"""
Base folder support
Copyright (C) 2002-2016 John Goerzen & contributors

   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 2 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, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
"""

import email
import os.path
import re
import time
from sys import exc_info

from email import policy
from email.parser import BytesParser
from email.utils import parsedate_tz, mktime_tz
from email.charset import Charset

from offlineimap import threadutil
from offlineimap.ui import getglobalui
from offlineimap.error import OfflineImapError
import offlineimap.accounts

# This is wrapper to workaround for:
# - https://bugs.python.org/issue32330
class EmailMessage(email.message.EmailMessage):
    def set_payload(self, payload, charset=None):
        if hasattr(payload, 'encode') and charset is not None:
            if not isinstance(charset, Charset):
                charset = Charset(charset)
            payload = payload.encode(charset.output_charset, 'replace')
            charset = None
        super().set_payload(payload, charset)

class BaseFolder:
    """
    Base Folder Class
    """
    __hash__ = None

    def __init__(self, name, repository):
        """
        :param name: Path & name of folder minus root or reference
        :param repository: Repository() in which the folder is.
        """

        self.ui = getglobalui()
        self.messagelist = {}
        # Use the built-in email libraries
        # Establish some policies
        default_policy = policy.default.clone(message_factory=EmailMessage)
        self.policy = {
          '7bit':
          default_policy.clone(cte_type='7bit', utf8=False, refold_source='none'),
          '7bit-RFC':
          default_policy.clone(cte_type='7bit', utf8=False, refold_source='none', linesep='\r\n'),
          '8bit':
          default_policy.clone(cte_type='8bit', utf8=True, refold_source='none'),
          '8bit-RFC':
          default_policy.clone(cte_type='8bit', utf8=True, refold_source='none', linesep='\r\n'),
        }
        # Parsers
        self.parser = {}
        for key in self.policy:
            self.parser[key] = BytesParser(policy=self.policy[key])
        # Save original name for folderfilter operations.
        self.ffilter_name = name
        # Top level dir name is always ''.
        self.root = None
        self.name = name if not name == self.getsep() else ''
        self.newmail_hook = None
        # Only set the newmail_hook if the IMAP folder is named 'INBOX'.
        if self.name == 'INBOX':
            self.newmail_hook = repository.newmail_hook
        self.have_newmail = False
        self.copy_ignoreUIDs = None  # List of UIDs to ignore.
        self.repository = repository
        self.visiblename = repository.nametrans(name)
        # In case the visiblename becomes '.' or '/' (top-level) we use
        # '' as that is the name that e.g. the Maildir scanning will
        # return for the top-level dir.
        if self.visiblename == self.getsep():
            self.visiblename = ''

        self.repoconfname = "Repository " + repository.name

        self.config = repository.getconfig()

        # Do we need to use mail timestamp for filename prefix?
        filename_use_mail_timestamp_global = self.config.getdefaultboolean(
            "general", "filename_use_mail_timestamp", False)
        self._filename_use_mail_timestamp = self.config.getdefaultboolean(
            self.repoconfname,
            "filename_use_mail_timestamp",
            filename_use_mail_timestamp_global)

        self._sync_deletes = self.config.getdefaultboolean(
            self.repoconfname, "sync_deletes", True)
        self._dofsync = self.config.getdefaultboolean("general", "fsync", True)

        # Determine if we're running static or dynamic folder filtering
        # and check filtering status.
        self._dynamic_folderfilter = self.config.getdefaultboolean(
            self.repoconfname, "dynamic_folderfilter", False)
        self._sync_this = repository.should_sync_folder(self.ffilter_name)
        if self._dynamic_folderfilter:
            self.ui.debug('', "Running dynamic folder filtering on '%s'[%s]" %
                          (self.ffilter_name, repository))
        elif not self._sync_this:
            self.ui.debug('', "Filtering out '%s'[%s] due to folderfilter" %
                          (self.ffilter_name, repository))

        # Passes for syncmessagesto.
        self.syncmessagesto_passes = [
            self.__syncmessagesto_copy,
            self.__syncmessagesto_delete,
            self.__syncmessagesto_flags,
        ]

    def getname(self):
        """Returns name"""
        return self.name

    def __str__(self):
        # FIXME: remove calls of this. We have getname().
        return self.name

    def __unicode__(self):
        # NOTE(sheeprine): Implicit call to this by UIBase deletingflags() which
        # fails if the str is utf-8
        return self.name.decode('utf-8')

    def __enter__(self):
        """Starts a transaction. This will postpone (guaranteed) saving to disk
        of all messages saved inside this transaction until its committed."""
        pass

    def __exit__(self, exc_type, exc_val, exc_tb):
        """Commits a transaction, all messages saved inside this transaction
        will only now be persisted to disk."""
        pass

    @property
    def accountname(self):
        """Account name as string"""

        return self.repository.accountname

    @property
    def sync_this(self):
        """Should this folder be synced or is it e.g. filtered out?"""

        if not self._dynamic_folderfilter:
            return self._sync_this
        else:
            return self.repository.should_sync_folder(self.ffilter_name)

    def dofsync(self):
        """
        Call and returns _dofsync()

        Returns: Call and returns _dofsync()

        """
        return self._dofsync

    def suggeststhreads(self):
        """Returns True if this folder suggests using threads for actions.

        Only IMAP returns True. This method must honor any CLI or configuration
        option."""

        return False

    def waitforthread(self):
        """Implements method that waits for thread to be usable.
        Should be implemented only for folders that suggest threads."""
        raise NotImplementedError

    def quickchanged(self, statusfolder):
        """ Runs quick check for folder changes and returns changed
        status: True -- changed, False -- not changed.

        :param statusfolder: keeps track of the last known folder state.
        """

        return True

    def getinstancelimitnamespace(self):
        """For threading folders, returns the instancelimitname for
        InstanceLimitedThreads."""

        raise NotImplementedError

    def storesmessages(self):
        """Should be true for any backend that actually saves message bodies.
        (Almost all of them).  False for the LocalStatus backend.  Saves
        us from having to slurp up messages just for localstatus purposes."""

        return 1

    def getvisiblename(self):
        """The nametrans-transposed name of the folder's name."""

        return self.visiblename

    def getexplainedname(self):
        """Name that shows both real and nametrans-mangled values."""

        if self.name == self.visiblename:
            return self.name
        else:
            return "%s [remote name %s]" % (self.visiblename, self.name)

    def getrepository(self):
        """Returns the repository object that this folder is within."""

        return self.repository

    def getroot(self):
        """Returns the root of the folder, in a folder-specific fashion."""

        return self.root

    def getsep(self):
        """Returns the separator for this folder type."""

        return self.sep

    def getfullname(self):
        """
        Returns the folder full name, using the getname(). If getroot() is set
        their value is concatenated to getname() using the separator

        Returns: The folder full name

        """
        if self.getroot():
            return self.getroot() + self.getsep() + self.getname()
        else:
            return self.getname()

    def getfolderbasename(self):
        """Return base file name of file to store Status/UID info in."""

        if not self.name:
            basename = '.'
        else:  # Avoid directory hierarchies and file names such as '/'.
            basename = self.name.replace('/', '.')
        # Replace with literal 'dot' if final path name is '.' as '.' is
        # an invalid file name.
        basename = re.sub(r'(^|\/)\.$', '\\1dot', basename)
        return basename

    def check_uidvalidity(self):
        """Tests if the cached UIDVALIDITY match the real current one

        If required it saves the UIDVALIDITY value. In this case the
        function is not threadsafe. So don't attempt to call it from
        concurrent threads.

        :returns: Boolean indicating the match. Returns True in case it
            implicitly saved the UIDVALIDITY."""

        if self.get_saveduidvalidity() is not None:
            return self.get_saveduidvalidity() == self.get_uidvalidity()
        else:
            self.save_uidvalidity()
            return True

    def _getuidfilename(self):
        """provides UIDVALIDITY cache filename for class internal purposes."""

        return os.path.join(self.repository.getuiddir(),
                            self.getfolderbasename())

    def get_saveduidvalidity(self):
        """Return the previously cached UIDVALIDITY value

        :returns: UIDVALIDITY as (long) number or None, if None had been
            saved yet."""

        if hasattr(self, '_base_saved_uidvalidity'):
            return self._base_saved_uidvalidity
        uidfilename = self._getuidfilename()
        if not os.path.exists(uidfilename):
            self._base_saved_uidvalidity = None
        else:
            file = open(uidfilename, "rt")
            self._base_saved_uidvalidity = int(file.readline().strip())
            file.close()
        return self._base_saved_uidvalidity

    def save_uidvalidity(self):
        """Save the UIDVALIDITY value of the folder to the cache

        This function is not threadsafe, so don't attempt to call it
        from concurrent threads."""

        newval = self.get_uidvalidity()
        uidfilename = self._getuidfilename()

        with open(uidfilename + ".tmp", "wt") as uidfile:
            uidfile.write("%d\n" % newval)

        # This is weird, os.rename on Windows raises an exception,
        # But not in Linux. In linux the file is overwritten.
        try:
            os.rename(uidfilename + ".tmp", uidfilename)
        except WindowsError:
            os.remove(uidfilename)
            os.rename(uidfilename + ".tmp", uidfilename)

        self._base_saved_uidvalidity = newval

    def get_uidvalidity(self):
        """Retrieve the current connections UIDVALIDITY value

        This function needs to be implemented by each Backend
        :returns: UIDVALIDITY as a (long) number."""

        raise NotImplementedError

    def cachemessagelist(self):
        """Cache the list of messages.

        Reads the message list from disk or network and stores it in memory for
        later use.  This list will not be re-read from disk or memory unless
        this function is called again."""

        raise NotImplementedError

    def ismessagelistempty(self):
        """Is the list of messages empty."""

        if len(list(self.messagelist.keys())) < 1:
            return True
        return False

    def dropmessagelistcache(self):
        """Empty everything we know about messages."""

        self.messagelist = {}

    def getmessagelist(self):
        """Gets the current message list.

        You must call cachemessagelist() before calling this function!"""

        return self.messagelist

    def msglist_item_initializer(self, uid):
        """Returns value for empty messagelist element with given UID.

        This function must initialize all fields of messagelist item
        and must be called every time when one creates new messagelist
        entry to ensure that all fields that must be present are present.

        :param uid: Message UID"""

        raise NotImplementedError

    def uidexists(self, uid):
        """Returns True if uid exists."""

        return uid in self.getmessagelist()

    def getmessageuidlist(self):
        """Gets a list of UIDs.

        You may have to call cachemessagelist() before calling this function!"""

        return sorted(self.getmessagelist().keys())

    def getmessagecount(self):
        """Gets the number of messages."""

        return len(self.getmessagelist())

    def getmessage(self, uid):
        """Returns an email message object."""

        raise NotImplementedError

    def getmaxage(self):
        """Return maxage.

        maxage is allowed to be either an integer or a date of the form
        YYYY-mm-dd. This returns a time_struct."""

        maxagestr = self.config.getdefault("Account %s" %
                                           self.accountname, "maxage", None)
        if maxagestr is None:
            return None
        # Is it a number?
        try:
            maxage = int(maxagestr)
            if maxage < 1:
                raise OfflineImapError("invalid maxage value %d" % maxage,
                                       OfflineImapError.ERROR.MESSAGE)
            return time.gmtime(time.time() - 60 * 60 * 24 * maxage)
        except ValueError:
            pass  # Maybe it was a date.
        # Is it a date string?
        try:
            date = time.strptime(maxagestr, "%Y-%m-%d")
            if date[0] < 1900:
                raise OfflineImapError("maxage led to year %d. "
                                       "Abort syncing." % date[0],
                                       OfflineImapError.ERROR.MESSAGE)
            if (time.mktime(date) - time.mktime(time.localtime())) > 0:
                raise OfflineImapError("maxage led to future date %s. "
                                       "Abort syncing." % maxagestr,
                                       OfflineImapError.ERROR.MESSAGE)
            return date
        except ValueError:
            raise OfflineImapError("invalid maxage value %s" % maxagestr,
                                   OfflineImapError.ERROR.MESSAGE)

    def getmaxsize(self):
        """
        Get the maxsize for account name. If not found, returns None.

        Returns: A string with the maxise of the account name

        """
        return self.config.getdefaultint("Account %s" %
                                         self.accountname, "maxsize", None)

    def getstartdate(self):
        """ Retrieve the value of the configuration option startdate """
        datestr = self.config.getdefault("Repository " + self.repository.name,
                                         'startdate', None)
        try:
            if not datestr:
                return None
            date = time.strptime(datestr, "%Y-%m-%d")
            if date[0] < 1900:
                raise OfflineImapError("startdate led to year %d. "
                                       "Abort syncing." % date[0],
                                       OfflineImapError.ERROR.MESSAGE)
            if (time.mktime(date) - time.mktime(time.localtime())) > 0:
                raise OfflineImapError("startdate led to future date %s. "
                                       "Abort syncing." % datestr,
                                       OfflineImapError.ERROR.MESSAGE)
            return date
        except ValueError:
            raise OfflineImapError("invalid startdate value %s",
                                   OfflineImapError.ERROR.MESSAGE)

    def get_min_uid_file(self):
        """
        Get the min UID file name. Create it if not found.

        Returns: Min UID file name.

        """
        startuiddir = os.path.join(self.config.getmetadatadir(),
                                   'Repository-' + self.repository.name,
                                   'StartUID')
        if not os.path.exists(startuiddir):
            os.mkdir(startuiddir, 0o700)
        return os.path.join(startuiddir, self.getfolderbasename())

    def save_min_uid(self, min_uid):
        """
        Save the min UID in the min uid file

        Args:
            min_uid: min_uid to save

        Returns: None

        """
        uidfile = self.get_min_uid_file()
        fd = open(uidfile, 'wt')
        fd.write(str(min_uid) + "\n")
        fd.close()

    def retrieve_min_uid(self):
        """
        Retrieve the min UID file

        Returns: min UID of file

        """
        uidfile = self.get_min_uid_file()
        if not os.path.exists(uidfile):
            return None
        try:
            fd = open(uidfile, 'rt')
            min_uid = int(fd.readline().strip())
            fd.close()
            return min_uid
        except:
            raise IOError("Can't read %s" % uidfile)

    def savemessage(self, uid, msg, flags, rtime):
        """Writes a new message, with the specified uid.

        If the uid is < 0: The backend should assign a new uid and
           return it.  In case it cannot assign a new uid, it returns
           the negative uid passed in WITHOUT saving the message.

           If the backend CAN assign a new uid, but cannot find out what
           this UID is (as is the case with some IMAP servers), it
           returns 0 but DOES save the message.

           IMAP backend should be the only one that can assign a new
           uid.

        If the uid is > 0, the backend should set the uid to this, if it can.
           If it cannot set the uid to that, it will save it anyway.
           It will return the uid assigned in any case.

        Note that savemessage() does not check against dryrun settings,
        so you need to ensure that savemessage is never called in a
        dryrun mode."""

        raise NotImplementedError

    def getmessagetime(self, uid):
        """Return the received time for the specified message."""

        raise NotImplementedError

    def getmessagemtime(self, uid):
        """Returns the message modification time of the specified message."""

        raise NotImplementedError

    def getmessageflags(self, uid):
        """Returns the flags for the specified message."""

        raise NotImplementedError

    def getmessagekeywords(self, uid):
        """Returns the keywords for the specified message."""

        raise NotImplementedError

    def savemessageflags(self, uid, flags):
        """Sets the specified message's flags to the given set.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        raise NotImplementedError

    def addmessageflags(self, uid, flags):
        """Adds the specified flags to the message's flag set.

        If a given flag is already present, it will not be duplicated.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        :param uid: Message UID
        :param flags: A set() of flags"""

        newflags = self.getmessageflags(uid) | flags
        self.savemessageflags(uid, newflags)

    def addmessagesflags(self, uidlist, flags):
        """Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        for uid in uidlist:
            if self.uidexists(uid):
                self.addmessageflags(uid, flags)

    def deletemessageflags(self, uid, flags):
        """Removes each flag given from the message's flag set.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        If a given flag is already removed, no action will be taken for that
        flag."""

        newflags = self.getmessageflags(uid) - flags
        self.savemessageflags(uid, newflags)

    def deletemessagesflags(self, uidlist, flags):
        """
        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        for uid in uidlist:
            self.deletemessageflags(uid, flags)

    def getmessagelabels(self, uid):
        """Returns the labels for the specified message."""

        raise NotImplementedError

    def savemessagelabels(self, uid, labels, ignorelabels=None, mtime=0):
        """Sets the specified message's labels to the given set.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        """
        If this function is implemented,
        then it should include this code:

        if ignorelabels is None:
            ignorelabels = set()
        """

        raise NotImplementedError

    def addmessagelabels(self, uid, labels):
        """Adds the specified labels to the message's labels set.  If a given
        label is already present, it will not be duplicated.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        :param uid: Message UID
        :param labels: A set() of labels"""

        newlabels = self.getmessagelabels(uid) | labels
        self.savemessagelabels(uid, newlabels)

    def addmessageslabels(self, uidlist, labels):
        """Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        :param uidlist: Message UID
        :param labels: Labels to add"""

        for uid in uidlist:
            self.addmessagelabels(uid, labels)

    def deletemessagelabels(self, uid, labels):
        """Removes each label given from the message's label set.

        If a given label is already removed, no action will be taken for that
        label.

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        :param uid: Message uid
        :param labels: Labels to delete"""

        newlabels = self.getmessagelabels(uid) - labels
        self.savemessagelabels(uid, newlabels)

    def deletemessageslabels(self, uidlist, labels):
        """
        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        for uid in uidlist:
            self.deletemessagelabels(uid, labels)

    def addmessageheader(self, msg, headername, headervalue):
        """Adds new header to the provided message.

        Arguments:
        - msg: message object
        - headername: name of the header to add
        - headervalue: value of the header to add

        Returns: None

        """

        self.ui.debug('', 'addmessageheader: called to add %s: %s' %
                      (headername, headervalue))

        msg.add_header(headername, headervalue)
        return

    def getmessageheader(self, msg, headername):
        """Return the value of an undefined occurrence of the given header.

        Header name is case-insensitive.

        Arguments:
        - msg: message object
        - headername: name of the header to be searched

        Returns: header value or None if no such header was found.
        """

        self.ui.debug('', 'getmessageheader: called to get %s' % headername)
        return msg.get(headername)

    def getmessageheaderlist(self, msg, headername):
        """Return a list of values for the given header.

        Header name is case-insensitive.

        Arguments:
        - msg: message object
        - headername: name of the header to be searched

        Returns: list of header values or empty list if no such header was
        found.
        """

        self.ui.debug('', 'getmessageheaderlist: called to get %s' % headername)
        return msg.get_all(headername, [])

    def deletemessageheaders(self, msg, header_list):
        """Deletes headers in the given list from the message.

        Arguments:
        - msg: message object
        - header_list: list of headers to be deleted or just the header name

        """

        if type(header_list) != type([]):
            header_list = [header_list]
        self.ui.debug('',
                      'deletemessageheaders: called to delete %s' % header_list)

        for h in header_list:
            del msg[h]

        return

    def get_message_date(self, msg, header="Date"):
        """Returns the Unix timestamp of the email message, derived from the
        Date field header by default.

        Arguments:
        - msg: message object
        - header: header to extract the date from 

        Returns: timestamp or `None` in the case of failure.
        """

        datetuple = parsedate_tz(msg.get(header))
        if datetuple is None:
            return None

        return mktime_tz(datetuple)

    def change_message_uid(self, uid, new_uid):
        """Change the message from existing uid to new_uid.

        If the backend supports it (IMAP does not).

        :param uid: Message UID
        :param new_uid: (optional) If given, the old UID will be changed
            to a new UID. This allows backends efficient renaming of
            messages if the UID has changed."""

        raise NotImplementedError

    def deletemessage(self, uid):
        """Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        raise NotImplementedError

    def deletemessages(self, uidlist):
        """Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode."""

        for uid in uidlist:
            self.deletemessage(uid)

    def copymessageto(self, uid, dstfolder, statusfolder, register=1):
        """Copies a message from self to dst if needed, updating the status

        Note that this function does not check against dryrun settings,
        so you need to ensure that it is never called in a
        dryrun mode.

        :param uid: uid of the message to be copied.
        :param dstfolder: A BaseFolder-derived instance
        :param statusfolder: A LocalStatusFolder instance
        :param register: whether we should register a new thread."
        :returns: Nothing on success, or raises an Exception."""

        # Sometimes, it could be the case that if a sync takes awhile,
        # a message might be deleted from the maildir before it can be
        # synced to the status cache.  This is only a problem with
        # self.getmessage().  So, don't call self.getmessage unless
        # really needed.
        if register:  # Output that we start a new thread.
            self.ui.registerthread(self.repository.account)

        try:
            message = None
            flags = self.getmessageflags(uid)
            rtime = self.getmessagetime(uid)

            # If any of the destinations actually stores the message body,
            # load it up.
            if dstfolder.storesmessages():
                message = self.getmessage(uid)
            # Succeeded? -> IMAP actually assigned a UID. If newid
            # remained negative, no server was willing to assign us an
            # UID. If newid is 0, saving succeeded, but we could not
            # retrieve the new UID. Ignore message in this case.
            new_uid = dstfolder.savemessage(uid, message, flags, rtime)
            if new_uid > 0:
                if new_uid != uid:
                    # Got new UID, change the local uid to match the new one.
                    self.change_message_uid(uid, new_uid)
                    statusfolder.deletemessage(uid)
                    # Got new UID, change the local uid.
                # Save uploaded status in the statusfolder.
                statusfolder.savemessage(new_uid, message, flags, rtime)
                # Check whether the mail has been seen.
                if 'S' not in flags:
                    self.have_newmail = True
            elif new_uid == 0:
                # Message was stored to dstfolder, but we can't find it's UID
                # This means we can't link current message to the one created
                # in IMAP. So we just delete local message and on next run
                # we'll sync it back
                # XXX This could cause infinite loop on syncing between two
                # IMAP servers ...
                self.deletemessage(uid)
            else:
                msg = "Trying to save msg (uid %d) on folder " \
                      "%s returned invalid uid %d" % \
                      (uid, dstfolder.getvisiblename(), new_uid)
                raise OfflineImapError(msg, OfflineImapError.ERROR.MESSAGE)
        except KeyboardInterrupt:  # Bubble up CTRL-C.
            raise
        except OfflineImapError as e:
            if e.severity > OfflineImapError.ERROR.MESSAGE:
                raise  # Bubble severe errors up.
            self.ui.error(e, exc_info()[2])
        except Exception as e:
            self.ui.error(e, exc_info()[2],
                          msg="Copying message %s [acc: %s]" %
                              (uid, self.accountname))
            raise  # Raise on unknown errors, so we can fix those.

    def _extract_message_id(self, raw_msg_bytes):
        """Extract the Message-ID from a bytes object containing a raw message.

        This function attempts to find the Message-ID for a message that has not
        been processed by the built-in email library, and is therefore NOT an
        email object.  If parsing the message fails (or is otherwise not
        needed), this utility can be useful to help provide a (hopefully) unique
        identifier in log messages to facilitate locating the message on disk.

        :param raw_msg_bytes: bytes object containing the raw email message.
        :returns: A tuple containing the contents of the Message-ID header if
        found (or <Unknown Message-ID> if not found) and a flag which is True if
        the Message-ID was in proper RFC format or False if it contained
        defects.
        """
        msg_header = re.split(b'[\r]?\n[\r]?\n', raw_msg_bytes)[0]
        try:
            msg_id = re.search(br"\nmessage-id:[\s]+(<[A-Za-z0-9!#$%&'*+-/=?^_`{}|~.@ ]+>)", 
                msg_header, re.IGNORECASE).group(1)
        except AttributeError:
            # No match - Likely not following RFC rules.  Try and find anything
            # that looks like it could be the Message-ID but flag it.
            _start_pos = msg_header.find(b'\nMessage-ID:')
            if _start_pos > 0:
                _end_pos = msg_header.find(b'\n',_start_pos+15)
                msg_id = msg_header[_start_pos+12:_end_pos].strip()
                return (msg_id, False)
            else:
                return (b"<Unknown Message-ID>", False)
        return (msg_id, True)

    def _quote_boundary_fix(self, raw_msg_bytes):
        """Modify a raw message to quote the boundary separator for multipart messages.

        This function quotes only the first occurrence of the boundary field in
        the email header, and quotes any boundary value.  Improperly quoted
        boundary fields can give the internal python email library issues.

        :param raw_msg_bytes: bytes object containing the raw email message.
        :returns: The raw byte stream containing the quoted boundary
        """
        # Use re.split to extract just the header, and search for the boundary in
        # the context-type header and extract just the boundary and characters per
        # RFC 2046 ( see https://tools.ietf.org/html/rfc2046#section-5.1.1 )
        # We don't cap the length to 70 characters, because we are just trying to
        # soft fix this message to resolve the python library looking for properly
        # quoted boundaries.
        try: boundary_field = \
            re.search(b"content-type:.*(boundary=[\"]?[A-Za-z0-9'()+_,-./:=? ]+[\"]?)",
              re.split(b'[\r]?\n[\r]?\n', raw_msg_bytes)[0],
              (re.IGNORECASE|re.DOTALL)).group(1)
        except AttributeError:
            # No match
            return raw_msg_bytes
        # get the boundary field, and strip off any trailing ws (against RFC rules, leading ws is OK)
        # if it was already quoted, well then there was nothing to fix
        boundary, value = boundary_field.split(b'=', 1)
        value = value.rstrip()
        # ord(b'"') == 34
        if value[0] == value[-1] == 34:
            # Sanity Check - Do not requote if already quoted.
            # A quoted boundary was the end goal so return the original
            #
            # No need to worry about if the original email did something like:
            # boundary="ahahah  " as the email library will trim the ws for us
            return raw_msg_bytes
        else:
            new_field = b''.join([boundary, b'="', value, b'"'])
            return(raw_msg_bytes.replace(boundary_field, new_field, 1))

    def __syncmessagesto_copy(self, dstfolder, statusfolder):
        """Pass1: Copy locally existing messages not on the other side.

        This will copy messages to dstfolder that exist locally but are
        not in the statusfolder yet. The strategy is:

        1) Look for messages present in self but not in statusfolder.
        2) invoke copymessageto() on those which:
           - If dstfolder doesn't have it yet, add them to dstfolder.
           - Update statusfolder.

        This function checks and protects us from action in dryrun mode."""

        # We have no new mail yet.
        self.have_newmail = False

        threads = []

        copylist = [uid for uid in self.getmessageuidlist()
                    if not statusfolder.uidexists(uid)]
        num_to_copy = len(copylist)

        # Honor 'copy_ignore_eval' configuration option.
        if self.copy_ignoreUIDs is not None:
            for uid in self.copy_ignoreUIDs:
                if uid in copylist:
                    copylist.remove(uid)
                    self.ui.ignorecopyingmessage(uid, self, dstfolder)

        if num_to_copy > 0 and self.repository.account.dryrun:
            self.ui.info("[DRYRUN] Copy {} messages from {}[{}] to {}".format(
                num_to_copy, self, self.repository, dstfolder.repository)
            )
            return

        with self:
            for num, uid in enumerate(copylist):
                # Bail out on CTRL-C or SIGTERM.
                if offlineimap.accounts.Account.abort_NOW_signal.is_set():
                    break

                if uid == 0:
                    msg = "Assertion that UID != 0 failed; ignoring message."
                    self.ui.warn(msg)
                    continue

                if uid > 0 and dstfolder.uidexists(uid):
                    # dstfolder has message with that UID already,
                    # only update status.
                    flags = self.getmessageflags(uid)
                    rtime = self.getmessagetime(uid)
                    statusfolder.savemessage(uid, None, flags, rtime)
                    continue

                self.ui.copyingmessage(uid, num + 1, num_to_copy, self,
                                       dstfolder)
                # Exceptions are caught in copymessageto().
                if self.suggeststhreads():
                    self.waitforthread()
                    thread = threadutil.InstanceLimitedThread(
                        self.getinstancelimitnamespace(),
                        target=self.copymessageto,
                        name="Copy message from %s:%s" % (self.repository,
                                                          self),
                        args=(uid, dstfolder, statusfolder)
                    )
                    thread.start()
                    threads.append(thread)
                else:
                    self.copymessageto(uid, dstfolder, statusfolder, register=0)
            for thread in threads:
                thread.join()  # Block until all "copy" threads are done.

        # Execute new mail hook if we have new mail.
        if self.have_newmail:
            if self.newmail_hook is not None:
                self.newmail_hook()

    def __syncmessagesto_delete(self, dstfolder, statusfolder):
        """Pass 2: Remove locally deleted messages on dst.

        Get all UIDs in statusfolder but not self. These are messages
        that were deleted in 'self'. Delete those from dstfolder and
        statusfolder.

        This function checks and protects us from action in dryrun mode.
        """
        # The list of messages to delete. If sync of deletions is disabled we
        # still remove stale entries from statusfolder (neither in local nor
        # remote).
        deletelist = [uid for uid in statusfolder.getmessageuidlist()
                      if uid >= 0 and
                      not self.uidexists(uid) and
                      (self._sync_deletes or not dstfolder.uidexists(uid))]

        if len(deletelist):
            # Delete in statusfolder first to play safe. In case of abort, we
            # won't lose message, we will just unnecessarily retransmit some.
            # Delete messages from statusfolder that were either deleted by the
            # user, or not being tracked (e.g. because of maxage).
            if not self.repository.account.dryrun:
                statusfolder.deletemessages(deletelist)
            # Filter out untracked messages.
            deletelist = [uid for uid in deletelist if dstfolder.uidexists(uid)]
            if len(deletelist):
                self.ui.deletingmessages(deletelist, [dstfolder])
                if not self.repository.account.dryrun:
                    dstfolder.deletemessages(deletelist)

    def combine_flags_and_keywords(self, uid, dstfolder):
        """Combine the message's flags and keywords using the mapping for the
        destination folder."""

        # Take a copy of the message flag set, otherwise
        # __syncmessagesto_flags() will fail because statusflags is actually a
        # reference to selfflags (which it should not, but I don't have time to
        # debug THAT).
        selfflags = set(self.getmessageflags(uid))

        try:
            keywordmap = dstfolder.getrepository().getkeywordmap()
            if keywordmap is None:
                return selfflags

            knownkeywords = set(keywordmap.keys())
            selfkeywords = self.getmessagekeywords(uid)

            if not knownkeywords >= selfkeywords:
                # Some of the message's keywords are not in the mapping, so
                # skip them.
                skipped_keywords = list(selfkeywords - knownkeywords)
                selfkeywords &= knownkeywords
                msg = "Unknown keywords skipped: %s\n" \
                      "You may want to change your configuration to include " \
                      "those\n" % skipped_keywords
                self.ui.warn(msg)

            keywordletterset = set([keywordmap[keyw] for keyw in selfkeywords])

            # Add the mapped keywords to the list of message flags.
            selfflags |= keywordletterset
        except NotImplementedError:
            pass

        return selfflags

    def __syncmessagesto_flags(self, dstfolder, statusfolder):
        """Pass 3: Flag synchronization.

        Compare flag mismatches in self with those in statusfolder. If
        msg has a valid UID and exists on dstfolder (has not e.g. been
        deleted there), sync the flag change to both dstfolder and
        statusfolder.

        This function checks and protects us from action in ryrun mode.
        """

        # For each flag, we store a list of uids to which it should be
        # added.  Then, we can call addmessagesflags() to apply them in
        # bulk, rather than one call per message.
        addflaglist = {}
        delflaglist = {}
        for uid in self.getmessageuidlist():
            # Ignore messages with negative UIDs missed by pass 1 and
            # don't do anything if the message has been deleted remotely
            if uid < 0 or not dstfolder.uidexists(uid):
                continue

            if statusfolder.uidexists(uid):
                statusflags = statusfolder.getmessageflags(uid)
            else:
                statusflags = set()

            selfflags = self.combine_flags_and_keywords(uid, dstfolder)

            addflags = selfflags - statusflags
            delflags = statusflags - selfflags

            for flag in addflags:
                if flag not in addflaglist:
                    addflaglist[flag] = []
                addflaglist[flag].append(uid)

            for flag in delflags:
                if flag not in delflaglist:
                    delflaglist[flag] = []
                delflaglist[flag].append(uid)

        for flag, uids in list(addflaglist.items()):
            self.ui.addingflags(uids, flag, dstfolder)
            if self.repository.account.dryrun:
                continue  # Don't actually add in a dryrun.
            dstfolder.addmessagesflags(uids, set(flag))
            statusfolder.addmessagesflags(uids, set(flag))

        for flag, uids in list(delflaglist.items()):
            self.ui.deletingflags(uids, flag, dstfolder)
            if self.repository.account.dryrun:
                continue  # Don't actually remove in a dryrun.
            dstfolder.deletemessagesflags(uids, set(flag))
            statusfolder.deletemessagesflags(uids, set(flag))

    def syncmessagesto(self, dstfolder, statusfolder):
        """Syncs messages in this folder to the destination dstfolder.

        This is the high level entry for syncing messages in one direction.
        Syncsteps are:

        Pass1: Copy locally existing messages
         Copy messages in self, but not statusfolder to dstfolder if not
         already in dstfolder. dstfolder might assign a new UID (e.g. if
         uploading to IMAP). Update statusfolder.

        Pass2: Remove locally deleted messages
         Get all UIDS in statusfolder but not self. These are messages
         that were deleted in 'self'. Delete those from dstfolder and
         statusfolder.

         After this pass, the message lists should be identical wrt the
         uids present (except for potential negative uids that couldn't
         be placed anywhere).

        Pass3: Synchronize flag changes
         Compare flag mismatches in self with those in statusfolder. If
         msg has a valid UID and exists on dstfolder (has not e.g. been
         deleted there), sync the flag change to both dstfolder and
         statusfolder.

        Pass4: Synchronize label changes (Gmail only)
         Compares label mismatches in self with those in statusfolder.
         If msg has a valid UID and exists on dstfolder, syncs the labels
         to both dstfolder and statusfolder.

        :param dstfolder: Folderinstance to sync the msgs to.
        :param statusfolder: LocalStatus instance to sync against.
        """

        for action in self.syncmessagesto_passes:
            # Bail out on CTRL-C or SIGTERM.
            if offlineimap.accounts.Account.abort_NOW_signal.is_set():
                break
            try:
                action(dstfolder, statusfolder)
            except KeyboardInterrupt:
                raise
            except OfflineImapError as e:
                if e.severity > OfflineImapError.ERROR.FOLDER:
                    raise
                msg = "while syncing %s [account %s]" % (self, self.accountname)
                self.ui.error(e, exc_info()[2], msg)
            except Exception as e:
                msg = "while syncing %s [account %s]" % (self, self.accountname)
                self.ui.error(e, exc_info()[2], msg)
                raise  # Raise unknown Exceptions so we can fix them.

    def __eq__(self, other):
        """Comparisons work either on string comparing folder names or
        on the same instance.

        MailDirFolder('foo') == 'foo' --> True
        a = MailDirFolder('foo'); a == b --> True
        MailDirFolder('foo') == 'moo' --> False
        MailDirFolder('foo') == IMAPFolder('foo') --> False
        MailDirFolder('foo') == MaildirFolder('foo') --> False
        """

        if isinstance(other, str):
            return other == self.name
        return id(self) == id(other)

    def __ne__(self, other):
        return not self.__eq__(other)
