#
# Copyright (C) 2020  FreeIPA Contributors see COPYING for license
#

# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

"""This tool prepares then sends email notifications to users
   whose passwords are expiring in the near future.
"""


from __future__ import absolute_import, print_function

import ast
import grp
import json
import os
import pwd
import logging
import smtplib

from collections import deque
from datetime import datetime, timedelta
from email.utils import formataddr, formatdate
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.header import Header
from email.utils import make_msgid

from ipaclient.install.client import is_ipa_client_installed
from ipaplatform.paths import paths
from ipalib import api, errors
from ipalib.install import sysrestore
from ipapython import admintool, ipaldap
from ipapython.dn import DN

from jinja2 import Environment, FileSystemLoader, TemplateSyntaxError


EPN_CONF = "/etc/ipa/epn.conf"
EPN_CONFIG = {
    "smtp_server": "localhost",
    "smtp_port": 25,
    "smtp_user": None,
    "smtp_password": None,
    "smtp_timeout": 60,
    "smtp_security": "none",
    "smtp_admin": "root@localhost",
    "mail_from": None,
    "notify_ttls": "28,14,7,3,1",
    "msg_charset": "utf8",
    "msg_subtype": "plain",
    "msg_subject": "Your password will expire soon.",
}

logger = logging.getLogger(__name__)


def drop_privileges(new_username="daemon", new_groupname="daemon"):
    """Drop privileges, defaults to daemon:daemon.
    """
    try:
        if os.getuid() != 0:
            return

        os.setgroups([])
        os.setgid(pwd.getpwnam(new_username).pw_uid)
        os.setuid(grp.getgrnam(new_groupname).gr_gid)

        if os.getuid() == 0:
            raise Exception()

        logger.debug(
            "Dropped privileges to user=%s, group=%s",
            new_username,
            new_groupname,
        )

    except Exception as e:
        logger.error(
            "Failed to drop privileges to %s, %s: %s",
            new_username,
            new_groupname,
            e,
        )


class EPNUserList:
    """Maintains a list of users whose passwords are expiring.
       Provides add(), check(), pop(), and json_print().
       From the outside, the list is considered always sorted:
       * displaying the list results in a sorted JSON representation thereof
       * pop() returns the "most urgent" item from the list.
       Internal implementation notes:
       * Uses a deque instead of a list for efficiency reasons
       * all add()-style methods MUST set _sorted to False.
       * all print() and pop-like methods MUST call _sort() first.
    """

    def __init__(self):
        self._sorted = False
        self._expiring_password_user_dq = deque()

    def __bool__(self):
        """If it quacks like a container...
        """
        return bool(self._expiring_password_user_dq)

    def __len__(self):
        """Return len(self)."""
        return len(self._expiring_password_user_dq)

    def add(self, entry):
        """Parses and appends an LDAP user entry with the uid, cn,
           givenname, sn, krbpasswordexpiration and mail attributes.
        """
        try:
            self._sorted = False
            self._expiring_password_user_dq.append(
                dict(
                    uid=str(entry["uid"].pop(0)),
                    cn=str(entry["cn"].pop(0)),
                    givenname=str(entry.get("givenname", "")),
                    sn=str(entry["sn"].pop(0)),
                    krbpasswordexpiration=str(
                        entry["krbpasswordexpiration"].pop(0)
                    ),
                    mail=str(entry["mail"]),
                )
            )
        except IndexError as e:
            logger.info("IPA-EPN: Could not parse entry: %s", e)

    def pop(self):
        """Returns the "most urgent" user to notify.
           In fact: popleft()
        """
        self._sort()
        try:
            return self._expiring_password_user_dq.popleft()
        except IndexError:
            return False

    def check(self):
        self.json_print(really_print=False)

    def json_print(self, really_print=True):
        """Dump self._expiring_password_user_dq to JSON.
           Check that the result can be re-rencoded to UTF-8.
           If really_print, print the result.
        """
        try:
            self._sort()
            temp_str = json.dumps(
                list(self._expiring_password_user_dq),
                indent=4,
                ensure_ascii=False,
            )
            temp_str.encode("utf8")
            if really_print:
                print(temp_str)
        except Exception as e:
            logger.error("IPA-EPN: unexpected error: %s", e)

    def _sort(self):
        if not self._sorted:
            if isinstance(self._expiring_password_user_dq, deque):
                self._expiring_password_user_dq = deque(
                    sorted(
                        self._expiring_password_user_dq,
                        key=lambda item: item["krbpasswordexpiration"],
                    )
                )
                self._sorted = True


class EPN(admintool.AdminTool):
    command_name = "IPA-EPN"
    log_file_name = paths.IPAEPN_LOG

    usage = "%prog [options]"
    description = "Expiring Password Notifications (EPN)"

    def __init__(self, options, args):
        super(EPN, self).__init__(options, args)
        self._conn = None
        self._expiring_password_user_list = EPNUserList()
        self._ldap_data = []
        self._date_ranges = []
        self._mailer = None
        self.env = None
        self.default_email_domain = None

    @classmethod
    def add_options(cls, parser):
        super(EPN, cls).add_options(parser, debug_option=True)
        parser.add_option(
            "--from-nbdays",
            dest="from_nbdays",
            action="store",
            default=None,
            help="minimal number of days",
        )
        parser.add_option(
            "--to-nbdays",
            dest="to_nbdays",
            action="store",
            default=None,
            help="maximal number of days",
        )
        parser.add_option(
            "--dry-run",
            dest="dry_run",
            action="store_true",
            default=False,
            help="Dry run mode. JSON ouput only.",
        )

    def validate_options(self):
        super(EPN, self).validate_options(needs_root=True)
        if self.options.to_nbdays:
            self.options.dry_run = True
        if self.options.from_nbdays and not self.options.to_nbdays:
            self.option_parser.error(
                "You cannot specify --from-nbdays without --to-nbdays"
            )

    def setup_logging(self, log_file_mode="a"):
        super(EPN, self).setup_logging(log_file_mode="a")

    def run(self):
        super(EPN, self).run()

        fstore = sysrestore.FileStore(paths.IPA_CLIENT_SYSRESTORE)
        if not is_ipa_client_installed(fstore):
            logger.error("IPA client is not configured on this system.")
            raise admintool.ScriptError()

        self._get_krb5_ticket()
        self._read_configuration()
        self._validate_configuration()
        self._parse_configuration()
        self._get_connection()
        self._read_ipa_configuration()
        drop_privileges()
        if self.options.to_nbdays:
            self._build_cli_date_ranges()
        for date_range in self._date_ranges:
            self._fetch_data_from_ldap(date_range)
            self._parse_ldap_data()
        if self.options.dry_run:
            self._pretty_print_data()
        else:
            self._mailer = MailUserAgent(
                security_protocol=api.env.smtp_security,
                smtp_hostname=api.env.smtp_server,
                smtp_port=api.env.smtp_port,
                smtp_timeout=api.env.smtp_timeout,
                smtp_username=api.env.smtp_user,
                smtp_password=api.env.smtp_password,
                x_mailer=self.command_name,
                msg_subtype=api.env.msg_subtype,
                msg_charset=api.env.msg_charset,
            )
            self._send_emails()

    def _get_date_range_from_nbdays(self, nbdays_end, nbdays_start=None):
        """Detects current time and returns a date range, given a number
           of days in the future.
           If only nbdays_end is specified, the range is 1d long.
        """
        now = datetime.utcnow()
        today_at_midnight = datetime.combine(now, datetime.min.time())
        range_end = today_at_midnight + timedelta(days=nbdays_end)
        if nbdays_start is not None:
            range_start = today_at_midnight + timedelta(days=nbdays_start)
        else:
            range_start = range_end - timedelta(days=1)

        logger.debug(
            "IPA-EPN: Current date: %s \n"
            "IPA-EPN: Date & time, today at midnight: %s \n"
            "IPA-EPN: Date range start: %s \n"
            "IPA-EPN: Date range end: %s \n",
            now,
            today_at_midnight,
            range_start,
            range_end,
        )
        return (range_start, range_end)

    def _datetime_to_generalized_time(self, dt):
        """Convert datetime to LDAP_GENERALIZED_TIME_FORMAT
           Note: Consider moving into ipalib.
        """
        dt = dt.timetuple()
        generalized_time_str = str(dt.tm_year) + "".join(
            "0" * (2 - len(str(item))) + str(item)
            for item in (
                dt.tm_mon,
                dt.tm_mday,
                dt.tm_hour,
                dt.tm_min,
                dt.tm_sec,
            )
        )
        return generalized_time_str + "Z"

    def _get_krb5_ticket(self):
        """Setup the environment to obtain a krb5 ticket for us using the
           system keytab.
           Uses CCACHE = MEMORY (limited to the current process).
        """
        os.environ.setdefault("KRB5_CLIENT_KTNAME", "/etc/krb5.keytab")
        os.environ["KRB5CCNAME"] = "MEMORY:"

    def _read_configuration(self):
        """Merge in the EPN configuration from /etc/ipa/epn.conf"""
        base_config = dict(
            context="epn", confdir=paths.ETC_IPA, in_server=False,
        )
        api.bootstrap(**base_config)
        api.env._merge(**EPN_CONFIG)

        if not api.isdone("finalize"):
            api.finalize()

    def _validate_configuration(self):
        """Examine the user-provided configuration.
        """
        if api.env.smtp_security.lower() not in ("none", "starttls", "ssl"):
            raise RuntimeError(
                "smtp_security must be one of: " "none, starttls or ssl"
            )
        if api.env.smtp_user is not None and api.env.smtp_password is None:
            raise RuntimeError("smtp_user set and smtp_password is not")
        if api.env.notify_ttls is None:
            raise RuntimeError("notify_ttls must be set in %s" % EPN_CONF)
        try:
            [int(k) for k in str(api.env.notify_ttls).split(',')]
        except ValueError as e:
            raise RuntimeError('Failed to parse notify_ttls: \'%s\': %s' %
                               (api.env.notify_ttls, e))

    def _parse_configuration(self):
        """
        """
        daylist = [int(k) for k in str(api.env.notify_ttls).split(',')]
        daylist.sort()

        for day in daylist:
            self._date_ranges.append(
                self._get_date_range_from_nbdays(
                    nbdays_start=None, nbdays_end=day + 1
                )
            )

        loader = FileSystemLoader(os.path.join(api.env.confdir, 'epn'))
        self.env = Environment(loader=loader)

    def _read_ipa_configuration(self):
        """Get the IPA configuration"""
        api.Backend.rpcclient.connect()
        result = api.Command.config_show()['result']
        self.default_email_domain = result.get('ipadefaultemaildomain',
                                               [None])[0]
        api.Backend.rpcclient.disconnect()

    def _get_connection(self):
        """Create a connection to LDAP and bind to it.
        """
        if self._conn is not None:
            return self._conn

        try:
            # LDAPI
            self._conn = ipaldap.LDAPClient.from_realm(api.env.realm)
            self._conn.external_bind()
        except Exception:
            try:
                # LDAP + GSSAPI
                self._conn = ipaldap.LDAPClient.from_hostname_secure(
                    api.env.server
                )
                self._conn.gssapi_bind()
            except Exception as e:
                logger.error(
                    "Unable to bind to LDAP server %s: %s",
                    self._conn.ldap_uri,
                    e,
                )

        return self._conn

    def _fetch_data_from_ldap(self, date_range):
        """Run a LDAP query to fetch a list of user entries whose passwords
           would expire in the near future. Store in self._ldap_data.
        """

        if self._conn is None:
            logger.error(
                "IPA-EPN: Connection to LDAP not established. Exiting."
            )

        search_base = DN(api.env.container_user, api.env.basedn)
        attrs_list = ["uid", "krbpasswordexpiration", "mail", "cn",
                      "gn", "surname"]

        search_filter = (
            "(&(!(nsaccountlock=TRUE)) \
            (krbpasswordexpiration<=%s) \
            (krbpasswordexpiration>=%s))"
            % (
                self._datetime_to_generalized_time(date_range[1]),
                self._datetime_to_generalized_time(date_range[0]),
            )
        )

        try:
            self._ldap_data = self._conn.get_entries(
                search_base,
                filter=search_filter,
                attrs_list=attrs_list,
                scope=self._conn.SCOPE_SUBTREE,
            )
        except errors.EmptyResult:
            logger.debug("Empty Result.")
        finally:
            logger.debug("%d entries found", len(self._ldap_data))

    def _parse_ldap_data(self):
        """Fill out self._expiring_password_user_list from data from ldap.
        """
        if self._ldap_data:
            for entry in self._ldap_data:
                self._expiring_password_user_list.add(entry)
            # Validate json.
            try:
                self._pretty_print_data(really_print=False)
            except Exception as e:
                logger.error("IPA-EPN: Could not create JSON: %s", e)
            finally:
                self._ldap_data = []

    def _pretty_print_data(self, really_print=True):
        """Dump self._expiring_password_user_list to JSON.
        """
        self._expiring_password_user_list.json_print(
            really_print=really_print
        )

    def _send_emails(self):
        if self._mailer is None:
            logger.error("IPA-EPN: mailer was not configured.")
            return
        else:
            try:
                template = self.env.get_template("expire_msg.template")
            except TemplateSyntaxError as e:
                raise RuntimeError("Parsing template %s failed: %s" %
                                   (e.filename, e))
            if api.env.mail_from:
                mail_from = api.env.mail_from
            else:
                mail_from = "noreply@%s" % self.default_email_domain
            while self._expiring_password_user_list:
                entry = self._expiring_password_user_list.pop()
                body = template.render(
                    uid=entry["uid"],
                    first=entry["givenname"],
                    last=entry["sn"],
                    fullname=entry["cn"],
                    expiration=entry["krbpasswordexpiration"],
                )
                self._mailer.send_message(
                    mail_subject=api.env.msg_subject,
                    mail_body=body,
                    subscribers=ast.literal_eval(entry["mail"]),
                    mail_from=mail_from,
                )
                now = datetime.utcnow()
                expdate = datetime.strptime(
                    entry["krbpasswordexpiration"],
                    '%Y-%m-%d %H:%M:%S')
                logger.debug(
                    "Notified %s (%s). Password expiring in %d days at %s.",
                    entry["mail"], entry["uid"], (expdate - now).days,
                    expdate)
            self._mailer.cleanup()

    def _build_cli_date_ranges(self):
        """When self.options.to_nbdays is set, override the date ranges read
           from the configuration file and build the date ranges from the CLI
           options.
        """
        self._date_ranges = []
        logger.debug("IPA-EPN: Ignoring configuration file ranges.")
        if self.options.from_nbdays is not None:
            self._date_ranges.append(
                self._get_date_range_from_nbdays(
                    nbdays_start=int(self.options.from_nbdays),
                    nbdays_end=int(self.options.to_nbdays),
                )
            )
        elif self.options.to_nbdays is not None:
            self._date_ranges.append(
                self._get_date_range_from_nbdays(
                    nbdays_start=None, nbdays_end=int(self.options.to_nbdays)
                )
            )


class MTAClient:
    """MTA Client class. Originally done for EPN.
    """

    def __init__(
        self,
        security_protocol="none",
        smtp_hostname="localhost",
        smtp_port=25,
        smtp_timeout=60,
        smtp_username=None,
        smtp_password=None,
    ):
        # We only support "none" (cleartext) for now.
        # Future values: "ssl", "starttls"
        self._security_protocol = security_protocol
        self._smtp_hostname = smtp_hostname
        self._smtp_port = smtp_port
        self._smtp_timeout = smtp_timeout
        self._username = smtp_username
        self._password = smtp_password

        # This should not be touched
        self._conn = None

        if (
            self._security_protocol == "none"
            and "localhost" not in self._smtp_hostname
        ):
            logger.error(
                "IPA-EPN: using cleartext for non-localhost SMTPd "
                "is not supported."
            )

        self._connect()

    def cleanup(self):
        self._disconnect()

    def send_message(self, message_str=None, subscribers=None):
        result = None
        try:
            result = self._conn.sendmail(
                api.env.smtp_admin, subscribers, message_str,
            )
        except Exception as e:
            logger.info("IPA-EPN: Failed to send mail: %s", e)
        finally:
            if result:
                for key in result:
                    logger.info(
                        "IPA-EPN: Failed to send mail to '%s': %s %s",
                        key,
                        result[key][0],
                        result[key][1],
                    )
                logger.info(
                    "IPA-EPN: Failed to send mail to at least one recipient"
                )

    def _connect(self):
        try:
            if self._security_protocol.lower() in ["none", "starttls"]:
                self._conn = smtplib.SMTP(
                    host=self._smtp_hostname,
                    port=self._smtp_port,
                    timeout=self._smtp_timeout,
                )
            else:
                self._conn = smtplib.SMTP_SSL(
                    host=self._smtp_hostname,
                    port=self._smtp_port,
                    timeout=self._smtp_timeout,
                )
        except smtplib.SMTPException as e:
            logger.error(
                "IPA-EPN: Unable to connect to %s:%s: %s",
                self._smtp_hostname,
                self._smtp_port,
                e,
            )

        try:
            self._conn.ehlo()
        except smtplib.SMTPException as e:
            logger.error(
                "IPA-EPN: EHLO failed for host %s:%s: %s",
                self._smtp_hostname,
                self._smtp_port,
                e,
            )

        if (
            self._conn.has_extn("STARTTLS")
            and self._security_protocol.lower() == "starttls"
        ):
            try:
                self._conn.starttls()
                self._conn.ehlo()
            except smtplib.SMTPException as e:
                logger.error(
                    "IPA-EPN: Unable to create an encrypted session to "
                    "%s:%s: %s",
                    self._smtp_hostname,
                    self._smtp_port,
                    e,
                )

        if self._username and self._password:
            if self._conn.has_extn("AUTH"):
                try:
                    self._conn.login(self._username, self._password)
                    if self._security_protocol == "none":
                        logger.warning(
                            "IPA-EPN: Username and Password "
                            "were sent in the clear."
                        )
                except smtplib.SMTPAuthenticationError:
                    raise RuntimeError(
                        "IPA-EPN: Authentication to %s:%s failed, "
                        "please check your username and/or password:" %
                        (self._smtp_hostname,
                         self._smtp_port,)
                    )
                except smtplib.SMTPException as e:
                    raise RuntimeError(
                        "IPA-EPN: SMTP Error at %s:%s:%s" %
                        (self._smtp_hostname,
                         self._smtp_port,
                         e,)
                    )
            else:
                err_str = (
                    "IPA-EPN: Server at %s:%s "
                    "does not support authentication." %
                    (self._smtp_hostname,
                     self._smtp_port,)
                )
                logger.error(err_str)

    def _disconnect(self):
        self._conn.quit()


class MailUserAgent:
    """The MUA class for EPN.
    """

    def __init__(
        self,
        security_protocol="none",
        smtp_hostname="localhost",
        smtp_port=25,
        smtp_timeout=60,
        smtp_username=None,
        smtp_password=None,
        x_mailer=None,
        msg_subtype="plain",
        msg_charset="utf8",
    ):

        self._x_mailer = x_mailer
        self._subject = None
        self._body = None
        self._subscribers = None

        self._subtype = msg_subtype
        self._charset = msg_charset

        self._msg = None
        self._message_str = None

        self._mta_client = MTAClient(
            security_protocol=security_protocol,
            smtp_hostname=smtp_hostname,
            smtp_port=smtp_port,
            smtp_timeout=smtp_timeout,
            smtp_username=smtp_username,
            smtp_password=smtp_password,
        )

    def cleanup(self):
        self._mta_client.cleanup()

    def send_message(
        self, mail_subject=None, mail_body=None, subscribers=None,
        mail_from=None
    ):
        """Given mail_subject, mail_body, and subscribers, composes
           the message and sends it.
        """
        if None in [mail_subject, mail_body, subscribers, mail_from]:
            logger.error("IPA-EPN: Tried to send an empty message.")
            return False
        self._compose_message(
            mail_subject=mail_subject,
            mail_body=mail_body,
            subscribers=subscribers,
            mail_from=mail_from,
        )
        self._mta_client.send_message(
            message_str=self._message_str, subscribers=subscribers
        )
        return True

    def _compose_message(
        self, mail_subject, mail_body, subscribers, mail_from
    ):
        """The composer creates a MIME multipart message.
        """

        self._subject = mail_subject
        self._body = mail_body
        self._subscribers = subscribers

        self._msg = MIMEMultipart(_charset=self._charset)
        self._msg["From"] = formataddr(("IPA-EPN", mail_from))
        self._msg["To"] = ", ".join(self._subscribers)
        self._msg["Date"] = formatdate(localtime=True)
        self._msg["Subject"] = Header(self._subject, self._charset)
        self._msg["Message-Id"] = make_msgid()
        self._msg.preamble = "Multipart message"
        if "X-Mailer" not in self._msg and self._x_mailer:
            self._msg.add_header("X-Mailer", self._x_mailer)
        self._msg.attach(
            MIMEText(
                self._body + "\n\n",
                _subtype=self._subtype,
                _charset=self._charset,
            )
        )
        self._message_str = self._msg.as_string()