] = open(mcu_file, 'a')
                    fd = mcu_files[mcu]
                    fd.write(line + '\n')
        for f in mcu_files:
            mcu_files[f].close()


    def remove_latest_log_symlinks(self):
        """Remove the conveninence symlinks 'latest' servod logs."""
        symlink_wildcard = '%s/latest*' % self.remote_log_dir
        cmd = 'rm ' + symlink_wildcard
        self.run(cmd, stderr_tee=None, ignore_status=True)

    def grab_logs(self, outdir):
        """Retrieve logs from servo_host to |outdir|/servod_{port}.{ts}/.

        This method first collects all logs on the servo_host side pertaining
        to this servod instance (port, instatiation). It glues them together
        into combined log.[level].txt files and extracts all available MCU
        console I/O from the logs into individual files e.g. servo_v4.txt

        All the output can be found in a directory inside |outdir| that
        this generates based on |LOG_DIR|, the servod port, and the instance
        timestamp on the servo_host side.

        @param outdir: directory to create a subdirectory into to place the
                       servod logs into.
        """
        # First, extract the timestamp. This cmd gives the real filename of
        # the latest aka current log file.
        cmd = ('if [ -f %(dir)s/latest.DEBUG ];'
               'then realpath %(dir)s/latest.DEBUG;'
               'elif [ -f %(dir)s/latest ];'
               'then realpath %(dir)s/latest;'
               'else exit %(code)d;'
               'fi' % {'dir': self.remote_log_dir,
                       'code': self.NO_SYMLINKS_CODE})
        res = self.run(cmd, stderr_tee=None, ignore_status=True)
        if res.exit_status != 0:
            if res.exit_status == self.NO_SYMLINKS_CODE:
                logging.warning('servod log latest symlinks not found. '
                                'This is likely due to an error starting up '
                                'servod. Ignoring..')
            else:
                logging.warning('Failed to find servod logs on servo host.')
                logging.warning(res.stderr.strip())
            return
        fname = os.path.basename(res.stdout.strip())
        # From the fname, ought to extract the timestamp using the TS_EXTRACTOR
        instance_ts = self.TS_EXTRACTOR.match(fname).group(self.TS_GROUP)
        # Create the local results log dir.
        log_dir = os.path.join(outdir, '%s_%s.%s' % (self.LOG_DIR,
                                                     str(self.servo_port),
                                                     instance_ts))
        logging.info('Saving servod logs to %s.', log_dir)
        os.mkdir(log_dir)
        # Now, get all files with that timestamp.
        cmd = 'find %s -maxdepth 1 -name "log.%s*"' % (self.remote_log_dir,
                                                       instance_ts)
        res = self.run(cmd, stderr_tee=None, ignore_status=True)
        files = res.stdout.strip().split()
        try:
            self.get_file(files, log_dir, try_rsync=False)

        except error.AutoservRunError as e:
            result = e.result_obj
            if result.exit_status != 0:
                stderr = result.stderr.strip()
                logging.warning("Couldn't retrieve servod logs. Ignoring: %s",
                                stderr or '\n%s' % result)
            return
        local_files = [os.path.join(log_dir, f) for f in os.listdir(log_dir)]
        # TODO(crrev.com/c/1793030): remove no-level case once CL is pushed
        for level_name in ('DEBUG', 'INFO', 'WARNING', ''):
            # Create the joint files for each loglevel. i.e log.DEBUG
            joint_file = self.JOINT_LOG_PREFIX
            if level_name:
                joint_file = '%s.%s' % (self.JOINT_LOG_PREFIX, level_name)
            # This helps with some online tools to avoid complaints about an
            # unknown filetype.
            joint_file = joint_file + '.txt'
            joint_path = os.path.join(log_dir, joint_file)
            files = [f for f in local_files if level_name in f]
            if not files:
                # TODO(crrev.com/c/1793030): remove no-level case once CL
                # is pushed
                continue
            # Extract compressed logs if any.
            compressed, extracted = self._extract_compressed_logs(log_dir,
                                                                  files)
            files = list(set(files) - set(compressed))
            files.extend(extracted)
            # Need to sort. As they all share the same timestamp, and
            # loglevel, the index itself is sufficient. The highest index
            # is the oldest file, therefore we need a descending sort.
            def sortkey(f, level=level_name):
                """Custom sortkey to sort based on rotation number int."""
                if f.endswith(level_name): return 0
                return int(f.split('.')[-1])

            files.sort(reverse=True, key=sortkey)
            # Just rename the first file rather than building from scratch.
            os.rename(files[0], joint_path)
            with open(joint_path, 'a') as joint_f:
                for logfile in files[1:]:
                    # Transfer the file to the joint file line by line.
                    with open(logfile, 'r') as log_f:
                        for line in log_f:
                            joint_f.write(line)
                    # File has been written over. Delete safely.
                    os.remove(logfile)
            # Need to remove all files form |local_files| so we don't
            # analyze them again.
            local_files = list(set(local_files) - set(files) - set(compressed))
        # Lastly, extract MCU logs from the joint logs.
        self._extract_mcu_logs(log_dir)


    def _lock(self):
        """lock servohost by touching a file.
        """
        logging.debug('Locking servohost %s by touching %s file',
                      self.hostname, self._lock_file)
        self.run('touch %s' % self._lock_file, ignore_status=True)
        self._is_locked = True


    def _unlock(self):
        """Unlock servohost by removing the lock file.
        """
        logging.debug('Unlocking servohost by removing %s file',
                      self._lock_file)
        self.run('rm %s' % self._lock_file, ignore_status=True)
        self._is_locked = False


    def close(self):
        """Close the associated servo and the host object."""
        if self._closed:
            logging.debug('ServoHost is already closed.')
            return
        if self._servo:
            outdir = None if not self.job else self.job.resultdir
            # In some cases when we run as lab-tools, the job object is None.
            self._servo.close(outdir)

        if self.job and not self.is_localhost():
            # Grab all logs from this servod instance before stopping servod.
            # TODO(crbug.com/1011516): once enabled, remove the check against
            # localhost and instead check against log-rotiation enablement.
            try:
                self.grab_logs(self.job.resultdir)
            except error.AutoservRunError as e:
                logging.info('Failed to grab servo logs due to: %s. '
                             'This error is forgiven.', str(e))

        if self._is_locked:
            # Remove the lock if the servohost has been locked.
            try:
                self._unlock()
            except error.AutoservSSHTimeout:
                logging.error('Unlock servohost failed due to ssh timeout.'
                              ' It may caused by servohost went down during'
                              ' the task.')
        # We want always stop servod after task to minimum the impact of bad
        # servod process interfere other servods.(see crbug.com/1028665)
        try:
            self.stop_servod()
        except error.AutoservRunError as e:
            logging.info("Failed to stop servod due to:\n%s\n"
                         "This error is forgiven.", str(e))

        super(ServoHost, self).close()
        # Mark closed.
        self._closed = True


    def get_servo_state(self):
        return SERVO_STATE_BROKEN if self._servo_state is None else self._servo_state


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, dut_host_info=None):
    """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(**servo_args)
    try:
        newhost.restart_servod(quick_startup=True)
    except error.AutoservSSHTimeout:
        logging.warning("Restart servod failed due ssh connection "
                        "to servohost timed out. This error is forgiven"
                        " here, we will retry in servo repair process.")
    except error.AutoservRunError as e:
        logging.warning("Restart servod failed due to:\n%s\n"
                        "This error is forgiven here, we will retry"
                        " in servo repair process.", str(e))

    # TODO(gregorynisbet): Clean all of this up.
    logging.debug('create_servo_host: attempt to set info store on '
                  'servo host')
    try:
        if dut_host_info is None:
            logging.debug('create_servo_host: dut_host_info is '
                          'None, skipping')
        else:
            newhost.set_dut_host_info(dut_host_info)
            logging.debug('create_servo_host: successfully set info '
                          'store')
    except Exception:
        logging.error("create_servo_host: (%s)", traceback.format_exc())

    # 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