# -*- coding: utf-8 -*-
# fetch.py
# Copyright (C) 2013 LEAP
#
# 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/>.
"""
Incoming mail fetcher.
"""
import logging
import json
import ssl
import threading
import time
import copy
from StringIO import StringIO

from email.parser import Parser
from email.generator import Generator
from email.utils import parseaddr

from twisted.python import log
from twisted.internet.task import LoopingCall
from twisted.internet.threads import deferToThread
from zope.proxy import sameProxiedObjects

from leap.common import events as leap_events
from leap.common.check import leap_assert, leap_assert_type
from leap.common.events.events_pb2 import IMAP_FETCHED_INCOMING
from leap.common.events.events_pb2 import IMAP_MSG_PROCESSING
from leap.common.events.events_pb2 import IMAP_MSG_DECRYPTED
from leap.common.events.events_pb2 import IMAP_MSG_SAVED_LOCALLY
from leap.common.events.events_pb2 import IMAP_MSG_DELETED_INCOMING
from leap.common.events.events_pb2 import IMAP_UNREAD_MAIL
from leap.common.mail import get_email_charset
from leap.keymanager import errors as keymanager_errors
from leap.keymanager.openpgp import OpenPGPKey
from leap.soledad.client import Soledad
from leap.soledad.common.crypto import ENC_SCHEME_KEY, ENC_JSON_KEY


logger = logging.getLogger(__name__)


class MalformedMessage(Exception):
    """
    Raised when a given message is not well formed.
    """
    pass


class LeapIncomingMail(object):
    """
    Fetches and process mail from the incoming pool.

    This object has public methods start_loop and stop that will
    actually initiate a LoopingCall with check_period recurrency.
    The LoopingCall itself will invoke the fetch method each time
    that the check_period expires.

    This loop will sync the soledad db with the remote server and
    process all the documents found tagged as incoming mail.
    """

    RECENT_FLAG = "\\Recent"

    INCOMING_KEY = "incoming"
    CONTENT_KEY = "content"

    LEAP_SIGNATURE_HEADER = 'X-Leap-Signature'
    """
    Header added to messages when they are decrypted by the IMAP fetcher,
    which states the validity of an eventual signature that might be included
    in the encrypted blob.
    """
    LEAP_SIGNATURE_VALID = 'valid'
    LEAP_SIGNATURE_INVALID = 'invalid'
    LEAP_SIGNATURE_COULD_NOT_VERIFY = 'could not verify'

    fetching_lock = threading.Lock()

    def __init__(self, keymanager, soledad, imap_account,
                 check_period, userid):

        """
        Initialize LeapIncomingMail..

        :param keymanager: a keymanager instance
        :type keymanager: keymanager.KeyManager

        :param soledad: a soledad instance
        :type soledad: Soledad

        :param imap_account: the account to fetch periodically
        :type imap_account: SoledadBackedAccount

        :param check_period: the period to fetch new mail, in seconds.
        :type check_period: int
        """

        leap_assert(keymanager, "need a keymanager to initialize")
        leap_assert_type(soledad, Soledad)
        leap_assert(check_period, "need a period to check incoming mail")
        leap_assert_type(check_period, int)
        leap_assert(userid, "need a userid to initialize")

        self._keymanager = keymanager
        self._soledad = soledad
        self.imapAccount = imap_account
        self._inbox = self.imapAccount.getMailbox('inbox')
        self._userid = userid

        self._loop = None
        self._check_period = check_period

        self._create_soledad_indexes()

    def _create_soledad_indexes(self):
        """
        Create needed indexes on soledad.
        """
        self._soledad.create_index("just-mail", "incoming")

    @property
    def _pkey(self):
        if sameProxiedObjects(self._keymanager, None):
            logger.warning('tried to get key, but null keymanager found')
            return None
        return self._keymanager.get_key(self._userid, OpenPGPKey, private=True)

    #
    # Public API: fetch, start_loop, stop.
    #

    def fetch(self):
        """
        Fetch incoming mail, to be called periodically.

        Calls a deferred that will execute the fetch callback
        in a separate thread
        """
        logger.debug("fetching mail for: %s %s" % (
            self._soledad.uuid, self._userid))
        if not self.fetching_lock.locked():
            d = deferToThread(self._sync_soledad)
            d.addCallbacks(self._signal_fetch_to_ui, self._sync_soledad_error)
            d.addCallbacks(self._process_doclist, self._sync_soledad_error)
            return d
        else:
            logger.debug("Already fetching mail.")

    def start_loop(self):
        """
        Starts a loop to fetch mail.
        """
        if self._loop is None:
            self._loop = LoopingCall(self.fetch)
            self._loop.start(self._check_period)
        else:
            logger.warning("Tried to start an already running fetching loop.")

    def stop(self):
        # XXX change the name to stop_loop, for consistency.
        """
        Stops the loop that fetches mail.
        """
        if self._loop and self._loop.running is True:
            self._loop.stop()
            self._loop = None

    #
    # Private methods.
    #

    # synchronize incoming mail

    def _sync_soledad(self):
        """
        Synchronizes with remote soledad.

        :returns: a list of LeapDocuments, or None.
        :rtype: iterable or None
        """
        with self.fetching_lock:
            log.msg('syncing soledad...')
            self._soledad.sync()
            log.msg('soledad synced.')
            doclist = self._soledad.get_from_index("just-mail", "*")

        return doclist

    def _signal_unread_to_ui(self):
        """
        Sends unread event to ui.
        """
        leap_events.signal(
            IMAP_UNREAD_MAIL, str(self._inbox.getUnseenCount()))

    def _signal_fetch_to_ui(self, doclist):
        """
        Sends leap events to ui.

        :param doclist: iterable with msg documents.
        :type doclist: iterable.
        :returns: doclist
        :rtype: iterable
        """
        fetched_ts = time.mktime(time.gmtime())
        num_mails = len(doclist)
        log.msg("there are %s mails" % (num_mails,))
        leap_events.signal(
            IMAP_FETCHED_INCOMING, str(num_mails), str(fetched_ts))
        self._signal_unread_to_ui()
        return doclist

    def _sync_soledad_error(self, failure):
        """
        Errback for sync errors.
        """
        # XXX should signal unrecoverable maybe.
        err = failure.value
        logger.error("error syncing soledad: %s" % (err,))
        if failure.check(ssl.SSLError):
            logger.warning('SSL Error while '
                           'syncing soledad: %r' % (err,))
        elif failure.check(Exception):
            logger.warning('Unknown error while '
                           'syncing soledad: %r' % (err,))

    def _log_err(self, failure):
        """
        Generic errback
        """
        err = failure.value
        logger.exception("error!: %r" % (err,))

    def _decryption_error(self, failure):
        """
        Errback for decryption errors.
        """
        # XXX should signal unrecoverable maybe.
        err = failure.value
        logger.error("error decrypting msg: %s" % (err,))

    def _saving_error(self, failure):
        """
        Errback for local save errors.
        """
        # XXX should signal unrecoverable maybe.
        err = failure.value
        logger.error("error saving msg locally: %s" % (err,))

    # process incoming mail.

    def _process_doclist(self, doclist):
        """
        Iterates through the doclist, checks if each doc
        looks like a message, and yields a deferred that will decrypt and
        process the message.

        :param doclist: iterable with msg documents.
        :type doclist: iterable.
        :returns: a list of deferreds for individual messages.
        """
        log.msg('processing doclist')
        if not doclist:
            logger.debug("no docs found")
            return
        num_mails = len(doclist)

        docs_cb = []
        for index, doc in enumerate(doclist):
            logger.debug("processing doc %d of %d" % (index + 1, num_mails))
            leap_events.signal(
                IMAP_MSG_PROCESSING, str(index), str(num_mails))
            keys = doc.content.keys()
            if self._is_msg(keys):
                # Ok, this looks like a legit msg.
                # Let's process it!
                # Deferred chain for individual messages

                # XXX use an IConsumer instead... ?
                d = deferToThread(self._decrypt_doc, doc)
                d.addCallback(self._process_decrypted_doc)
                d.addErrback(self._log_err)
                d.addCallback(self._add_message_locally)
                d.addErrback(self._log_err)
                docs_cb.append(d)
            else:
                # Ooops, this does not.
                logger.debug('This does not look like a proper msg.')
        return docs_cb

    #
    # operations on individual messages
    #

    def _is_msg(self, keys):
        """
        Checks if the keys of a dictionary match the signature
        of the document type we use for messages.

        :param keys: iterable containing the strings to match.
        :type keys: iterable of strings.
        :rtype: bool
        """
        return ENC_SCHEME_KEY in keys and ENC_JSON_KEY in keys

    def _decrypt_doc(self, doc):
        """
        Decrypt the contents of a document.

        :param doc: A document containing an encrypted message.
        :type doc: SoledadDocument

        :return: A tuple containing the document and the decrypted message.
        :rtype: (SoledadDocument, str)
        """
        log.msg('decrypting msg')
        success = False

        try:
            decrdata = self._keymanager.decrypt(
                doc.content[ENC_JSON_KEY],
                self._pkey)
            success = True
        except Exception as exc:
            # XXX move this to errback !!!
            logger.error("Error while decrypting msg: %r" % (exc,))
            decrdata = ""
        leap_events.signal(IMAP_MSG_DECRYPTED, "1" if success else "0")
        return doc, decrdata

    def _process_decrypted_doc(self, msgtuple):
        """
        Process a document containing a succesfully decrypted message.

        :param msgtuple: a tuple consisting of a SoledadDocument
                         instance containing the incoming message
                         and data, the json-encoded, decrypted content of the
                         incoming message
        :type msgtuple: (SoledadDocument, str)
        :return: a SoledadDocument and the processed data.
        :rtype: (doc, data)
        """
        log.msg('processing decrypted doc')
        doc, data = msgtuple
        msg = json.loads(data)
        if not isinstance(msg, dict):
            return False
        if not msg.get(self.INCOMING_KEY, False):
            return False

        # ok, this is an incoming message
        rawmsg = msg.get(self.CONTENT_KEY, None)
        if not rawmsg:
            return False
        data = self._maybe_decrypt_msg(rawmsg)
        return doc, data

    def _maybe_decrypt_msg(self, data):
        """
        Tries to decrypt a gpg message if data looks like one.

        :param data: the text to be decrypted.
        :type data: unicode
        :return: data, possibly descrypted.
        :rtype: str
        """
        log.msg('maybe decrypting doc')
        leap_assert_type(data, unicode)

        # parse the original message
        parser = Parser()
        encoding = get_email_charset(data)
        data = data.encode(encoding)
        msg = parser.parsestr(data)

        # try to obtain sender public key
        senderPubkey = None
        fromHeader = msg.get('from', None)
        if fromHeader is not None:
            _, senderAddress = parseaddr(fromHeader)
            try:
                senderPubkey = self._keymanager.get_key_from_cache(
                    senderAddress, OpenPGPKey)
            except keymanager_errors.KeyNotFound:
                pass

        valid_sig = False  # we will add a header saying if sig is valid
        if msg.get_content_type() == 'multipart/encrypted':
            decrmsg, valid_sig = self._decrypt_multipart_encrypted_msg(
                msg, encoding, senderPubkey)
        else:
            decrmsg, valid_sig = self._maybe_decrypt_inline_encrypted_msg(
                msg, encoding, senderPubkey)

        # add x-leap-signature header
        if senderPubkey is None:
            decrmsg.add_header(
                self.LEAP_SIGNATURE_HEADER,
                self.LEAP_SIGNATURE_COULD_NOT_VERIFY)
        else:
            decrmsg.add_header(
                self.LEAP_SIGNATURE_HEADER,
                self.LEAP_SIGNATURE_VALID if valid_sig else
                self.LEAP_SIGNATURE_INVALID,
                pubkey=senderPubkey.key_id)

        return decrmsg.as_string()

    def _decrypt_multipart_encrypted_msg(self, msg, encoding, senderPubkey):
        """
        Decrypt a message with content-type 'multipart/encrypted'.

        :param msg: The original encrypted message.
        :type msg: Message
        :param encoding: The encoding of the email message.
        :type encoding: str
        :param senderPubkey: The key of the sender of the message.
        :type senderPubkey: OpenPGPKey

        :return: A unitary tuple containing a decrypted message.
        :rtype: (Message)
        """
        log.msg('decrypting multipart encrypted msg')
        msg = copy.deepcopy(msg)
        # sanity check
        payload = msg.get_payload()
        if len(payload) != 2:
            raise MalformedMessage(
                'Multipart/encrypted messages should have exactly 2 body '
                'parts (instead of %d).' % len(payload))
        if payload[0].get_content_type() != 'application/pgp-encrypted':
            raise MalformedMessage(
                "Multipart/encrypted messages' first body part should "
                "have content type equal to 'application/pgp-encrypted' "
                "(instead of %s)." % payload[0].get_content_type())
        if payload[1].get_content_type() != 'application/octet-stream':
            raise MalformedMessage(
                "Multipart/encrypted messages' second body part should "
                "have content type equal to 'octet-stream' (instead of "
                "%s)." % payload[1].get_content_type())
        # parse message and get encrypted content
        pgpencmsg = msg.get_payload()[1]
        encdata = pgpencmsg.get_payload()
        # decrypt or fail gracefully
        try:
            decrdata, valid_sig = self._decrypt_and_verify_data(
                encdata, senderPubkey)
        except keymanager_errors.DecryptError as e:
            logger.warning('Failed to decrypt encrypted message (%s). '
                           'Storing message without modifications.' % str(e))
            return msg, False  # return original message
        # decrypted successully, now fix encoding and parse
        try:
            decrdata = decrdata.encode(encoding)
        except (UnicodeEncodeError, UnicodeDecodeError) as e:
            logger.error("Unicode error {0}".format(e))
            decrdata = decrdata.encode(encoding, 'replace')
        parser = Parser()
        decrmsg = parser.parsestr(decrdata)
        # remove original message's multipart/encrypted content-type
        del(msg['content-type'])
        # replace headers back in original message
        for hkey, hval in decrmsg.items():
            try:
                # this will raise KeyError if header is not present
                msg.replace_header(hkey, hval)
            except KeyError:
                msg[hkey] = hval
        # replace payload by unencrypted payload
        msg.set_payload(decrmsg.get_payload())
        return msg, valid_sig

    def _maybe_decrypt_inline_encrypted_msg(self, origmsg, encoding,
                                            senderPubkey):
        """
        Possibly decrypt an inline OpenPGP encrypted message.

        :param origmsg: The original, possibly encrypted message.
        :type origmsg: Message
        :param encoding: The encoding of the email message.
        :type encoding: str
        :param senderPubkey: The key of the sender of the message.
        :type senderPubkey: OpenPGPKey

        :return: A unitary tuple containing a decrypted message.
        :rtype: (Message)
        """
        log.msg('maybe decrypting inline encrypted msg')
        # serialize the original message
        buf = StringIO()
        g = Generator(buf)
        g.flatten(origmsg)
        data = buf.getvalue()
        # handle exactly one inline PGP message
        PGP_BEGIN = "-----BEGIN PGP MESSAGE-----"
        PGP_END = "-----END PGP MESSAGE-----"
        valid_sig = False
        if PGP_BEGIN in data:
            begin = data.find(PGP_BEGIN)
            end = data.find(PGP_END)
            pgp_message = data[begin:end + len(PGP_END)]
            try:
                decrdata, valid_sig = self._decrypt_and_verify_data(
                    pgp_message, senderPubkey)
                # replace encrypted by decrypted content
                data = data.replace(pgp_message, decrdata)
            except keymanager_errors.DecryptError:
                logger.warning('Failed to decrypt potential inline encrypted '
                               'message. Storing message as is...')
        # if message is not encrypted, return raw data
        if isinstance(data, unicode):
            data = data.encode(encoding, 'replace')
        parser = Parser()
        return parser.parsestr(data), valid_sig

    def _decrypt_and_verify_data(self, data, senderPubkey):
        """
        Decrypt C{data} using our private key and attempt to verify a
        signature using C{senderPubkey}.

        :param data: The text to be decrypted.
        :type data: unicode
        :param senderPubkey: The public key of the sender of the message.
        :type senderPubkey: OpenPGPKey

        :return: The decrypted data and a boolean stating whether the
                 signature could be verified.
        :rtype: (str, bool)

        :raise DecryptError: Raised if failed to decrypt.
        """
        log.msg('decrypting and verifying data')
        valid_sig = False
        try:
            decrdata = self._keymanager.decrypt(
                data, self._pkey,
                verify=senderPubkey)
            if senderPubkey is not None:
                valid_sig = True
        except keymanager_errors.InvalidSignature:
            decrdata = self._keymanager.decrypt(
                data, self._pkey)
        return decrdata, valid_sig

    def _add_message_locally(self, msgtuple):
        """
        Adds a message to local inbox and delete it from the incoming db
        in soledad.

        :param msgtuple: a tuple consisting of a SoledadDocument
                         instance containing the incoming message
                         and data, the json-encoded, decrypted content of the
                         incoming message
        :type msgtuple: (SoledadDocument, str)
        """
        log.msg('adding message to local db')
        doc, data = msgtuple
        self._inbox.addMessage(data, (self.RECENT_FLAG,))
        leap_events.signal(IMAP_MSG_SAVED_LOCALLY)
        doc_id = doc.doc_id
        self._soledad.delete_doc(doc)
        log.msg("deleted doc %s from incoming" % doc_id)
        leap_events.signal(IMAP_MSG_DELETED_INCOMING)
        self._signal_unread_to_ui()