base.py

# Copyright (C) 2010 Google Inc. All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following disclaimer
# in the documentation and/or other materials provided with the
# distribution.
#     * Neither the Google name nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
"""Abstract base class for Port classes.

The Port classes encapsulate Port-specific (platform-specific) behavior
in the web test infrastructure.
"""

import time
import collections
import json
import logging
import optparse
import os
import re
import sys
import tempfile
from collections import defaultdict

import six
from six.moves import zip_longest

from blinkpy.common import exit_codes
from blinkpy.common import find_files
from blinkpy.common import read_checksum_from_png
from blinkpy.common import path_finder
from blinkpy.common.memoized import memoized
from blinkpy.common.system.executive import ScriptError
from blinkpy.common.system.path import abspath_to_uri
from blinkpy.w3c.wpt_manifest import WPTManifest, MANIFEST_NAME
from blinkpy.web_tests.layout_package.bot_test_expectations import BotTestExpectationsFactory
from blinkpy.web_tests.models.test_configuration import TestConfiguration
from blinkpy.web_tests.models.test_run_results import TestRunException
from blinkpy.web_tests.models.typ_types import TestExpectations, ResultType
from blinkpy.web_tests.port import driver
from blinkpy.web_tests.port import server_process
from blinkpy.web_tests.port.factory import PortFactory
from blinkpy.web_tests.servers import apache_http
from blinkpy.web_tests.servers import pywebsocket
from blinkpy.web_tests.servers import wptserve
from blinkpy.web_tests.skia_gold import blink_skia_gold_properties as sgp
from blinkpy.web_tests.skia_gold import blink_skia_gold_session_manager as sgsm

_log = logging.getLogger(__name__)

# Path relative to the build directory.
CONTENT_SHELL_FONTS_DIR = "test_fonts"

FONT_FILES = [
    [[CONTENT_SHELL_FONTS_DIR], 'Ahem.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Arimo-Bold.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Arimo-BoldItalic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Arimo-Italic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Arimo-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Cousine-Bold.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Cousine-BoldItalic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Cousine-Italic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Cousine-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'DejaVuSans.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'GardinerModBug.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'GardinerModCat.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Garuda.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Gelasio-Bold.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Gelasio-BoldItalic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Gelasio-Italic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Gelasio-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Lohit-Devanagari.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Lohit-Gurmukhi.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Lohit-Tamil.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'MuktiNarrow.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'NotoColorEmoji.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'NotoSansCJKjp-Regular.otf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'NotoSansKhmer-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'NotoSansSymbols2-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'NotoSansTibetan-Regular.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Tinos-Bold.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Tinos-BoldItalic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Tinos-Italic.ttf', None],
    [[CONTENT_SHELL_FONTS_DIR], 'Tinos-Regular.ttf', None],
]

# This is the fingerprint of wpt's certificate found in
# blinkpy/third_party/wpt/certs.  The following line is updated by
# update_cert.py.
WPT_FINGERPRINT = 'Nxvaj3+bY3oVrTc+Jp7m3E3sB1n3lXtnMDCyBsqEXiY='
# One for 127.0.0.1.sxg.pem
SXG_FINGERPRINT = '55qC1nKu2A88ESbFmk5sTPQS/ScG+8DD7P+2bgFA9iM='
# And one for external/wpt/signed-exchange/resources/127.0.0.1.sxg.pem
SXG_WPT_FINGERPRINT = '0Rt4mT6SJXojEMHTnKnlJ/hBKMBcI4kteBlhR1eTTdk='

# A convervative rule for names that are valid for file or directory names.
VALID_FILE_NAME_REGEX = re.compile(r'^[\w\-=]+$')

# This sub directory will be inside the results directory and it will
# contain all the disc artifacts created by web tests
ARTIFACTS_SUB_DIR = 'layout-test-results'


class Port(object):
    """Abstract class for Port-specific hooks for the web_test package."""

    # Subclasses override this. This should indicate the basic implementation
    # part of the port name, e.g., 'mac', 'win', 'gtk'; there is one unique
    # value per class.
    # FIXME: Rename this to avoid confusion with the "full port name".
    port_name = None

    # Test paths use forward slash as separator on all platforms.
    TEST_PATH_SEPARATOR = '/'

    ALL_BUILD_TYPES = ('debug', 'release')

    CONTENT_SHELL_NAME = 'content_shell'

    # Update the first line in third_party/blink/web_tests/TestExpectations and
    # the documentation in docs/testing/web_test_expectations.md when this list
    # changes.
    ALL_SYSTEMS = (
        ('mac10.12', 'x86'),
        ('mac10.13', 'x86'),
        ('mac10.14', 'x86'),
        ('mac10.15', 'x86'),
        ('mac11', 'x86'),
        ('mac11-arm64', 'arm64'),
        ('win7', 'x86'),
        ('win10.20h2', 'x86'),
        ('trusty', 'x86_64'),
        ('fuchsia', 'x86_64'),
    )

    CONFIGURATION_SPECIFIER_MACROS = {
        'mac': [
            'mac10.12', 'mac10.13', 'mac10.14', 'mac10.15', 'mac11',
            'mac11-arm64'
        ],
        'win': ['win7', 'win10.20h2'],
        'linux': ['trusty'],
        'fuchsia': ['fuchsia'],
    }

    # List of ports open on the host that the tests will connect to. When tests
    # run on a separate machine (Android and Fuchsia) these ports need to be
    # forwarded back to the host.
    # 8000, 8080 and 8443 are for http/https tests;
    # 8880 is for websocket tests (see apache_http.py and pywebsocket.py).
    # 8001, 8081, 8444, and 8445 are for http/https WPT;
    # 9001 and 9444 are for websocket WPT (see wptserve.py).
    SERVER_PORTS = [8000, 8001, 8080, 8081, 8443, 8444, 8445, 8880, 9001, 9444]

    FALLBACK_PATHS = {}

    SUPPORTED_VERSIONS = []

    # URL to the build requirements page.
    BUILD_REQUIREMENTS_URL = ''

    # The suffixes of baseline files (not extensions).
    BASELINE_SUFFIX = '-expected'
    BASELINE_MISMATCH_SUFFIX = '-expected-mismatch'

    # All of the non-reftest baseline extensions we use.
    BASELINE_EXTENSIONS = ('.wav', '.txt', '.png')

    FLAG_EXPECTATIONS_PREFIX = 'FlagExpectations'

    # The following is used for concetenating WebDriver test names.
    WEBDRIVER_SUBTEST_SEPARATOR = '>>'

    # The following is used for concetenating WebDriver test names in pytest format.
    WEBDRIVER_SUBTEST_PYTEST_SEPARATOR = '::'

    # The following two constants must match. When adding a new WPT root, also
    # remember to add an alias rule to //third_party/wpt_tools/wpt.config.json.
    # WPT_DIRS maps WPT roots on the file system to URL prefixes on wptserve.
    # The order matters: '/' MUST be the last URL prefix.
    WPT_DIRS = collections.OrderedDict([
        ('wpt_internal', '/wpt_internal/'),
        ('external/wpt', '/'),
    ])
    # WPT_REGEX captures: 1. the root directory of WPT relative to web_tests
    # (without a trailing slash), 2. the path of the test within WPT (without a
    # leading slash).
    WPT_REGEX = re.compile(
        r'^(?:virtual/[^/]+/)?(external/wpt|wpt_internal)/(.*)$')

    # Because this is an abstract base class, arguments to functions may be
    # unused in this class - pylint: disable=unused-argument

    @classmethod
    def latest_platform_fallback_path(cls):
        return cls.FALLBACK_PATHS[cls.SUPPORTED_VERSIONS[-1]]

    @classmethod
    def determine_full_port_name(cls, host, options, port_name):
        """Return a fully-specified port name that can be used to construct objects."""
        # Subclasses will usually override this.
        assert port_name.startswith(cls.port_name)
        return port_name

    def __init__(self, host, port_name, options=None, **kwargs):

        # This value is the "full port name", and may be different from
        # cls.port_name by having version modifiers appended to it.
        self._name = port_name

        # These are default values that should be overridden in a subclasses.
        self._version = ''
        self._architecture = 'x86'

        # FIXME: Ideally we'd have a package-wide way to get a well-formed
        # options object that had all of the necessary options defined on it.
        self._options = options or optparse.Values()

        self.host = host
        self._executive = host.executive
        self._filesystem = host.filesystem
        self._path_finder = path_finder.PathFinder(host.filesystem)

        self._http_server = None
        self._websocket_server = None
        self._wpt_server = None
        self._image_differ = None
        self.server_process_constructor = server_process.ServerProcess  # This can be overridden for testing.
        self._http_lock = None  # FIXME: Why does this live on the port object?
        self._dump_reader = None

        if not hasattr(options, 'configuration') or not options.configuration:
            self.set_option_default('configuration',
                                    self.default_configuration())
        if not hasattr(options, 'target') or not options.target:
            self.set_option_default('target', self._options.configuration)
        if not hasattr(options, 'no_virtual_tests'):
            self.set_option_default('no_virtual_tests', False)
        self._test_configuration = None
        self._results_directory = None
        self._virtual_test_suites = None
        self._used_expectation_files = None

        self._skia_gold_temp_dir = None
        self._skia_gold_session_manager = None
        self._skia_gold_properties = None

    def __del__(self):
        if self._skia_gold_temp_dir:
            self._filesystem.rmtree(self._skia_gold_temp_dir,
                                    ignore_errors=True)

    def __str__(self):
        return 'Port{name=%s, version=%s, architecture=%s, test_configuration=%s}' % (
            self._name, self._version, self._architecture,
            self._test_configuration)

    def get_platform_tags(self):
        """Returns system condition tags that are used to find active expectations
           for a test run on a specific system"""
        return frozenset([
            self._options.configuration.lower(), self._version, self.port_name,
            self._architecture
        ])

    @memoized
    def flag_specific_config_name(self):
        """Returns the name of the flag-specific configuration which best matches
           self._specified_additional_driver_flags(), or the first specified flag
           with leading '-'s stripped if no match in the configuration is found.
        """
        specified_flags = self._specified_additional_driver_flags()
        if not specified_flags:
            return None

        best_match = None
        configs = self._flag_specific_configs()
        for name in configs:
            # To match the specified flags must start with all config args.
            args = configs[name]
            if specified_flags[:len(args)] != args:
                continue
            # The first config matching the highest number of specified flags wins.
            if not best_match or len(configs[best_match]) < len(args):
                best_match = name

        if best_match:
            return best_match
        # If no match, fallback to the old mode: using the name of the first specified flag.
        return specified_flags[0].lstrip('-')

    @memoized
    def _flag_specific_configs(self):
        """Reads configuration from FlagSpecificConfig and returns a dictionary from name to args."""
        config_file = self._filesystem.join(self.web_tests_dir(),
                                            'FlagSpecificConfig')
        if not self._filesystem.exists(config_file):
            return {}

        try:
            json_configs = json.loads(
                self._filesystem.read_text_file(config_file))
        except ValueError as error:
            raise ValueError('{} is not a valid JSON file: {}'.format(
                config_file, error))

        configs = {}
        for config in json_configs:
            name = config['name']
            args = config['args']
            if not VALID_FILE_NAME_REGEX.match(name):
                raise ValueError(
                    '{}: name "{}" contains invalid characters'.format(
                        config_file, name))
            if name in configs:
                raise ValueError('{} contains duplicated name {}.'.format(
                    config_file, name))
            if args in configs.values():
                raise ValueError(
                    '{}: name "{}" has the same args as another entry.'.format(
                        config_file, name))
            configs[name] = args
        return configs

    def _specified_additional_driver_flags(self):
        """Returns the list of additional driver flags specified by the user in
           the following ways, concatenated:
           1. Flags in web_tests/additional-driver-flag.setting.
           2. flags expanded from --flag-specific=<name> based on flag-specific config.
           3. Zero or more flags passed by --additional-driver-flag.
        """
        flags = []
        flag_file = self._filesystem.join(self.web_tests_dir(),
                                          'additional-driver-flag.setting')
        if self._filesystem.exists(flag_file):
            flags = self._filesystem.read_text_file(flag_file).split()

        flag_specific_option = self.get_option('flag_specific')
        if flag_specific_option:
            configs = self._flag_specific_configs()
            assert flag_specific_option in configs, '{} is not defined in FlagSpecificConfig'.format(
                flag_specific_option)
            flags += configs[flag_specific_option]

        flags += self.get_option('additional_driver_flag', [])
        return flags

    def additional_driver_flags(self):
        flags = self._specified_additional_driver_flags()
        if self.driver_name() == self.CONTENT_SHELL_NAME:
            flags += [
                '--run-web-tests',
                '--ignore-certificate-errors-spki-list=' + WPT_FINGERPRINT +
                ',' + SXG_FINGERPRINT + ',' + SXG_WPT_FINGERPRINT,
                # Required for WebTransport tests.
                '--origin-to-force-quic-on=web-platform.test:11000',
                '--user-data-dir'
            ]
            if self.get_option('nocheck_sys_deps', False):
                flags.append('--disable-system-font-check')

        # If we're already repeating the tests more than once, then we're not
        # particularly concerned with speed. Resetting the shell between tests
        # increases test run time by 2-5X, but provides more consistent results
        # [less state leaks between tests].
        if (self.get_option('reset_shell_between_tests')
                or (self.get_option('repeat_each')
                    and self.get_option('repeat_each') > 1)
                or (self.get_option('iterations')
                    and self.get_option('iterations') > 1)):
            flags += ['--reset-shell-between-tests']
        return flags

    def supports_per_test_timeout(self):
        return False

    def default_smoke_test_only(self):
        return False

    def _default_timeout_ms(self):
        return 6000

    def timeout_ms(self):
        timeout_ms = self._default_timeout_ms()
        if self.get_option('configuration') == 'Debug':
            # Debug is about 5x slower than Release.
            return 5 * timeout_ms
        if self._build_has_dcheck_always_on():
            # Release with DCHECK is also slower than pure Release.
            return 2 * timeout_ms
        return timeout_ms

    def skia_gold_temp_dir(self):
        return self._skia_gold_temp_dir

    def skia_gold_properties(self):
        if not self._skia_gold_properties:
            self._skia_gold_properties = sgp.BlinkSkiaGoldProperties(
                self._options)
        return self._skia_gold_properties

    def skia_gold_session_manager(self):
        if not self._skia_gold_session_manager:
            self._skia_gold_temp_dir = self._filesystem.mkdtemp()
            self._skia_gold_session_manager = sgsm.BlinkSkiaGoldSessionManager(
                str(self._skia_gold_temp_dir), self.skia_gold_properties())
        return self._skia_gold_session_manager

    def skia_gold_json_keys(self):
        return {
            'configuration': self._options.configuration.lower(),
            'version': self._version,
            'port': self.port_name,
            'architecture': self._architecture,
            'ignore': '1',
        }

    @memoized
    def _build_has_dcheck_always_on(self):
        args_gn_file = self._build_path('args.gn')
        if not self._filesystem.exists(args_gn_file):
            _log.error('Unable to find %s', args_gn_file)
            return False
        contents = self._filesystem.read_text_file(args_gn_file)
        return bool(
            re.search(r'^\s*dcheck_always_on\s*=\s*true\s*(#.*)?$', contents,
                      re.MULTILINE))

    def driver_stop_timeout(self):
        """Returns the amount of time in seconds to wait before killing the process in driver.stop()."""
        # We want to wait for at least 3 seconds, but if we are really slow, we
        # want to be slow on cleanup as well (for things like ASAN, Valgrind, etc.)
        return (3.0 * float(self.get_option('timeout_ms', '0')) /
                self._default_timeout_ms())

    def default_batch_size(self):
        """Returns the default batch size to use for this port."""
        if self.get_option('enable_sanitizer'):
            # ASAN/MSAN/TSAN use more memory than regular content_shell. Their
            # memory usage may also grow over time, up to a certain point.
            # Relaunching the driver periodically helps keep it under control.
            return 40
        # The default is infinite batch size.
        return 0

    def default_child_processes(self):
        """Returns the number of child processes to use for this port."""
        return self._executive.cpu_count()

    def default_max_locked_shards(self):
        """Returns the number of "locked" shards to run in parallel (like the http tests)."""
        max_locked_shards = int(self.default_child_processes()) // 4
        if not max_locked_shards:
            return 1
        return max_locked_shards

    def baseline_version_dir(self):
        """Returns the absolute path to the platform-and-version-specific results."""
        baseline_search_paths = self.baseline_search_path()
        return baseline_search_paths[0]

    def baseline_flag_specific_dir(self):
        """If --additional-driver-flag is specified, returns the absolute path to the flag-specific
           platform-independent results. Otherwise returns None."""
        flag_specific_path = self._flag_specific_baseline_search_path()
        return flag_specific_path[-1] if flag_specific_path else None

    def baseline_search_path(self):
        return (self.get_option('additional_platform_directory', []) +
                self._flag_specific_baseline_search_path() +
                self._compare_baseline() +
                list(self.default_baseline_search_path()))

    def default_baseline_search_path(self):
        """Returns a list of absolute paths to directories to search under for baselines.

        The directories are searched in order.
        """
        return map(self._absolute_baseline_path,
                   self.FALLBACK_PATHS[self.version()])

    @memoized
    def _compare_baseline(self):
        factory = PortFactory(self.host)
        target_port = self.get_option('compare_port')
        if target_port:
            return factory.get(target_port).default_baseline_search_path()
        return []

    def _check_file_exists(self,
                           path_to_file,
                           file_description,
                           override_step=None,
                           more_logging=True):
        """Verifies that the file is present where expected, or logs an error.

        Args:
            file_name: The (human friendly) name or description of the file
                you're looking for (e.g., "HTTP Server"). Used for error logging.
            override_step: An optional string to be logged if the check fails.
            more_logging: Whether or not to log the error messages.

        Returns:
            True if the file exists, else False.
        """
        if not self._filesystem.exists(path_to_file):
            if more_logging:
                _log.error('Unable to find %s', file_description)
                _log.error('    at %s', path_to_file)
                if override_step:
                    _log.error('    %s', override_step)
                    _log.error('')
            return False
        return True

    def check_build(self, needs_http, printer):
        if not self._check_file_exists(self._path_to_driver(), 'test driver'):
            return exit_codes.UNEXPECTED_ERROR_EXIT_STATUS

        if not self._check_driver_build_up_to_date(
                self.get_option('configuration')):
            return exit_codes.UNEXPECTED_ERROR_EXIT_STATUS

        if not self._check_file_exists(self._path_to_image_diff(),
                                       'image_diff'):
            return exit_codes.UNEXPECTED_ERROR_EXIT_STATUS

        if self._dump_reader and not self._dump_reader.check_is_functional():
            return exit_codes.UNEXPECTED_ERROR_EXIT_STATUS

        if needs_http and not self.check_httpd():
            return exit_codes.UNEXPECTED_ERROR_EXIT_STATUS

        return exit_codes.OK_EXIT_STATUS

    def check_sys_deps(self):
        """Checks whether the system is properly configured.

        Most checks happen during invocation of the driver prior to running
        tests. This can be overridden to run custom checks.

        Returns:
            An exit status code.
        """
        return exit_codes.OK_EXIT_STATUS

    def check_httpd(self):
        httpd_path = self.path_to_apache()
        if httpd_path:
            try:
                env = self.setup_environ_for_server()
                if self._executive.run_command(
                    [httpd_path, '-v'], env=env, return_exit_code=True) != 0:
                    _log.error('httpd seems broken. Cannot run http tests.')
                    return False
                return True
            except OSError as e:
                _log.error('while trying to run: ' + httpd_path)
                _log.error('httpd launch error: ' + repr(e))
        _log.error('No httpd found. Cannot run http tests.')
        return False

    def do_text_results_differ(self, expected_text, actual_text):
        return expected_text != actual_text

    def do_audio_results_differ(self, expected_audio, actual_audio):
        return expected_audio != actual_audio

    def diff_image(self,
                   expected_contents,
                   actual_contents,
                   max_channel_diff=None,
                   max_pixels_diff=None):
        """Compares two images and returns an (image diff, error string) pair.

        If an error occurs (like image_diff isn't found, or crashes), we log an
        error and return True (for a diff).
        """
        # If only one of them exists, return that one.
        if not actual_contents and not expected_contents:
            return (None, None)
        if not actual_contents:
            return (expected_contents, None)
        if not expected_contents:
            return (actual_contents, None)

        tempdir = self._filesystem.mkdtemp()

        expected_filename = self._filesystem.join(str(tempdir), 'expected.png')
        self._filesystem.write_binary_file(expected_filename,
                                           expected_contents)

        actual_filename = self._filesystem.join(str(tempdir), 'actual.png')
        self._filesystem.write_binary_file(actual_filename, actual_contents)

        diff_filename = self._filesystem.join(str(tempdir), 'diff.png')

        executable = self._path_to_image_diff()
        # Although we are handed 'old', 'new', image_diff wants 'new', 'old'.
        command = [
            executable, '--diff', actual_filename, expected_filename,
            diff_filename
        ]
        # Notifies image_diff to allow a tolerance when calculating the pixel
        # diff. To account for variances when the tests are ran on an actual
        # GPU.
        if self.get_option('fuzzy_diff'):
            command.append('--fuzzy-diff')
        # The max_channel_diff and max_pixels_diff arguments are used by WPT
        # tests for fuzzy reftests. See
        # https://web-platform-tests.org/writing-tests/reftests.html#fuzzy-matching
        if max_channel_diff is not None:
            command.append('--fuzzy-max-channel-diff={}'.format('-'.join(
                map(str, max_channel_diff))))
        if max_pixels_diff is not None:
            command.append('--fuzzy-max-pixels-diff={}'.format('-'.join(
                map(str, max_pixels_diff))))

        result = None
        err_str = None
        try:
            output = self._executive.run_command(command)
            # Log the output, to enable user debugging of a diff hidden by fuzzy
            # expectations. This is useful when tightening fuzzy bounds.
            if output:
                _log.debug(output)
        except ScriptError as error:
            if error.exit_code == 1:
                result = self._filesystem.read_binary_file(diff_filename)
                # Log the output, to enable user debugging of the diff.
                if error.output:
                    _log.debug(error.output)
            else:
                err_str = 'Image diff returned an exit code of %s. See http://crbug.com/278596' % error.exit_code
        except OSError as error:
            err_str = 'error running image diff: %s' % error
        finally:
            self._filesystem.rmtree(str(tempdir))

        return (result, err_str or None)

    def driver_name(self):
        if self.get_option('driver_name'):
            return self.get_option('driver_name')
        return self.CONTENT_SHELL_NAME

    def expected_baselines_by_extension(self, test_name):
        """Returns a dict mapping baseline suffix to relative path for each baseline in a test.

        For reftests, it returns ".==" or ".!=" instead of the suffix.
        """
        # FIXME: The name similarity between this and expected_baselines()
        # below, is unfortunate. We should probably rename them both.
        baseline_dict = {}
        reference_files = self.reference_files(test_name)
        if reference_files:
            # FIXME: How should this handle more than one type of reftest?
            baseline_dict['.' + reference_files[0][0]] = \
                self.relative_test_filename(reference_files[0][1])

        for extension in self.BASELINE_EXTENSIONS:
            path = self.expected_filename(
                test_name, extension, return_default=False)
            baseline_dict[extension] = self.relative_test_filename(
                path) if path else path

        return baseline_dict

    def output_filename(self, test_name, suffix, extension):
        """Generates the output filename for a test.

        This method gives a proper filename for various outputs of a test,
        including baselines and actual results. Usually, the output filename
        follows the pattern: test_name_without_ext+suffix+extension, but when
        the test name contains query strings, e.g. external/wpt/foo.html?wss,
        test_name_without_ext is mangled to be external/wpt/foo_wss.

        It is encouraged to use this method instead of writing another mangling.

        Args:
            test_name: The name of a test.
            suffix: A suffix string to add before the extension
                (e.g. "-expected").
            extension: The extension of the output file (starting with .).

        Returns:
            A string, the output filename.
        """
        # WPT names might contain query strings, e.g. external/wpt/foo.html?wss,
        # in which case we mangle test_name_root (the part of a path before the
        # last extension point) to external/wpt/foo_wss, and the output filename
        # becomes external/wpt/foo_wss-expected.txt.
        index = test_name.find('?')
        if index != -1:
            test_name_root, _ = self._filesystem.splitext(test_name[:index])
            query_part = test_name[index:]
            test_name_root += self._filesystem.sanitize_filename(query_part)
        else:
            test_name_root, _ = self._filesystem.splitext(test_name)
        return test_name_root + suffix + extension

    def expected_baselines(self,
                           test_name,
                           extension,
                           all_baselines=False,
                           match=True):
        """Given a test name, finds where the baseline results are located.

        Return values will be in the format appropriate for the current
        platform (e.g., "\\" for path separators on Windows). If the results
        file is not found, then None will be returned for the directory,
        but the expected relative pathname will still be returned.

        This routine is generic but lives here since it is used in
        conjunction with the other baseline and filename routines that are
        platform specific.

        Args:
            test_name: Name of test file (usually a relative path under web_tests/)
            extension: File extension of the expected results, including dot;
                e.g. '.txt' or '.png'.  This should not be None, but may be an
                empty string.
            all_baselines: If True, return an ordered list of all baseline paths
                for the given platform. If False, return only the first one.
            match: Whether the baseline is a match or a mismatch.

        Returns:
            A list of (platform_dir, results_filename) pairs, where
                platform_dir - abs path to the top of the results tree (or test
                    tree)
                results_filename - relative path from top of tree to the results
                    file
                (port.join() of the two gives you the full path to the file,
                    unless None was returned.)
        """
        baseline_filename = self.output_filename(
            test_name,
            self.BASELINE_SUFFIX if match else self.BASELINE_MISMATCH_SUFFIX,
            extension)
        baseline_search_path = self.baseline_search_path()

        baselines = []
        for platform_dir in baseline_search_path:
            if self._filesystem.exists(
                    self._filesystem.join(platform_dir, baseline_filename)):
                baselines.append((platform_dir, baseline_filename))

            if not all_baselines and baselines:
                return baselines

        # If it wasn't found in a platform directory, return the expected
        # result in the test directory, even if no such file actually exists.
        platform_dir = self.web_tests_dir()
        if self._filesystem.exists(
                self._filesystem.join(platform_dir, baseline_filename)):
            baselines.append((platform_dir, baseline_filename))

        if baselines:
            return baselines

        return [(None, baseline_filename)]

    def expected_filename(self,
                          test_name,
                          extension,
                          return_default=True,
                          fallback_base_for_virtual=True,
                          match=True):
        """Given a test name, returns an absolute path to its expected results.

        If no expected results are found in any of the searched directories,
        the directory in which the test itself is located will be returned.
        The return value is in the format appropriate for the platform
        (e.g., "\\" for path separators on windows).

        This routine is generic but is implemented here to live alongside
        the other baseline and filename manipulation routines.

        Args:
            test_name: Name of test file (usually a relative path under web_tests/)
            extension: File extension of the expected results, including dot;
                e.g. '.txt' or '.png'.  This should not be None, but may be an
                empty string.
            return_default: If True, returns the path to the generic expectation
                if nothing else is found; if False, returns None.
            fallback_base_for_virtual: For virtual test only. When no virtual
                specific baseline is found, if this parameter is True, fallback
                to find baselines of the base test; if False, depending on
                |return_default|, returns the generic virtual baseline or None.
            match: Whether the baseline is a match or a mismatch.

        Returns:
            An absolute path to its expected results, or None if not found.
        """
        # The [0] means the first expected baseline (which is the one to be
        # used) in the fallback paths.
        platform_dir, baseline_filename = self.expected_baselines(
            test_name, extension, match=match)[0]
        if platform_dir:
            return self._filesystem.join(platform_dir, baseline_filename)

        if fallback_base_for_virtual:
            actual_test_name = self.lookup_virtual_test_base(test_name)
            if actual_test_name:
                return self.expected_filename(
                    actual_test_name, extension, return_default, match=match)

        if return_default:
            return self._filesystem.join(self.web_tests_dir(),
                                         baseline_filename)
        return None

    def fallback_expected_filename(self, test_name, extension):
        """Given a test name, returns an absolute path to its next fallback baseline.
        Args:
            same as expected_filename()
        Returns:
            An absolute path to the next fallback baseline, or None if not found.
        """
        baselines = self.expected_baselines(
            test_name, extension, all_baselines=True)
        if len(baselines) < 2:
            actual_test_name = self.lookup_virtual_test_base(test_name)
            if actual_test_name:
                if len(baselines) == 0:
                    return self.fallback_expected_filename(
                        actual_test_name, extension)
                # In this case, baselines[0] is the current baseline of the
                # virtual test, so the first base test baseline is the fallback
                # baseline of the virtual test.
                return self.expected_filename(
                    actual_test_name, extension, return_default=False)
            return None

        platform_dir, baseline_filename = baselines[1]
        if platform_dir:
            return self._filesystem.join(platform_dir, baseline_filename)
        return None

    def expected_checksum(self, test_name):
        """Returns the checksum of the image we expect the test to produce,
        or None if it is a text-only test.
        """
        png_path = self.expected_filename(test_name, '.png')

        if self._filesystem.exists(png_path):
            with self._filesystem.open_binary_file_for_reading(
                    png_path) as filehandle:
                return read_checksum_from_png.read_checksum(filehandle)

        return None

    def expected_image(self, test_name):
        """Returns the image we expect the test to produce."""
        baseline_path = self.expected_filename(test_name, '.png')
        if not self._filesystem.exists(baseline_path):
            return None
        return self._filesystem.read_binary_file(baseline_path)

    def expected_audio(self, test_name):
        baseline_path = self.expected_filename(test_name, '.wav')
        if not self._filesystem.exists(baseline_path):
            return None
        return self._filesystem.read_binary_file(baseline_path)

    def expected_text(self, test_name):
        """Returns the text output we expect the test to produce, or None
        if we don't expect there to be any text output.

        End-of-line characters are normalized to '\n'.
        """
        # FIXME: DRT output is actually utf-8, but since we don't decode the
        # output from DRT (instead treating it as a binary string), we read the
        # baselines as a binary string, too.
        baseline_path = self.expected_filename(test_name, '.txt')
        if not self._filesystem.exists(baseline_path):
            return None
        text = self._filesystem.read_binary_file(baseline_path)
        return text.replace(b'\r\n', b'\n')

    def expected_subtest_failure(self, test_name):
        baseline = self.expected_text(test_name)
        if baseline:
            baseline = baseline.decode('utf8', 'replace')
            if re.search(r"^(FAIL|NOTRUN|TIMEOUT)", baseline, re.MULTILINE):
                return True
        return False

    def expected_harness_error(self, test_name):
        baseline = self.expected_text(test_name)
        if baseline:
            baseline = baseline.decode('utf8', 'replace')
            if re.search(r"^Harness Error\.", baseline, re.MULTILINE):
                return True
        return False

    def reference_files(self, test_name):
        """Returns a list of expectation (== or !=) and filename pairs"""

        # Try to find -expected.* or -expected-mismatch.* in the same directory.
        reftest_list = []
        for expectation in ('==', '!='):
            for extension in Port.supported_file_extensions:
                path = self.expected_filename(
                    test_name, extension, match=(expectation == '=='))
                if self._filesystem.exists(path):
                    reftest_list.append((expectation, path))
        if reftest_list:
            return reftest_list

        # Try to extract information from MANIFEST.json.
        match = self.WPT_REGEX.match(test_name)
        if not match:
            return []
        wpt_path = match.group(1)
        path_in_wpt = match.group(2)
        for expectation, ref_path_in_wpt in self.wpt_manifest(
                wpt_path).extract_reference_list(path_in_wpt):
            ref_absolute_path = self._filesystem.join(
                self.web_tests_dir(), wpt_path + ref_path_in_wpt)
            reftest_list.append((expectation, ref_absolute_path))
        return reftest_list

    def tests(self, paths=None):
        """Returns all tests or tests matching supplied paths.

        Args:
            paths: Array of paths to match. If supplied, this function will only
                return tests matching at least one path in paths.

        Returns:
            An array of test paths and test names. The latter are web platform
            tests that don't correspond to file paths but are valid tests,
            for instance a file path test.any.js could correspond to two test
            names: test.any.html and test.any.worker.html.
        """
        tests = self.real_tests(paths)

        if paths:
            if not self._options.no_virtual_tests:
                tests.extend(self._virtual_tests_matching_paths(paths))
            if (any(wpt_path in path for wpt_path in self.WPT_DIRS
                    for path in paths)
                    # TODO(robertma): Remove this special case when external/wpt is moved to wpt.
                    or any('external' in path for path in paths)):
                tests.extend(self._wpt_test_urls_matching_paths(paths))
        else:
            # '/' is used instead of filesystem.sep as the WPT manifest always
            # uses '/' for paths (it is not OS dependent).
            wpt_tests = [
                wpt_path + '/' + test for wpt_path in self.WPT_DIRS
                for test in self.wpt_manifest(wpt_path).all_urls()
            ]
            tests_by_dir = defaultdict(list)
            for test in tests + wpt_tests:
                dirname = os.path.dirname(test) + '/'
                tests_by_dir[dirname].append(test)

            if not self._options.no_virtual_tests:
                tests.extend(self._all_virtual_tests(tests_by_dir))
            tests.extend(wpt_tests)
        return tests

    def real_tests_from_dict(self, paths, tests_by_dir):
        """Find all real tests in paths, using results saved in dict."""
        files = []
        for path in paths:
            if self._has_supported_extension_for_all(path):
                files.append(path)
                continue
            path = path + '/' if path[-1] != '/' else path
            for key, value in tests_by_dir.items():
                if key.startswith(path):
                    files.extend(value)
        return files

    def real_tests(self, paths):
        """Find all real tests in paths except WPT."""
        # When collecting test cases, skip these directories.
        skipped_directories = set([
            'platform', 'resources', 'support', 'script-tests', 'reference',
            'reftest'
        ])
        # Also ignore all WPT directories. Note that this is only an
        # optimization; is_non_wpt_test_file should skip WPT regardless.
        skipped_directories |= set(self.WPT_DIRS)
        files = find_files.find(self._filesystem, self.web_tests_dir(), paths, skipped_directories,
                                lambda _, dirname, filename: self.is_non_wpt_test_file(dirname, filename),
                                self.test_key)
        return [self.relative_test_filename(f) for f in files]

    @staticmethod
    def is_reference_html_file(filesystem, dirname, filename):
        # TODO(robertma): We probably do not need prefixes/suffixes other than
        # -expected{-mismatch} any more. Or worse, there might be actual tests
        # with these prefixes/suffixes.
        if filename.startswith('ref-') or filename.startswith('notref-'):
            return True
        filename_without_ext, _ = filesystem.splitext(filename)
        for suffix in ['-expected', '-expected-mismatch', '-ref', '-notref']:
            if filename_without_ext.endswith(suffix):
                return True
        return False

    # When collecting test cases, we include any file with these extensions.
    supported_file_extensions = set([
        '.html',
        '.xml',
        '.xhtml',
        '.xht',
        '.pl',
        '.htm',
        '.php',
        '.svg',
        '.mht',
        '.pdf',
    ])

    def _has_supported_extension_for_all(self, filename):
        extension = self._filesystem.splitext(filename)[1]
        if 'inspector-protocol' in filename and extension == '.js':
            return True
        if 'devtools' in filename and extension == '.js':
            return True
        return extension in self.supported_file_extensions

    def _has_supported_extension(self, filename):
        """Returns True if filename is one of the file extensions we want to run a test on."""
        extension = self._filesystem.splitext(filename)[1]
        return extension in self.supported_file_extensions

    def is_non_wpt_test_file(self, dirname, filename):
        # Convert dirname to a relative path to web_tests with slashes
        # normalized and ensure it has a trailing slash.
        normalized_test_dir = self.relative_test_filename(
            dirname) + self.TEST_PATH_SEPARATOR
        if any(
                normalized_test_dir.startswith(d + self.TEST_PATH_SEPARATOR)
                for d in self.WPT_DIRS):
            return False
        extension = self._filesystem.splitext(filename)[1]
        if 'inspector-protocol' in dirname and extension == '.js':
            return True
        if 'devtools' in dirname and extension == '.js':
            return True
        return (self._has_supported_extension(filename)
                and not Port.is_reference_html_file(self._filesystem, dirname,
                                                    filename))

    @memoized
    def wpt_manifest(self, path):
        assert path in self.WPT_DIRS
        # Convert '/' to the platform-specific separator.
        path = self._filesystem.normpath(path)
        manifest_path = self._filesystem.join(self.web_tests_dir(), path,
                                              MANIFEST_NAME)
        if not self._filesystem.exists(manifest_path) or self.get_option(
                'manifest_update', False):
            _log.debug('Generating MANIFEST.json for %s...', path)
            WPTManifest.ensure_manifest(self, path)
        return WPTManifest(self.host, manifest_path)

    def is_wpt_crash_test(self, test_name):
        """Returns whether a WPT test is a crashtest.

        See https://web-platform-tests.org/writing-tests/crashtest.html.
        """
        match = self.WPT_REGEX.match(test_name)
        if not match:
            return False
        wpt_path = match.group(1)
        path_in_wpt = match.group(2)
        return self.wpt_manifest(wpt_path).is_crash_test(path_in_wpt)

    def is_slow_wpt_test(self, test_name):
        # When DCHECK is enabled, idlharness tests run 5-6x slower due to the
        # amount of JavaScript they use (most web_tests run very little JS).
        # This causes flaky timeouts for a lot of them, as a 0.5-1s test becomes
        # close to the default 6s timeout.
        if (self.is_wpt_idlharness_test(test_name)
                and self._build_has_dcheck_always_on()):
            return True

        match = self.WPT_REGEX.match(test_name)
        if not match:
            return False
        wpt_path = match.group(1)
        path_in_wpt = match.group(2)
        return self.wpt_manifest(wpt_path).is_slow_test(path_in_wpt)

    def get_wpt_fuzzy_metadata(self, test_name):
        """Returns the fuzzy metadata for the given WPT test.

        The metadata is a pair of lists, (maxDifference, totalPixels), where
        each list is a [min, max] range, inclusive. If the test is not a WPT
        test or has no fuzzy metadata, returns (None, None).

        See https://web-platform-tests.org/writing-tests/reftests.html#fuzzy-matching
        """
        match = self.WPT_REGEX.match(test_name)
        if not match:
            return None, None
        wpt_path = match.group(1)
        path_in_wpt = match.group(2)
        return self.wpt_manifest(wpt_path).extract_fuzzy_metadata(path_in_wpt)

    def get_file_path_for_wpt_test(self, test_name):
        """Returns the real file path for the given WPT test.

        Or None if the test is not a WPT.
        """
        match = self.WPT_REGEX.match(test_name)
        if not match:
            return None
        wpt_path = match.group(1)
        path_in_wpt = match.group(2)
        file_path_in_wpt = self.wpt_manifest(wpt_path).file_path_for_test_url(
            path_in_wpt)
        if not file_path_in_wpt:
            return None
        return self._filesystem.join(wpt_path, file_path_in_wpt)

    def test_key(self, test_name):
        """Turns a test name into a pair of sublists: the natural sort key of the
        dirname, and the natural sort key of the basename.

        This can be used when sorting paths so that files in a directory.
        directory are kept together rather than being mixed in with files in
        subdirectories.
        """
        dirname, basename = self.split_test(test_name)
        return (self._natural_sort_key(dirname + self.TEST_PATH_SEPARATOR),
                self._natural_sort_key(basename))

    def _natural_sort_key(self, string_to_split):
        """Turns a string into a list of string and number chunks.

        For example: "z23a" -> ["z", 23, "a"]
        This can be used to implement "natural sort" order. See:
        http://www.codinghorror.com/blog/2007/12/sorting-for-humans-natural-sort-order.html
        http://nedbatchelder.com/blog/200712.html#e20071211T054956
        """

        def tryint(val):
            try:
                return int(val)
            except ValueError:
                return val

        return [tryint(chunk) for chunk in re.split(r'(\d+)', string_to_split)]

    def test_dirs(self):
        """Returns the list of top-level test directories."""
        web_tests_dir = self.web_tests_dir()
        fs = self._filesystem
        return [
            d for d in fs.listdir(web_tests_dir)
            if fs.isdir(fs.join(web_tests_dir, d))
        ]

    @memoized
    def test_isfile(self, test_name):
        """Returns True if the test name refers to an existing test file."""
        # Used by test_expectations.py to apply rules to a file.
        if self._filesystem.isfile(self.abspath_for_test(test_name)):
            return True
        base = self.lookup_virtual_test_base(test_name)
        return base and self._filesystem.isfile(self.abspath_for_test(base))

    @memoized
    def test_isdir(self, test_name):
        """Returns True if the test name refers to an existing directory of tests."""
        # Used by test_expectations.py to apply rules to whole directories.
        if self._filesystem.isdir(self.abspath_for_test(test_name)):
            return True
        base = self.lookup_virtual_test_base(test_name)
        return base and self._filesystem.isdir(self.abspath_for_test(base))

    @memoized
    def test_exists(self, test_name):
        """Returns True if the test name refers to an existing test directory or file."""
        # Used by lint_test_expectations.py to determine if an entry refers to a
        # valid test.
        if self.is_wpt_test(test_name):
            # A virtual WPT test must have valid virtual prefix and base.
            if test_name.startswith('virtual/'):
                return bool(self.lookup_virtual_test_base(test_name))
            # Otherwise treat any WPT test as existing regardless of their real
            # existence on the file system.
            # TODO(crbug.com/959958): Actually check existence of WPT tests.
            return True
        return self.test_isfile(test_name) or self.test_isdir(test_name)

    def split_test(self, test_name):
        """Splits a test name into the 'directory' part and the 'basename' part."""
        index = test_name.rfind(self.TEST_PATH_SEPARATOR)
        if index < 1:
            return ('', test_name)
        return (test_name[0:index], test_name[index:])

    def normalize_test_name(self, test_name):
        """Returns a normalized version of the test name or test directory."""
        if test_name.endswith('/'):
            return test_name
        if self.test_isdir(test_name):
            return test_name + '/'
        return test_name

    def driver_cmd_line(self):
        """Prints the DRT (DumpRenderTree) command that will be used."""
        return self.create_driver(0).cmd_line([])

    def update_baseline(self, baseline_path, data):
        """Updates the baseline for a test.

        Args:
            baseline_path: the actual path to use for baseline, not the path to
              the test. This function is used to update either generic or
              platform-specific baselines, but we can't infer which here.
            data: contents of the baseline.
        """
        self._filesystem.write_binary_file(baseline_path, data)

    def _path_from_chromium_base(self, *comps):
        return self._path_finder.path_from_chromium_base(*comps)

    def _perf_tests_dir(self):
        return self._path_finder.perf_tests_dir()

    def web_tests_dir(self):
        custom_web_tests_dir = self.get_option('layout_tests_directory')
        if custom_web_tests_dir:
            return self._filesystem.abspath(custom_web_tests_dir)
        return self._path_finder.web_tests_dir()

    def skips_test(self, test):
        """Checks whether the given test is skipped for this port.

        Returns True if the test is skipped because the port runs smoke tests
        only or because the test is marked as Skip in NeverFixTest (otherwise
        the test is only marked as Skip indicating a temporary skip).
        """
        return self.skipped_due_to_smoke_tests(
            test) or self.skipped_in_never_fix_tests(test)

    @memoized
    def _tests_from_file(self, filename):
        tests = set()
        file_contents = self._filesystem.read_text_file(filename)
        for line in file_contents.splitlines():
            line = line.strip()
            if line.startswith('#') or not line:
                continue
            tests.add(line)
        return tests

    def skipped_due_to_smoke_tests(self, test):
        """Checks if the test is skipped based on the set of Smoke tests.

        Returns True if this port runs only smoke tests, and the test is not
        in the smoke tests file; returns False otherwise.
        """
        if not self.default_smoke_test_only():
            return False
        smoke_test_filename = self.path_to_smoke_tests_file()
        if not self._filesystem.exists(smoke_test_filename):
            return False
        smoke_tests = self._tests_from_file(smoke_test_filename)
        return test not in smoke_tests

    def path_to_smoke_tests_file(self):
        return self._filesystem.join(self.web_tests_dir(), 'SmokeTests')

    def skipped_in_never_fix_tests(self, test):
        """Checks if the test is marked as Skip in NeverFixTests for this port.

        Skip in NeverFixTests indicate we will never fix the failure and
        permanently skip the test. Only Skip lines are allowed in NeverFixTests.
        Some lines in NeverFixTests are platform-specific.

        Note: this will not work with skipped directories. See also the same
        issue with update_all_test_expectations_files in test_importer.py.
        """
        # Note: The parsing logic here (reading the file, constructing a
        # parser, etc.) is very similar to blinkpy/w3c/test_copier.py.
        path = self.path_to_never_fix_tests_file()
        contents = self._filesystem.read_text_file(path)
        test_expectations = TestExpectations(tags=self.get_platform_tags())
        test_expectations.parse_tagged_list(contents)
        return ResultType.Skip in test_expectations.expectations_for(
            test).results

    def path_to_never_fix_tests_file(self):
        return self._filesystem.join(self.web_tests_dir(), 'NeverFixTests')

    def name(self):
        """Returns a name that uniquely identifies this particular type of port.

        This is the full port name including both base port name and version,
        and can be passed to PortFactory.get() to instantiate a port.
        """
        return self._name

    def operating_system(self):
        raise NotImplementedError

    def version(self):
        """Returns a string indicating the version of a given platform

        For example, "win10" or "trusty". This is used to help identify the
        exact port when parsing test expectations, determining search paths,
        and logging information.
        """
        return self._version

    def architecture(self):
        return self._architecture

    def python3_command(self):
        """Returns the correct command to use to run python3.

        This exists because Windows has inconsistent behavior between the bots
        and local developer machines, such that determining which python3 name
        to use is non-trivial. See https://crbug.com/1155616.

        Once blinkpy runs under python3, this can be removed in favour of
        callers using sys.executable.
        """
        if six.PY3:
            # Prefer sys.executable when the current script runs under python3.
            # The current script might be running with vpython3 and in that case
            # using the same executable will share the same virtualenv.
            return sys.executable
        return 'python3'

    def get_option(self, name, default_value=None):
        return getattr(self._options, name, default_value)

    def set_option_default(self, name, default_value):
        return self._options.ensure_value(name, default_value)

    def relative_test_filename(self, filename):
        """Returns a Unix-style path for a filename relative to web_tests.

        Ports may legitimately return absolute paths here if no relative path
        makes sense.
        """
        # Ports that run on windows need to override this method to deal with
        # filenames with backslashes in them.
        if filename.startswith(self.web_tests_dir()):
            return self.host.filesystem.relpath(filename, self.web_tests_dir())
        else:
            return self.host.filesystem.abspath(filename)

    @memoized
    def abspath_for_test(self, test_name):
        """Returns the full path to the file for a given test name.

        This is the inverse of relative_test_filename().
        """
        return self._filesystem.join(self.web_tests_dir(), test_name)

    @memoized
    def args_for_test(self, test_name):
        args = self._lookup_virtual_test_args(test_name)
        tracing_categories = self.get_option('enable_tracing')
        if tracing_categories:
            args.append('--trace-startup=' + tracing_categories)
            # Do not finish the trace until the test is finished.
            args.append('--trace-startup-duration=0')
            # Append the current time to the output file name to ensure that
            # the subsequent repetitions of the test do not overwrite older
            # trace files.
            current_time = time.strftime("%Y-%m-%d-%H-%M-%S")
            file_name = 'trace_layout_test_{}_{}.json'.format(
                self._filesystem.sanitize_filename(test_name), current_time)
            args.append('--trace-startup-file=' + file_name)
        return args

    @memoized
    def name_for_test(self, test_name):
        test_base = self.lookup_virtual_test_base(test_name)
        if test_base and not self._filesystem.exists(
                self.abspath_for_test(test_name)):
            return test_base
        return test_name

    def bot_test_times_path(self):
        # TODO(crbug.com/1030434): For the not_site_per_process_blink_web_tests step on linux,
        # an exception is raised when merging the bot times json files. This happens  whenever they
        # are outputted into the results directory. Temporarily we will return the bot times json
        # file relative to the target directory.
        return self._build_path('webkit_test_times', 'bot_times_ms.json')

    def results_directory(self):
        """Returns the absolute path directory which will store all web tests outputted
        files. It may include a sub directory for artifacts and it may store performance test results."""
        if not self._results_directory:
            option_val = self.get_option(
                'results_directory') or self.default_results_directory()
            assert not self._filesystem.basename(option_val) == 'layout-test-results', (
                'crbug.com/1026494, crbug.com/1027708: The layout-test-results sub directory should '
                'not be passed as part of the --results-directory command line argument.')
            self._results_directory = self._filesystem.abspath(option_val)
        return self._results_directory

    def artifacts_directory(self):
        """Returns path to artifacts sub directory of the results directory. This
        directory will store test artifacts, which may include actual and expected
        output from web tests."""
        return self._filesystem.join(self.results_directory(),
                                     ARTIFACTS_SUB_DIR)

    def perf_results_directory(self):
        return self.results_directory()

    def inspector_build_directory(self):
        return self._build_path('gen', 'third_party', 'devtools-frontend',
                                'src', 'front_end')

    def generated_sources_directory(self):
        return self._build_path('gen')

    def apache_config_directory(self):
        return self._path_finder.path_from_blink_tools('apache_config')

    def default_results_directory(self):
        """Returns the absolute path to the build directory."""
        return self._build_path()

    def setup_test_run(self):
        """Performs port-specific work at the beginning of a test run."""
        # Delete the disk cache if any to ensure a clean test run.
        dump_render_tree_binary_path = self._path_to_driver()
        cachedir = self._filesystem.dirname(dump_render_tree_binary_path)
        cachedir = self._filesystem.join(cachedir, 'cache')
        if self._filesystem.exists(cachedir):
            self._filesystem.rmtree(cachedir)

        if self._dump_reader:
            self._filesystem.maybe_make_directory(
                self._dump_reader.crash_dumps_directory())

    def num_workers(self, requested_num_workers):
        """Returns the number of available workers (possibly less than the number requested)."""
        return requested_num_workers

    def clean_up_test_run(self):
        """Performs port-specific work at the end of a test run."""
        if self._image_differ:
            self._image_differ.stop()
            self._image_differ = None

    def setup_environ_for_server(self):
        # We intentionally copy only a subset of the environment when
        # launching subprocesses to ensure consistent test results.
        clean_env = {}
        variables_to_copy = [
            'CHROME_DEVEL_SANDBOX',
            'CHROME_IPC_LOGGING',
            'ASAN_OPTIONS',
            'TSAN_OPTIONS',
            'MSAN_OPTIONS',
            'LSAN_OPTIONS',
            'UBSAN_OPTIONS',
            'VALGRIND_LIB',
            'VALGRIND_LIB_INNER',
            'TMPDIR',
        ]
        if 'TMPDIR' not in self.host.environ:
            self.host.environ['TMPDIR'] = tempfile.gettempdir()
        # CGIs are run directory-relative so they need an absolute TMPDIR
        self.host.environ['TMPDIR'] = self._filesystem.abspath(
            self.host.environ['TMPDIR'])
        if self.host.platform.is_linux() or self.host.platform.is_freebsd():
            variables_to_copy += [
                'XAUTHORITY', 'HOME', 'LANG', 'LD_LIBRARY_PATH',
                'DBUS_SESSION_BUS_ADDRESS', 'XDG_DATA_DIRS', 'XDG_RUNTIME_DIR'
            ]
            clean_env['DISPLAY'] = self.host.environ.get('DISPLAY', ':1')
        if self.host.platform.is_mac():
            variables_to_copy += [
                'HOME',
            ]
        if self.host.platform.is_win():
            variables_to_copy += [
                'PATH',
            ]

        for variable in variables_to_copy:
            if variable in self.host.environ:
                clean_env[variable] = self.host.environ[variable]

        for string_variable in self.get_option('additional_env_var', []):
            [name, value] = string_variable.split('=', 1)
            clean_env[name] = value

        return clean_env

    def show_results_html_file(self, results_filename):
        """Displays the given HTML file in a user's browser."""
        return self.host.user.open_url(
            abspath_to_uri(self.host.platform, results_filename))

    def create_driver(self, worker_number, no_timeout=False):
        """Returns a newly created Driver subclass for starting/stopping the
        test driver.
        """
        return self._driver_class()(self, worker_number, no_timeout=no_timeout)

    def requires_http_server(self):
        # Does the port require an HTTP server for running tests? This could
        # be the case when the tests aren't run on the host platform.
        return False

    def start_http_server(self,
                          additional_dirs,
                          number_of_drivers,
                          output_dir=''):
        """Start a web server. Raise an error if it can't start or is already running.

        Ports can stub this out if they don't need a web server to be running.
        """
        assert not self._http_server, 'Already running an http server.'
        output_dir = output_dir or self.artifacts_directory()
        server = apache_http.ApacheHTTP(
            self,
            output_dir,
            additional_dirs=additional_dirs,
            number_of_servers=(number_of_drivers * 4))
        server.start()
        self._http_server = server

    def start_websocket_server(self, output_dir=''):
        """Start a web server. Raise an error if it can't start or is already running.

        Ports can stub this out if they don't need a websocket server to be running.
        """
        assert not self._websocket_server, 'Already running a websocket server.'
        output_dir = output_dir or self.artifacts_directory()
        server = pywebsocket.PyWebSocket(
            self,
            output_dir,
            python_executable=self._options.python_executable)
        server.start()
        self._websocket_server = server

    @staticmethod
    def is_wpt_test(test):
        """Whether a test is considered a web-platform-tests test."""
        return Port.WPT_REGEX.match(test)

    @staticmethod
    def is_wpt_idlharness_test(test_file):
        """Returns whether a WPT test is (probably) an idlharness test.

        There are no rules in WPT that can be used to identify idlharness tests
        without examining the file contents (which would be expensive). This
        method utilizes a filename heuristic, based on the convention of
        including 'idlharness' in the appropriate test names.
        """
        match = Port.WPT_REGEX.match(test_file)
        if not match:
            return False
        filename = match.group(2).split('/')[-1]
        return 'idlharness' in filename

    @staticmethod
    def should_use_wptserve(test):
        return Port.is_wpt_test(test)

    def start_wptserve(self, output_dir=''):
        """Starts a WPT web server.

        Raises an error if it can't start or is already running.
        """
        assert not self._wpt_server, 'Already running a WPT server.'
        output_dir = output_dir or self.artifacts_directory()
        # We currently don't support any output mechanism for the WPT server.
        server = wptserve.WPTServe(self, output_dir)
        server.start()
        self._wpt_server = server

    def stop_wptserve(self):
        """Shuts down the WPT server if it is running."""
        if self._wpt_server:
            self._wpt_server.stop()
            self._wpt_server = None

    def http_server_requires_http_protocol_options_unsafe(self):
        httpd_path = self.path_to_apache()
        intentional_syntax_error = 'INTENTIONAL_SYNTAX_ERROR'
        # yapf: disable
        cmd = [
            httpd_path,
            '-t',
            '-f', self.path_to_apache_config_file(),
            '-C', 'ServerRoot "%s"' % self.apache_server_root(),
            '-C', 'HttpProtocolOptions Unsafe',
            '-C', intentional_syntax_error
        ]
        # yapf: enable
        env = self.setup_environ_for_server()

        def error_handler(err):
            pass

        output = self._executive.run_command(
            cmd, env=env, error_handler=error_handler)
        # If apache complains about the intentional error, it apparently
        # accepted the HttpProtocolOptions directive, and we should add it.
        return intentional_syntax_error in output

    def http_server_supports_ipv6(self):
        # Apache < 2.4 on win32 does not support IPv6.
        return not self.host.platform.is_win()

    def stop_http_server(self):
        """Shuts down the http server if it is running."""
        if self._http_server:
            self._http_server.stop()
            self._http_server = None

    def stop_websocket_server(self):
        """Shuts down the websocket server if it is running."""
        if self._websocket_server:
            self._websocket_server.stop()
            self._websocket_server = None

    #
    # TEST EXPECTATION-RELATED METHODS
    #

    def test_configuration(self):
        """Returns the current TestConfiguration for the port."""
        if not self._test_configuration:
            self._test_configuration = TestConfiguration(
                self._version, self._architecture,
                self._options.configuration.lower())
        return self._test_configuration

    # FIXME: Belongs on a Platform object.
    @memoized
    def all_test_configurations(self):
        """Returns a list of TestConfiguration instances, representing all available
        test configurations for this port.
        """
        return self._generate_all_test_configurations()

    # FIXME: Belongs on a Platform object.
    def configuration_specifier_macros(self):
        """Ports may provide a way to abbreviate configuration specifiers to conveniently
        refer to them as one term or alias specific values to more generic ones. For example:

        (vista, win7) -> win # Abbreviate all Windows versions into one namesake.
        (precise, trusty) -> linux  # Change specific name of Linux distro to a more generic term.

        Returns a dictionary, each key representing a macro term ('win', for example),
        and value being a list of valid configuration specifiers (such as ['vista', 'win7']).
        """
        return self.CONFIGURATION_SPECIFIER_MACROS

    def _generate_all_test_configurations(self):
        """Returns a sequence of the TestConfigurations the port supports."""
        # By default, we assume we want to test every graphics type in
        # every configuration on every system.
        test_configurations = []
        for version, architecture in self.ALL_SYSTEMS:
            for build_type in self.ALL_BUILD_TYPES:
                test_configurations.append(
                    TestConfiguration(version, architecture, build_type))
        return test_configurations

    def _flag_specific_expectations_path(self):
        config_name = self.flag_specific_config_name()
        if config_name:
            return self.path_to_flag_specific_expectations_file(config_name)

    def _flag_specific_baseline_search_path(self):
        config_name = self.flag_specific_config_name()
        if not config_name:
            return []
        flag_dir = self._filesystem.join(self.web_tests_dir(), 'flag-specific',
                                         config_name)
        platform_dirs = [
            self._filesystem.join(flag_dir, 'platform', platform_dir)
            for platform_dir in self.FALLBACK_PATHS[self.version()]
        ]
        return platform_dirs + [flag_dir]

    def expectations_dict(self):
        """Returns an OrderedDict of name -> expectations strings.

        The names are expected to be (but not required to be) paths in the
        filesystem. If the name is a path, the file can be considered updatable
        for things like rebaselining, so don't use names that are paths if
        they're not paths.

        Generally speaking the ordering should be files in the filesystem in
        cascade order (TestExpectations followed by Skipped, if the port honors
        both formats), then any built-in expectations (e.g., from compile-time
        exclusions), then --additional-expectations options.
        """
        # FIXME: rename this to test_expectations() once all the callers are
        # updated to know about the ordered dict.
        expectations = collections.OrderedDict()

        default_expectations_files = set(self.default_expectations_files())
        ignore_default = self.get_option('ignore_default_expectations', False)
        for path in self.used_expectations_files():
            is_default = path in default_expectations_files
            if ignore_default and is_default:
                continue
            path_exists = self._filesystem.exists(path)
            if is_default:
                if path_exists:
                    expectations[path] = self._filesystem.read_text_file(path)
            else:
                if path_exists:
                    _log.debug(
                        "reading additional_expectations from path '%s'", path)
                    expectations[path] = self._filesystem.read_text_file(path)
                else:
                    # TODO(rmhasan): Fix additional expectation paths for
                    # not_site_per_process_blink_web_tests, then change this
                    # back to raising exceptions for incorrect expectation
                    # paths.
                    _log.warning(
                        "additional_expectations path '%s' does not exist",
                        path)
        return expectations

    def all_expectations_dict(self):
        """Returns an OrderedDict of name -> expectations strings."""
        expectations = self.expectations_dict()

        flag_path = self._filesystem.join(self.web_tests_dir(),
                                          self.FLAG_EXPECTATIONS_PREFIX)
        if not self._filesystem.exists(flag_path):
            return expectations

        for (_, _, filenames) in self._filesystem.walk(flag_path):
            if 'README.txt' in filenames:
                filenames.remove('README.txt')
            if 'PRESUBMIT.py' in filenames:
                filenames.remove('PRESUBMIT.py')
            for filename in filenames:
                path = self._filesystem.join(flag_path, filename)
                try:
                    expectations[path] = self._filesystem.read_text_file(path)
                except UnicodeDecodeError:
                    _log.error('Failed to read expectations file: \'%s\'',
                               path)
                    raise

        return expectations

    def bot_expectations(self):
        if not self.get_option('ignore_flaky_tests'):
            return {}

        full_port_name = self.determine_full_port_name(
            self.host, self._options, self.port_name)
        builder_category = self.get_option('ignore_builder_category', 'layout')
        factory = BotTestExpectationsFactory(self.host.builders)
        # FIXME: This only grabs release builder's flakiness data. If we're running debug,
        # when we should grab the debug builder's data.
        expectations = factory.expectations_for_port(full_port_name,
                                                     builder_category)

        if not expectations:
            return {}

        ignore_mode = self.get_option('ignore_flaky_tests')
        if ignore_mode == 'very-flaky' or ignore_mode == 'maybe-flaky':
            return expectations.flakes_by_path(ignore_mode == 'very-flaky')
        if ignore_mode == 'unexpected':
            return expectations.unexpected_results_by_path()
        _log.warning("Unexpected ignore mode: '%s'.", ignore_mode)
        return {}

    def default_expectations_files(self):
        """Returns a list of paths to expectations files that apply by default.

        There are other "test expectations" files that may be applied if
        the --additional-expectations flag is passed; those aren't included
        here.
        """
        return filter(None, [
            self.path_to_generic_test_expectations_file(),
            self.path_to_webdriver_expectations_file(),
            self._filesystem.join(self.web_tests_dir(), 'NeverFixTests'),
            self._filesystem.join(self.web_tests_dir(),
                                  'StaleTestExpectations'),
            self._filesystem.join(self.web_tests_dir(), 'SlowTests')
        ])

    def used_expectations_files(self):
        """Returns a list of paths to expectation files that are used."""
        if self._used_expectation_files is None:
            self._used_expectation_files = list(
                self.default_expectations_files())
            flag_specific = self._flag_specific_expectations_path()
            if flag_specific:
                self._used_expectation_files.append(flag_specific)
            for path in self.get_option('additional_expectations', []):
                expanded_path = self._filesystem.expanduser(path)
                abs_path = self._filesystem.abspath(expanded_path)
                self._used_expectation_files.append(abs_path)
        return self._used_expectation_files

    def extra_expectations_files(self):
        """Returns a list of paths to test expectations not loaded by default.

        These paths are passed via --additional-expectations on some builders.
        """
        return [
            self._filesystem.join(self.web_tests_dir(), 'ASANExpectations'),
            self._filesystem.join(self.web_tests_dir(), 'LeakExpectations'),
            self._filesystem.join(self.web_tests_dir(), 'MSANExpectations'),
        ]

    @memoized
    def path_to_generic_test_expectations_file(self):
        return self._filesystem.join(self.web_tests_dir(), 'TestExpectations')

    @memoized
    def path_to_webdriver_expectations_file(self):
        return self._filesystem.join(self.web_tests_dir(),
                                     'WebDriverExpectations')

    def path_to_flag_specific_expectations_file(self, flag_specific):
        return self._filesystem.join(self.web_tests_dir(),
                                     self.FLAG_EXPECTATIONS_PREFIX,
                                     flag_specific)

    def repository_path(self):
        """Returns the repository path for the chromium code base."""
        return self._path_from_chromium_base('build')

    def default_configuration(self):
        return 'Release'

    def clobber_old_port_specific_results(self):
        pass

    # FIXME: This does not belong on the port object.
    @memoized
    def path_to_apache(self):
        """Returns the full path to the apache binary.

        This is needed only by ports that use the apache_http_server module.
        """
        raise NotImplementedError('Port.path_to_apache')

    def apache_server_root(self):
        """Returns the root that the apache binary is installed to.

        This is used for the ServerRoot directive.
        """
        executable = self.path_to_apache()
        return self._filesystem.dirname(self._filesystem.dirname(executable))

    def path_to_apache_config_file(self):
        """Returns the full path to the apache configuration file.

        If the WEBKIT_HTTP_SERVER_CONF_PATH environment variable is set, its
        contents will be used instead.

        This is needed only by ports that use the apache_http_server module.
        """
        config_file_from_env = self.host.environ.get(
            'WEBKIT_HTTP_SERVER_CONF_PATH')
        if config_file_from_env:
            if not self._filesystem.exists(config_file_from_env):
                raise IOError(
                    '%s was not found on the system' % config_file_from_env)
            return config_file_from_env

        config_file_name = self._apache_config_file_name_for_platform()
        return self._filesystem.join(self.apache_config_directory(),
                                     config_file_name)

    def _apache_version(self):
        config = self._executive.run_command([self.path_to_apache(), '-v'])
        # Log version including patch level.
        _log.debug(
            'Found apache version %s',
            re.sub(
                r'(?:.|\n)*Server version: Apache/(\d+\.\d+(?:\.\d+)?)(?:.|\n)*',
                r'\1', config))
        return re.sub(r'(?:.|\n)*Server version: Apache/(\d+\.\d+)(?:.|\n)*',
                      r'\1', config)

    def _apache_config_file_name_for_platform(self):
        if self.host.platform.is_linux():
            distribution = self.host.platform.linux_distribution()

            custom_configurations = ['arch', 'debian', 'fedora', 'redhat']
            if distribution in custom_configurations:
                return '%s-httpd-%s.conf' % (distribution,
                                             self._apache_version())

        return 'apache2-httpd-' + self._apache_version() + '.conf'

    def _path_to_driver(self, target=None):
        """Returns the full path to the test driver."""
        return self._build_path(target, self.driver_name())

    def _path_to_image_diff(self):
        """Returns the full path to the image_diff binary, or None if it is not available.

        This is likely used only by diff_image()
        """
        return self._build_path('image_diff')

    def _absolute_baseline_path(self, platform_dir):
        """Return the absolute path to the top of the baseline tree for a
        given platform directory.
        """
        return self._filesystem.join(self.web_tests_dir(), 'platform',
                                     platform_dir)

    def _driver_class(self):
        """Returns the port's driver implementation."""
        return driver.Driver

    def output_contains_sanitizer_messages(self, output):
        if not output:
            return None
        if (b'AddressSanitizer' in output) or (b'MemorySanitizer' in output):
            return True
        return False

    def _get_crash_log(self, name, pid, stdout, stderr, newer_than):
        if self.output_contains_sanitizer_messages(stderr):
            # Running the symbolizer script can take a lot of memory, so we need to
            # serialize access to it across all the concurrently running drivers.

            llvm_symbolizer_path = self._path_from_chromium_base(
                'third_party', 'llvm-build', 'Release+Asserts', 'bin',
                'llvm-symbolizer')
            if self._filesystem.exists(llvm_symbolizer_path):
                env = self.host.environ.copy()
                env['LLVM_SYMBOLIZER_PATH'] = llvm_symbolizer_path
            else:
                env = None
            sanitizer_filter_path = self._path_from_chromium_base(
                'tools', 'valgrind', 'asan', 'asan_symbolize.py')
            sanitizer_strip_path_prefix = 'Release/../../'
            if self._filesystem.exists(sanitizer_filter_path):
                stderr = self._executive.run_command([
                    'flock', sys.executable, sanitizer_filter_path,
                    sanitizer_strip_path_prefix
                ],
                                                     input=stderr,
                                                     decode_output=False,
                                                     env=env)

        name_str = name or '<unknown process name>'
        pid_str = str(pid or '<unknown>')

        # We require stdout and stderr to be bytestrings, not character strings.
        if stdout:
            stdout_lines = stdout.decode('utf8', 'replace').splitlines()
        else:
            stdout_lines = [u'<empty>']

        if stderr:
            stderr_lines = stderr.decode('utf8', 'replace').splitlines()
        else:
            stderr_lines = [u'<empty>']

        return (stderr,
                ('crash log for %s (pid %s):\n%s\n%s\n' %
                 (name_str, pid_str, '\n'.join(
                     ('STDOUT: ' + l) for l in stdout_lines), '\n'.join(
                         ('STDERR: ' + l)
                         for l in stderr_lines))).encode('utf8', 'replace'),
                self._get_crash_site(stderr_lines))

    def _get_crash_site(self, stderr_lines):
        # [blah:blah:blah:FATAL:
        prefix_re = r'\[[\w:/.]*FATAL:'
        # crash_file.ext(line)
        site_re = r'(?P<site>[\w_]*\.[\w_]*\(\d*\))'
        # ] blah failed
        suffix_re = r'\]\s*(Check failed|Security DCHECK failed)'
        pattern = re.compile(prefix_re + site_re + suffix_re)
        for line in stderr_lines:
            match = pattern.search(line)
            if match:
                return match.group('site')
        return None

    def look_for_new_crash_logs(self, crashed_processes, start_time):
        pass

    def look_for_new_samples(self, unresponsive_processes, start_time):
        pass

    def sample_process(self, name, pid):
        pass

    def virtual_test_suites(self):
        if self._virtual_test_suites is None:
            path_to_virtual_test_suites = self._filesystem.join(
                self.web_tests_dir(), 'VirtualTestSuites')
            assert self._filesystem.exists(path_to_virtual_test_suites), \
                path_to_virtual_test_suites + ' not found'
            try:
                test_suite_json = json.loads(
                    self._filesystem.read_text_file(
                        path_to_virtual_test_suites))
                self._virtual_test_suites = []
                for json_config in test_suite_json:
                    vts = VirtualTestSuite(**json_config)
                    if any(vts.full_prefix == s.full_prefix
                           for s in self._virtual_test_suites):
                        raise ValueError(
                            '{} contains entries with the same prefix: {!r}. Please combine them'
                            .format(path_to_virtual_test_suites, json_config))
                    self._virtual_test_suites.append(vts)
            except ValueError as error:
                raise ValueError('{} is not a valid JSON file: {}'.format(
                    path_to_virtual_test_suites, error))
        return self._virtual_test_suites

    def _all_virtual_tests(self, tests_by_dir):
        tests = []

        for suite in self.virtual_test_suites():
            if suite.bases:
                tests.extend(map(lambda x: suite.full_prefix + x,
                             self.real_tests_from_dict(suite.bases, tests_by_dir)))
        return tests

    def _get_bases_for_suite_with_paths(self, suite, paths):
        """Returns a set of bases of the virutual suite that are referenced by
        paths. E.g. given a virtual test suite `foo` with the following bases:
          bar/baz
          bar/quu
          qux
        and given paths of [virtual/foo/bar], this method would return
          [bar/baz, bar/quu]

        Given paths of [virtual/foo/bar/baz/test.html], the return would be
        [bar/baz]
        """

        real_paths = [p.replace(suite.full_prefix, '', 1) for p in paths \
            if p.startswith(suite.full_prefix)]
        # Test for paths that are under the suite's bases, so that we don't run
        # a non-existent test.
        bases = set()
        for real_path in real_paths:
            for base in suite.bases:
                if real_path.startswith(base) or base.startswith(real_path):
                    bases.add(base)

        return list(bases)

    def _virtual_tests_for_suite_with_paths(self, suite, paths):
        if not suite.bases:
            return []

        bases = self._get_bases_for_suite_with_paths(suite, paths)

        if not bases:
            return []

        tests = []
        tests.extend(
            map(lambda x: suite.full_prefix + x, self.real_tests(bases)))

        wpt_bases = []
        for base in bases:
            if any(base.startswith(wpt_dir) for wpt_dir in self.WPT_DIRS):
                wpt_bases.append(base)

        if wpt_bases:
            tests.extend(
                self._wpt_test_urls_matching_paths(
                    wpt_bases, [suite.full_prefix] * len(wpt_bases)))

        return tests

    def _virtual_tests_matching_paths(self, paths):
        tests = []
        normalized_paths = [self.normalize_test_name(p) for p in paths]
        for suite in self.virtual_test_suites():
            virtual_paths = [
                p for p in normalized_paths if p.startswith(suite.full_prefix)
            ]
            if not virtual_paths:
                continue
            for test in self._virtual_tests_for_suite_with_paths(
                    suite, virtual_paths):
                if any(test.startswith(p) for p in normalized_paths):
                    tests.append(test)

        if any(self._path_has_wildcard(path) for path in paths):
            _log.warning(
                'WARNING: Wildcards in paths are not supported for virtual test suites.'
            )

        return tests

    def _path_has_wildcard(self, path):
        return '*' in path

    def _wpt_test_urls_matching_paths(self, filter_paths, virtual_prefixes=[]):
        """Returns a set of paths that are tests to be run from the
        web-platform-test manifest files.

        filter_paths: A list of strings that are prefix matched against the
            list of tests in the WPT manifests. Only tests that match are returned.
        virtual_prefixes: A list of prefixes corresponding to paths in |filter_paths|.
            If present, each test path output should have its virtual prefix
            prepended to the resulting path to the test.
        """
        # Generate the manifest files if needed and then read them. Do this once
        # for this whole method as the file is large and generation/loading is
        # slow.
        wpts = [(wpt_path, self.wpt_manifest(wpt_path))
                for wpt_path in self.WPT_DIRS]

        tests = []
        # This walks through the set of paths where we should look for tests.
        # For each path, a map can be provided that we replace 'path' with in
        # the result.
        for filter_path, virtual_prefix in zip_longest(filter_paths,
                                                       virtual_prefixes):
            # This is to make sure "external[\\/]?" can also match to
            # external/wpt.
            # TODO(robertma): Remove this special case when external/wpt is
            # moved to wpt.
            if filter_path.rstrip('\\/').endswith('external'):
                filter_path = self._filesystem.join(filter_path, 'wpt')
            # '/' is used throughout this function instead of filesystem.sep as
            # the WPT manifest always uses '/' for paths (it is not OS
            # dependent).
            if self._filesystem.sep != '/':
                filter_path = filter_path.replace(self._filesystem.sep, '/')

            # Drop empty path components.
            filter_path = filter_path.replace('//', '/')

            # We now have in |filter_path| a path to an actual test directory or file
            # on disk, in unix format, relative to the root of the web_tests
            # directory.

            for wpt_path, wpt_manifest in wpts:
                # If the |filter_path| is not inside a WPT dir, then we will
                # match no tests in the manifest.
                if not filter_path.startswith(wpt_path):
                    continue
                # Drop the WPT prefix (including the joining '/') from |path|.
                filter_path_from_wpt = filter_path[len(wpt_path) + 1:]

                # An empty filter matches everything.
                if filter_path_from_wpt:
                    # If the filter is to a specific test file that ends with .js,
                    # we match that against tests with any extension by dropping
                    # the extension from the filter.
                    #
                    # Else, when matching a directory, ensure the filter ends in '/'
                    # to only match the exact directory name and not directories
                    # with the filter as a prefix.
                    if wpt_manifest.is_test_file(filter_path_from_wpt):
                        filter_path_from_wpt = re.sub(r'\.js$', '.',
                                                      filter_path_from_wpt)
                    elif not wpt_manifest.is_test_url(filter_path_from_wpt):
                        filter_path_from_wpt = filter_path_from_wpt.rstrip(
                            '/') + '/'

                # We now have a path to an actual test directory or file on
                # disk, in unix format, relative to the WPT directory.
                #
                # Look for all tests in the manifest that are under the relative
                # |filter_path_from_wpt|.
                for test_path_from_wpt in wpt_manifest.all_urls():
                    assert not test_path_from_wpt.startswith('/')
                    assert not test_path_from_wpt.endswith('/')

                    # Drop empty path components.
                    test_path_from_wpt = test_path_from_wpt.replace('//', '/')

                    if test_path_from_wpt.startswith(filter_path_from_wpt):
                        # The result is a test path from the root web test
                        # directory. If a |virtual_prefix| was given, we prepend
                        # that to the result.
                        prefix = virtual_prefix if virtual_prefix else ''
                        tests.append(prefix + wpt_path + '/' +
                                     test_path_from_wpt)
        return tests

    def _lookup_virtual_suite(self, test_name):
        if not test_name.startswith('virtual/'):
            return None
        for suite in self.virtual_test_suites():
            if test_name.startswith(suite.full_prefix):
                return suite
        return None

    def lookup_virtual_test_base(self, test_name):
        suite = self._lookup_virtual_suite(test_name)
        if not suite:
            return None
        assert test_name.startswith(suite.full_prefix)
        maybe_base = self.normalize_test_name(
            test_name[len(suite.full_prefix):])
        for base in suite.bases:
            normalized_base = self.normalize_test_name(base)
            if normalized_base.startswith(maybe_base) or maybe_base.startswith(
                    normalized_base):
                return maybe_base
        return None

    def _lookup_virtual_test_args(self, test_name):
        normalized_test_name = self.normalize_test_name(test_name)
        for suite in self.virtual_test_suites():
            if normalized_test_name.startswith(suite.full_prefix):
                return suite.args
        return []

    def _build_path(self, *comps):
        """Returns a path from the build directory."""
        return self._build_path_with_target(self._options.target, *comps)

    def _build_path_with_target(self, target, *comps):
        target = target or self.get_option('target')
        return self._filesystem.join(
            self._path_from_chromium_base(),
            self.get_option('build_directory') or 'out', target, *comps)

    def _check_driver_build_up_to_date(self, target):
        # FIXME: We should probably get rid of this check altogether as it has
        # outlived its usefulness in a GN-based world, but for the moment we
        # will just check things if they are using the standard Debug or Release
        # target directories.
        if target not in ('Debug', 'Release'):
            return True

        try:
            debug_path = self._path_to_driver('Debug')
            release_path = self._path_to_driver('Release')

            debug_mtime = self._filesystem.mtime(debug_path)
            release_mtime = self._filesystem.mtime(release_path)

            if (debug_mtime > release_mtime and target == 'Release'
                    or release_mtime > debug_mtime and target == 'Debug'):
                most_recent_binary = 'Release' if target == 'Debug' else 'Debug'
                _log.warning(
                    'You are running the %s binary. However the %s binary appears to be more recent. '
                    'Please pass --%s.', target, most_recent_binary,
                    most_recent_binary.lower())
                _log.warning('')
        # This will fail if we don't have both a debug and release binary.
        # That's fine because, in this case, we must already be running the
        # most up-to-date one.
        except OSError:
            pass
        return True

    def _get_font_files(self):
        """Returns list of font files that should be used by the test."""
        # TODO(sergeyu): Currently FONT_FILES is valid only on Linux. Make it
        # usable on other platforms if necessary.
        result = []
        for (font_dirs, font_file, package) in FONT_FILES:
            exists = False
            for font_dir in font_dirs:
                font_path = self._filesystem.join(font_dir, font_file)
                if not self._filesystem.isabs(font_path):
                    font_path = self._build_path(font_path)
                if self._check_file_exists(font_path, '', more_logging=False):
                    result.append(font_path)
                    exists = True
                    break
            if not exists:
                message = 'You are missing %s under %s.' % (font_file,
                                                            font_dirs)
                if package:
                    message += ' Try installing %s. See build instructions.' % package

                _log.error(message)
                raise TestRunException(exit_codes.SYS_DEPS_EXIT_STATUS,
                                       message)
        return result

    @staticmethod
    def split_webdriver_test_name(test_name):
        """Splits a WebDriver test name into a filename and a subtest name and
        returns both of them. E.g.

        test.py>>foo.html -> (test.py, foo.html)
        test.py           -> (test.py, None)
        """
        separator_index = test_name.find(Port.WEBDRIVER_SUBTEST_SEPARATOR)
        if separator_index == -1:
            return (test_name, None)
        webdriver_test_name = test_name[:separator_index]
        separator_len = len(Port.WEBDRIVER_SUBTEST_SEPARATOR)
        subtest_suffix = test_name[separator_index + separator_len:]
        return (webdriver_test_name, subtest_suffix)

    @staticmethod
    def add_webdriver_subtest_suffix(test_name, subtest_name):
        """Appends a subtest name to a WebDriver test name. E.g.

        (test.py, foo.html) -> test.py>>foo.html
        (test.py, None)     -> test.py
        """
        if subtest_name:
            return test_name + Port.WEBDRIVER_SUBTEST_SEPARATOR + subtest_name
        return test_name

    @staticmethod
    def split_webdriver_subtest_pytest_name(test_name):
        """Splits a WebDriver test name in pytest format into a filename and a subtest name and
        returns both of them. E.g.

        test.py::foo.html -> (test.py, foo.html)
        test.py           -> (test.py, None)
        """
        names_after_split = test_name.split(
            Port.WEBDRIVER_SUBTEST_PYTEST_SEPARATOR)

        assert len(names_after_split) <= 2, \
            "%s has a length greater than 2 after split by ::" % (test_name)
        if len(names_after_split) == 1:
            return (names_after_split[0], None)

        return (names_after_split[0], names_after_split[1])

    @staticmethod
    def add_webdriver_subtest_pytest_suffix(test_name, subtest_name):
        if subtest_name is None:
            return test_name
        return test_name + Port.WEBDRIVER_SUBTEST_PYTEST_SEPARATOR + subtest_name


class VirtualTestSuite(object):
    def __init__(self, prefix=None, bases=None, args=None):
        assert VALID_FILE_NAME_REGEX.match(prefix), \
            "Virtual test suite prefix '{}' contains invalid characters".format(prefix)
        assert isinstance(bases, list)
        assert args
        assert isinstance(args, list)
        self.full_prefix = 'virtual/' + prefix + '/'
        self.bases = bases
        self.args = args

    def __repr__(self):
        return "VirtualTestSuite('%s', %s, %s)" % (self.full_prefix,
                                                   self.bases, self.args)