"""Renewable certificates storage."""
import datetime
import logging
import os
import re

import configobj
import parsedatetime
import pytz
import six

import certbot
from certbot import cli
from certbot import constants
from certbot import crypto_util
from certbot import errors
from certbot import error_handler
from certbot import util

logger = logging.getLogger(__name__)

ALL_FOUR = ("cert", "privkey", "chain", "fullchain")
CURRENT_VERSION = util.get_strict_version(certbot.__version__)


def config_with_defaults(config=None):
    """Merge supplied config, if provided, on top of builtin defaults."""
    defaults_copy = configobj.ConfigObj(constants.RENEWER_DEFAULTS)
    defaults_copy.merge(config if config is not None else configobj.ConfigObj())
    return defaults_copy


def add_time_interval(base_time, interval, textparser=parsedatetime.Calendar()):
    """Parse the time specified time interval, and add it to the base_time

    The interval can be in the English-language format understood by
    parsedatetime, e.g., '10 days', '3 weeks', '6 months', '9 hours', or
    a sequence of such intervals like '6 months 1 week' or '3 days 12
    hours'. If an integer is found with no associated unit, it is
    interpreted by default as a number of days.

    :param datetime.datetime base_time: The time to be added with the interval.
    :param str interval: The time interval to parse.

    :returns: The base_time plus the interpretation of the time interval.
    :rtype: :class:`datetime.datetime`"""

    if interval.strip().isdigit():
        interval += " days"

    # try to use the same timezone, but fallback to UTC
    tzinfo = base_time.tzinfo or pytz.UTC

    return textparser.parseDT(interval, base_time, tzinfo=tzinfo)[0]


def write_renewal_config(o_filename, n_filename, target, relevant_data):
    """Writes a renewal config file with the specified name and values.

    :param str o_filename: Absolute path to the previous version of config file
    :param str n_filename: Absolute path to the new destination of config file
    :param dict target: Maps ALL_FOUR to their symlink paths
    :param dict relevant_data: Renewal configuration options to save

    :returns: Configuration object for the new config file
    :rtype: configobj.ConfigObj

    """
    config = configobj.ConfigObj(o_filename)
    config["version"] = certbot.__version__
    for kind in ALL_FOUR:
        config[kind] = target[kind]

    if "renewalparams" not in config:
        config["renewalparams"] = {}
        config.comments["renewalparams"] = ["",
                                            "Options used in "
                                            "the renewal process"]

    config["renewalparams"].update(relevant_data)

    for k in config["renewalparams"].keys():
        if k not in relevant_data:
            del config["renewalparams"][k]

    if "renew_before_expiry" not in config:
        default_interval = constants.RENEWER_DEFAULTS["renew_before_expiry"]
        config.initial_comment = ["renew_before_expiry = " + default_interval]

    # TODO: add human-readable comments explaining other available
    #       parameters
    logger.debug("Writing new config %s.", n_filename)
    with open(n_filename, "w") as f:
        config.write(outfile=f)
    return config


def update_configuration(lineagename, target, cli_config):
    """Modifies lineagename's config to contain the specified values.

    :param str lineagename: Name of the lineage being modified
    :param dict target: Maps ALL_FOUR to their symlink paths
    :param .RenewerConfiguration cli_config: parsed command line
        arguments

    :returns: Configuration object for the updated config file
    :rtype: configobj.ConfigObj

    """
    config_filename = os.path.join(
        cli_config.renewal_configs_dir, lineagename) + ".conf"
    temp_filename = config_filename + ".new"

    # If an existing tempfile exists, delete it
    if os.path.exists(temp_filename):
        os.unlink(temp_filename)

    # Save only the config items that are relevant to renewal
    values = relevant_values(vars(cli_config.namespace))
    write_renewal_config(config_filename, temp_filename, target, values)
    os.rename(temp_filename, config_filename)

    return configobj.ConfigObj(config_filename)


def get_link_target(link):
    """Get an absolute path to the target of link.

    :param str link: Path to a symbolic link

    :returns: Absolute path to the target of link
    :rtype: str

    """
    target = os.readlink(link)
    if not os.path.isabs(target):
        target = os.path.join(os.path.dirname(link), target)
    return os.path.abspath(target)


def _relevant(option):
    """
    Is this option one that could be restored for future renewal purposes?
    :param str option: the name of the option

    :rtype: bool
    """
    # The list() here produces a list of the plugin names as strings.
    from certbot import renewal
    from certbot.plugins import disco as plugins_disco
    plugins = list(plugins_disco.PluginsRegistry.find_all())
    return (option in renewal.STR_CONFIG_ITEMS
            or option in renewal.INT_CONFIG_ITEMS
            or any(option.startswith(x + "_") for x in plugins))


def relevant_values(all_values):
    """Return a new dict containing only items relevant for renewal.

    :param dict all_values: The original values.

    :returns: A new dictionary containing items that can be used in renewal.
    :rtype dict:

    """
    return dict(
        (option, value)
        for option, value in six.iteritems(all_values)
        if _relevant(option) and cli.option_was_set(option, value))


class RenewableCert(object):  # pylint: disable=too-many-instance-attributes
    """Renewable certificate.

    Represents a lineage of certificates that is under the management of
    Certbot, indicated by the existence of an associated renewal
    configuration file.

    Note that the notion of "current version" for a lineage is
    maintained on disk in the structure of symbolic links, and is not
    explicitly stored in any instance variable in this object. The
    RenewableCert object is able to determine information about the
    current (or other) version by accessing data on disk, but does not
    inherently know any of this information except by examining the
    symbolic links as needed. The instance variables mentioned below
    point to symlinks that reflect the notion of "current version" of
    each managed object, and it is these paths that should be used when
    configuring servers to use the certificate managed in a lineage.
    These paths are normally within the "live" directory, and their
    symlink targets -- the actual cert files -- are normally found
    within the "archive" directory.

    :ivar str cert: The path to the symlink representing the current
        version of the certificate managed by this lineage.
    :ivar str privkey: The path to the symlink representing the current
        version of the private key managed by this lineage.
    :ivar str chain: The path to the symlink representing the current version
        of the chain managed by this lineage.
    :ivar str fullchain: The path to the symlink representing the
        current version of the fullchain (combined chain and cert)
        managed by this lineage.
    :ivar configobj.ConfigObj configuration: The renewal configuration
        options associated with this lineage, obtained from parsing the
        renewal configuration file and/or systemwide defaults.

    """
    def __init__(self, config_filename, cli_config):
        """Instantiate a RenewableCert object from an existing lineage.

        :param str config_filename: the path to the renewal config file
            that defines this lineage.
        :param .RenewerConfiguration: parsed command line arguments

        :raises .CertStorageError: if the configuration file's name didn't end
            in ".conf", or the file is missing or broken.

        """
        self.cli_config = cli_config
        if not config_filename.endswith(".conf"):
            raise errors.CertStorageError(
                "renewal config file name must end in .conf")
        self.lineagename = os.path.basename(
            config_filename[:-len(".conf")])

        # self.configuration should be used to read parameters that
        # may have been chosen based on default values from the
        # systemwide renewal configuration; self.configfile should be
        # used to make and save changes.
        try:
            self.configfile = configobj.ConfigObj(config_filename)
        except configobj.ConfigObjError:
            raise errors.CertStorageError(
                "error parsing {0}".format(config_filename))
        # TODO: Do we actually use anything from defaults and do we want to
        #       read further defaults from the systemwide renewal configuration
        #       file at this stage?
        self.configuration = config_with_defaults(self.configfile)

        if not all(x in self.configuration for x in ALL_FOUR):
            raise errors.CertStorageError(
                "renewal config file {0} is missing a required "
                "file reference".format(self.configfile))

        conf_version = self.configuration.get("version")
        if (conf_version is not None and
                util.get_strict_version(conf_version) > CURRENT_VERSION):
            logger.warning(
                "Attempting to parse the version %s renewal configuration "
                "file found at %s with version %s of Certbot. This might not "
                "work.", conf_version, config_filename, certbot.__version__)

        self.cert = self.configuration["cert"]
        self.privkey = self.configuration["privkey"]
        self.chain = self.configuration["chain"]
        self.fullchain = self.configuration["fullchain"]
        self.live_dir = os.path.dirname(self.cert)

        self._fix_symlinks()
        self._check_symlinks()

    def _check_symlinks(self):
        """Raises an exception if a symlink doesn't exist"""
        for kind in ALL_FOUR:
            link = getattr(self, kind)
            if not os.path.islink(link):
                raise errors.CertStorageError(
                    "expected {0} to be a symlink".format(link))
            target = get_link_target(link)
            if not os.path.exists(target):
                raise errors.CertStorageError("target {0} of symlink {1} does "
                                              "not exist".format(target, link))

    def _consistent(self):
        """Are the files associated with this lineage self-consistent?

        :returns: Whether the files stored in connection with this
            lineage appear to be correct and consistent with one
            another.
        :rtype: bool

        """
        # Each element must be referenced with an absolute path
        for x in (self.cert, self.privkey, self.chain, self.fullchain):
            if not os.path.isabs(x):
                logger.debug("Element %s is not referenced with an "
                             "absolute path.", x)
                return False

        # Each element must exist and be a symbolic link
        for x in (self.cert, self.privkey, self.chain, self.fullchain):
            if not os.path.islink(x):
                logger.debug("Element %s is not a symbolic link.", x)
                return False
        for kind in ALL_FOUR:
            link = getattr(self, kind)
            target = get_link_target(link)

            # Each element's link must point within the cert lineage's
            # directory within the official archive directory
            desired_directory = os.path.join(
                self.cli_config.archive_dir, self.lineagename)
            if not os.path.samefile(os.path.dirname(target),
                                    desired_directory):
                logger.debug("Element's link does not point within the "
                             "cert lineage's directory within the "
                             "official archive directory. Link: %s, "
                             "target directory: %s, "
                             "archive directory: %s.",
                             link, os.path.dirname(target), desired_directory)
                return False

            # The link must point to a file that exists
            if not os.path.exists(target):
                logger.debug("Link %s points to file %s that does not exist.",
                             link, target)
                return False

            # The link must point to a file that follows the archive
            # naming convention
            pattern = re.compile(r"^{0}([0-9]+)\.pem$".format(kind))
            if not pattern.match(os.path.basename(target)):
                logger.debug("%s does not follow the archive naming "
                             "convention.", target)
                return False

            # It is NOT required that the link's target be a regular
            # file (it may itself be a symlink). But we should probably
            # do a recursive check that ultimately the target does
            # exist?
        # XXX: Additional possible consistency checks (e.g.
        #      cryptographic validation of the chain being a chain,
        #      the chain matching the cert, and the cert matching
        #      the subject key)
        # XXX: All four of the targets are in the same directory
        #      (This check is redundant with the check that they
        #      are all in the desired directory!)
        #      len(set(os.path.basename(self.current_target(x)
        #      for x in ALL_FOUR))) == 1
        return True

    def _fix(self):
        """Attempt to fix defects or inconsistencies in this lineage.

        .. todo:: Currently unimplemented.

        """
        # TODO: Figure out what kinds of fixes are possible.  For
        #       example, checking if there is a valid version that
        #       we can update the symlinks to.  (Maybe involve
        #       parsing keys and certs to see if they exist and
        #       if a key corresponds to the subject key of a cert?)

    # TODO: In general, the symlink-reading functions below are not
    #       cautious enough about the possibility that links or their
    #       targets may not exist.  (This shouldn't happen, but might
    #       happen as a result of random tampering by a sysadmin, or
    #       filesystem errors, or crashes.)

    def _previous_symlinks(self):
        """Returns the kind and path of all symlinks used in recovery.

        :returns: list of (kind, symlink) tuples
        :rtype: list

        """
        previous_symlinks = []
        for kind in ALL_FOUR:
            link_dir = os.path.dirname(getattr(self, kind))
            link_base = "previous_{0}.pem".format(kind)
            previous_symlinks.append((kind, os.path.join(link_dir, link_base)))

        return previous_symlinks

    def _fix_symlinks(self):
        """Fixes symlinks in the event of an incomplete version update.

        If there is no problem with the current symlinks, this function
        has no effect.

        """
        previous_symlinks = self._previous_symlinks()
        if all(os.path.exists(link[1]) for link in previous_symlinks):
            for kind, previous_link in previous_symlinks:
                current_link = getattr(self, kind)
                if os.path.lexists(current_link):
                    os.unlink(current_link)
                os.symlink(os.readlink(previous_link), current_link)

        for _, link in previous_symlinks:
            if os.path.exists(link):
                os.unlink(link)

    def current_target(self, kind):
        """Returns full path to which the specified item currently points.

        :param str kind: the lineage member item ("cert", "privkey",
            "chain", or "fullchain")

        :returns: The path to the current version of the specified
            member.
        :rtype: str or None

        """
        if kind not in ALL_FOUR:
            raise errors.CertStorageError("unknown kind of item")
        link = getattr(self, kind)
        if not os.path.exists(link):
            logger.debug("Expected symlink %s for %s does not exist.",
                         link, kind)
            return None
        return get_link_target(link)

    def current_version(self, kind):
        """Returns numerical version of the specified item.

        For example, if kind is "chain" and the current chain link
        points to a file named "chain7.pem", returns the integer 7.

        :param str kind: the lineage member item ("cert", "privkey",
            "chain", or "fullchain")

        :returns: the current version of the specified member.
        :rtype: int

        """
        if kind not in ALL_FOUR:
            raise errors.CertStorageError("unknown kind of item")
        pattern = re.compile(r"^{0}([0-9]+)\.pem$".format(kind))
        target = self.current_target(kind)
        if target is None or not os.path.exists(target):
            logger.debug("Current-version target for %s "
                         "does not exist at %s.", kind, target)
            target = ""
        matches = pattern.match(os.path.basename(target))
        if matches:
            return int(matches.groups()[0])
        else:
            logger.debug("No matches for target %s.", kind)
            return None

    def version(self, kind, version):
        """The filename that corresponds to the specified version and kind.

        .. warning:: The specified version may not exist in this
           lineage. There is no guarantee that the file path returned
           by this method actually exists.

        :param str kind: the lineage member item ("cert", "privkey",
            "chain", or "fullchain")
        :param int version: the desired version

        :returns: The path to the specified version of the specified member.
        :rtype: str

        """
        if kind not in ALL_FOUR:
            raise errors.CertStorageError("unknown kind of item")
        where = os.path.dirname(self.current_target(kind))
        return os.path.join(where, "{0}{1}.pem".format(kind, version))

    def available_versions(self, kind):
        """Which alternative versions of the specified kind of item exist?

        The archive directory where the current version is stored is
        consulted to obtain the list of alternatives.

        :param str kind: the lineage member item (
            ``cert``, ``privkey``, ``chain``, or ``fullchain``)

        :returns: all of the version numbers that currently exist
        :rtype: `list` of `int`

        """
        if kind not in ALL_FOUR:
            raise errors.CertStorageError("unknown kind of item")
        where = os.path.dirname(self.current_target(kind))
        files = os.listdir(where)
        pattern = re.compile(r"^{0}([0-9]+)\.pem$".format(kind))
        matches = [pattern.match(f) for f in files]
        return sorted([int(m.groups()[0]) for m in matches if m])

    def newest_available_version(self, kind):
        """Newest available version of the specified kind of item?

        :param str kind: the lineage member item (``cert``,
            ``privkey``, ``chain``, or ``fullchain``)

        :returns: the newest available version of this member
        :rtype: int

        """
        return max(self.available_versions(kind))

    def latest_common_version(self):
        """Newest version for which all items are available?

        :returns: the newest available version for which all members
            (``cert, ``privkey``, ``chain``, and ``fullchain``) exist
        :rtype: int

        """
        # TODO: this can raise CertStorageError if there is no version overlap
        #       (it should probably return None instead)
        # TODO: this can raise a spurious AttributeError if the current
        #       link for any kind is missing (it should probably return None)
        versions = [self.available_versions(x) for x in ALL_FOUR]
        return max(n for n in versions[0] if all(n in v for v in versions[1:]))

    def next_free_version(self):
        """Smallest version newer than all full or partial versions?

        :returns: the smallest version number that is larger than any
            version of any item currently stored in this lineage
        :rtype: int

        """
        # TODO: consider locking/mutual exclusion between updating processes
        # This isn't self.latest_common_version() + 1 because we don't want
        # collide with a version that might exist for one file type but not
        # for the others.
        return max(self.newest_available_version(x) for x in ALL_FOUR) + 1

    def has_pending_deployment(self):
        """Is there a later version of all of the managed items?

        :returns: ``True`` if there is a complete version of this
            lineage with a larger version number than the current
            version, and ``False`` otherwis
        :rtype: bool

        """
        # TODO: consider whether to assume consistency or treat
        #       inconsistent/consistent versions differently
        smallest_current = min(self.current_version(x) for x in ALL_FOUR)
        return smallest_current < self.latest_common_version()

    def _update_link_to(self, kind, version):
        """Make the specified item point at the specified version.

        (Note that this method doesn't verify that the specified version
        exists.)

        :param str kind: the lineage member item ("cert", "privkey",
            "chain", or "fullchain")
        :param int version: the desired version

        """
        if kind not in ALL_FOUR:
            raise errors.CertStorageError("unknown kind of item")
        link = getattr(self, kind)
        filename = "{0}{1}.pem".format(kind, version)
        # Relative rather than absolute target directory
        target_directory = os.path.dirname(os.readlink(link))
        # TODO: it could be safer to make the link first under a temporary
        #       filename, then unlink the old link, then rename the new link
        #       to the old link; this ensures that this process is able to
        #       create symlinks.
        # TODO: we might also want to check consistency of related links
        #       for the other corresponding items
        os.unlink(link)
        os.symlink(os.path.join(target_directory, filename), link)

    def update_all_links_to(self, version):
        """Change all member objects to point to the specified version.

        :param int version: the desired version

        """
        with error_handler.ErrorHandler(self._fix_symlinks):
            previous_links = self._previous_symlinks()
            for kind, link in previous_links:
                os.symlink(self.current_target(kind), link)

            for kind in ALL_FOUR:
                self._update_link_to(kind, version)

            for _, link in previous_links:
                os.unlink(link)

    def names(self, version=None):
        """What are the subject names of this certificate?

        (If no version is specified, use the current version.)

        :param int version: the desired version number
        :returns: the subject names
        :rtype: `list` of `str`
        :raises .CertStorageError: if could not find cert file.

        """
        if version is None:
            target = self.current_target("cert")
        else:
            target = self.version("cert", version)
        if target is None:
            raise errors.CertStorageError("could not find cert file")
        with open(target) as f:
            return crypto_util.get_names_from_cert(f.read())

    def autodeployment_is_enabled(self):
        """Is automatic deployment enabled for this cert?

        If autodeploy is not specified, defaults to True.

        :returns: True if automatic deployment is enabled
        :rtype: bool

        """
        return ("autodeploy" not in self.configuration or
                self.configuration.as_bool("autodeploy"))

    def should_autodeploy(self, interactive=False):
        """Should this lineage now automatically deploy a newer version?

        This is a policy question and does not only depend on whether
        there is a newer version of the cert. (This considers whether
        autodeployment is enabled, whether a relevant newer version
        exists, and whether the time interval for autodeployment has
        been reached.)

        :param bool interactive: set to True to examine the question
            regardless of whether the renewal configuration allows
            automated deployment (for interactive use). Default False.

        :returns: whether the lineage now ought to autodeploy an
            existing newer cert version
        :rtype: bool

        """
        if interactive or self.autodeployment_is_enabled():
            if self.has_pending_deployment():
                interval = self.configuration.get("deploy_before_expiry",
                                                  "5 days")
                expiry = crypto_util.notAfter(self.current_target("cert"))
                now = pytz.UTC.fromutc(datetime.datetime.utcnow())
                if expiry < add_time_interval(now, interval):
                    return True
        return False

    def ocsp_revoked(self, version=None):
        # pylint: disable=no-self-use,unused-argument
        """Is the specified cert version revoked according to OCSP?

        Also returns True if the cert version is declared as intended
        to be revoked according to Let's Encrypt OCSP extensions.
        (If no version is specified, uses the current version.)

        This method is not yet implemented and currently always returns
        False.

        :param int version: the desired version number

        :returns: whether the certificate is or will be revoked
        :rtype: bool

        """
        # XXX: This query and its associated network service aren't
        # implemented yet, so we currently return False (indicating that the
        # certificate is not revoked).
        return False

    def autorenewal_is_enabled(self):
        """Is automatic renewal enabled for this cert?

        If autorenew is not specified, defaults to True.

        :returns: True if automatic renewal is enabled
        :rtype: bool

        """
        return ("autorenew" not in self.configuration or
                self.configuration.as_bool("autorenew"))

    def should_autorenew(self, interactive=False):
        """Should we now try to autorenew the most recent cert version?

        This is a policy question and does not only depend on whether
        the cert is expired. (This considers whether autorenewal is
        enabled, whether the cert is revoked, and whether the time
        interval for autorenewal has been reached.)

        Note that this examines the numerically most recent cert version,
        not the currently deployed version.

        :param bool interactive: set to True to examine the question
            regardless of whether the renewal configuration allows
            automated renewal (for interactive use). Default False.

        :returns: whether an attempt should now be made to autorenew the
            most current cert version in this lineage
        :rtype: bool

        """
        if interactive or self.autorenewal_is_enabled():
            # Consider whether to attempt to autorenew this cert now

            # Renewals on the basis of revocation
            if self.ocsp_revoked(self.latest_common_version()):
                logger.debug("Should renew, certificate is revoked.")
                return True

            # Renews some period before expiry time
            default_interval = constants.RENEWER_DEFAULTS["renew_before_expiry"]
            interval = self.configuration.get("renew_before_expiry", default_interval)
            expiry = crypto_util.notAfter(self.version(
                "cert", self.latest_common_version()))
            now = pytz.UTC.fromutc(datetime.datetime.utcnow())
            if expiry < add_time_interval(now, interval):
                logger.debug("Should renew, less than %s before certificate "
                             "expiry %s.", interval,
                             expiry.strftime("%Y-%m-%d %H:%M:%S %Z"))
                return True
        return False

    @classmethod
    def new_lineage(cls, lineagename, cert, privkey, chain, cli_config):
        # pylint: disable=too-many-locals
        """Create a new certificate lineage.

        Attempts to create a certificate lineage -- enrolled for
        potential future renewal -- with the (suggested) lineage name
        lineagename, and the associated cert, privkey, and chain (the
        associated fullchain will be created automatically). Optional
        configurator and renewalparams record the configuration that was
        originally used to obtain this cert, so that it can be reused
        later during automated renewal.

        Returns a new RenewableCert object referring to the created
        lineage. (The actual lineage name, as well as all the relevant
        file paths, will be available within this object.)

        :param str lineagename: the suggested name for this lineage
            (normally the current cert's first subject DNS name)
        :param str cert: the initial certificate version in PEM format
        :param str privkey: the private key in PEM format
        :param str chain: the certificate chain in PEM format
        :param .RenewerConfiguration cli_config: parsed command line
            arguments

        :returns: the newly-created RenewalCert object
        :rtype: :class:`storage.renewableCert`

        """

        # Examine the configuration and find the new lineage's name
        for i in (cli_config.renewal_configs_dir, cli_config.archive_dir,
                  cli_config.live_dir):
            if not os.path.exists(i):
                os.makedirs(i, 0o700)
                logger.debug("Creating directory %s.", i)
        config_file, config_filename = util.unique_lineage_name(
            cli_config.renewal_configs_dir, lineagename)
        if not config_filename.endswith(".conf"):
            raise errors.CertStorageError(
                "renewal config file name must end in .conf")

        # Determine where on disk everything will go
        # lineagename will now potentially be modified based on which
        # renewal configuration file could actually be created
        lineagename = os.path.basename(config_filename)[:-len(".conf")]
        archive = os.path.join(cli_config.archive_dir, lineagename)
        live_dir = os.path.join(cli_config.live_dir, lineagename)
        if os.path.exists(archive):
            raise errors.CertStorageError(
                "archive directory exists for " + lineagename)
        if os.path.exists(live_dir):
            raise errors.CertStorageError(
                "live directory exists for " + lineagename)
        os.mkdir(archive)
        os.mkdir(live_dir)
        logger.debug("Archive directory %s and live "
                     "directory %s created.", archive, live_dir)
        relative_archive = os.path.join("..", "..", "archive", lineagename)

        # Put the data into the appropriate files on disk
        target = dict([(kind, os.path.join(live_dir, kind + ".pem"))
                       for kind in ALL_FOUR])
        for kind in ALL_FOUR:
            os.symlink(os.path.join(relative_archive, kind + "1.pem"),
                       target[kind])
        with open(target["cert"], "w") as f:
            logger.debug("Writing certificate to %s.", target["cert"])
            f.write(cert)
        with open(target["privkey"], "w") as f:
            logger.debug("Writing private key to %s.", target["privkey"])
            f.write(privkey)
            # XXX: Let's make sure to get the file permissions right here
        with open(target["chain"], "w") as f:
            logger.debug("Writing chain to %s.", target["chain"])
            f.write(chain)
        with open(target["fullchain"], "w") as f:
            # assumes that OpenSSL.crypto.dump_certificate includes
            # ending newline character
            logger.debug("Writing full chain to %s.", target["fullchain"])
            f.write(cert + chain)

        # Document what we've done in a new renewal config file
        config_file.close()

        # Save only the config items that are relevant to renewal
        values = relevant_values(vars(cli_config.namespace))

        new_config = write_renewal_config(config_filename, config_filename, target, values)
        return cls(new_config.filename, cli_config)

    def save_successor(self, prior_version, new_cert,
                       new_privkey, new_chain, cli_config):
        """Save new cert and chain as a successor of a prior version.

        Returns the new version number that was created.

        .. note:: this function does NOT update links to deploy this
                  version

        :param int prior_version: the old version to which this version
            is regarded as a successor (used to choose a privkey, if the
            key has not changed, but otherwise this information is not
            permanently recorded anywhere)
        :param str new_cert: the new certificate, in PEM format
        :param str new_privkey: the new private key, in PEM format,
            or ``None``, if the private key has not changed
        :param str new_chain: the new chain, in PEM format
        :param .RenewerConfiguration cli_config: parsed command line
            arguments

        :returns: the new version number that was created
        :rtype: int

        """
        # XXX: assumes official archive location rather than examining links
        # XXX: consider using os.open for availability of os.O_EXCL
        # XXX: ensure file permissions are correct; also create directories
        #      if needed (ensuring their permissions are correct)
        # Figure out what the new version is and hence where to save things

        self.cli_config = cli_config
        target_version = self.next_free_version()
        archive = self.cli_config.archive_dir
        # XXX if anyone ever moves a renewal configuration file, this will
        # break... perhaps prefix should be the dirname of the previous
        # cert.pem?
        prefix = os.path.join(archive, self.lineagename)
        target = dict(
            [(kind,
              os.path.join(prefix, "{0}{1}.pem".format(kind, target_version)))
             for kind in ALL_FOUR])

        # Distinguish the cases where the privkey has changed and where it
        # has not changed (in the latter case, making an appropriate symlink
        # to an earlier privkey version)
        if new_privkey is None:
            # The behavior below keeps the prior key by creating a new
            # symlink to the old key or the target of the old key symlink.
            old_privkey = os.path.join(
                prefix, "privkey{0}.pem".format(prior_version))
            if os.path.islink(old_privkey):
                old_privkey = os.readlink(old_privkey)
            else:
                old_privkey = "privkey{0}.pem".format(prior_version)
            logger.debug("Writing symlink to old private key, %s.", old_privkey)
            os.symlink(old_privkey, target["privkey"])
        else:
            with open(target["privkey"], "w") as f:
                logger.debug("Writing new private key to %s.", target["privkey"])
                f.write(new_privkey)

        # Save everything else
        with open(target["cert"], "w") as f:
            logger.debug("Writing certificate to %s.", target["cert"])
            f.write(new_cert)
        with open(target["chain"], "w") as f:
            logger.debug("Writing chain to %s.", target["chain"])
            f.write(new_chain)
        with open(target["fullchain"], "w") as f:
            logger.debug("Writing full chain to %s.", target["fullchain"])
            f.write(new_cert + new_chain)

        symlinks = dict((kind, self.configuration[kind]) for kind in ALL_FOUR)
        # Update renewal config file
        self.configfile = update_configuration(
            self.lineagename, symlinks, cli_config)
        self.configuration = config_with_defaults(self.configfile)

        return target_version
