zulip/zerver/lib/bugdown/__init__.py

# Zulip's main markdown implementation.  See docs/subsystems/markdown.md for
# detailed documentation on our markdown syntax.
import datetime
import functools
import html
import logging
import os
import re
import time
import urllib
import urllib.parse
from collections import defaultdict, deque
from dataclasses import dataclass
from io import StringIO
from typing import (
    Any,
    Callable,
    Dict,
    Generic,
    Iterable,
    List,
    Optional,
    Set,
    Tuple,
    TypeVar,
    Union,
)
from typing.re import Match, Pattern
from xml.etree import ElementTree as etree
from xml.etree.ElementTree import Element, SubElement

import ahocorasick
import dateutil.parser
import dateutil.tz
import markdown
import pytz
import requests
from django.conf import settings
from django.db.models import Q
from hyperlink import parse
from markdown.extensions import codehilite, nl2br, sane_lists, tables
from typing_extensions import TypedDict

from zerver.lib import mention as mention
from zerver.lib.bugdown import fenced_code
from zerver.lib.bugdown.fenced_code import FENCE_RE
from zerver.lib.cache import NotFoundInCache, cache_with_key
from zerver.lib.camo import get_camo_url
from zerver.lib.emoji import (
    codepoint_to_name,
    emoticon_regex,
    name_to_codepoint,
    translate_emoticons,
)
from zerver.lib.exceptions import BugdownRenderingException
from zerver.lib.mention import extract_user_group, possible_mentions, possible_user_group_mentions
from zerver.lib.tex import render_tex
from zerver.lib.thumbnail import user_uploads_or_external
from zerver.lib.timeout import TimeoutExpired, timeout
from zerver.lib.timezone import get_common_timezones
from zerver.lib.url_encoding import encode_stream, hash_util_encode
from zerver.lib.url_preview import preview as link_preview
from zerver.models import (
    MAX_MESSAGE_LENGTH,
    Message,
    Realm,
    UserGroup,
    UserGroupMembership,
    UserProfile,
    all_realm_filters,
    get_active_streams,
    realm_filters_for_realm,
)

ReturnT = TypeVar('ReturnT')

def one_time(method: Callable[[], ReturnT]) -> Callable[[], ReturnT]:
    '''
        Use this decorator with extreme caution.
        The function you wrap should have no dependency
        on any arguments (no args, no kwargs) nor should
        it depend on any global state.
    '''
    val = None

    def cache_wrapper() -> ReturnT:
        nonlocal val
        if val is None:
            val = method()
        return val
    return cache_wrapper

class FullNameInfo(TypedDict):
    id: int
    email: str
    full_name: str

DbData = Dict[str, Any]

# Format version of the bugdown rendering; stored along with rendered
# messages so that we can efficiently determine what needs to be re-rendered
version = 1

_T = TypeVar('_T')
ElementStringNone = Union[Element, Optional[str]]

AVATAR_REGEX = r'!avatar\((?P<email>[^)]*)\)'
GRAVATAR_REGEX = r'!gravatar\((?P<email>[^)]*)\)'
EMOJI_REGEX = r'(?P<syntax>:[\w\-\+]+:)'

def verbose_compile(pattern: str) -> Any:
    return re.compile(
        f"^(.*?){pattern}(.*?)$",
        re.DOTALL | re.UNICODE | re.VERBOSE,
    )

def normal_compile(pattern: str) -> Any:
    return re.compile(
        fr"^(.*?){pattern}(.*)$",
        re.DOTALL | re.UNICODE,
    )

STREAM_LINK_REGEX = r"""
                     (?<![^\s'"\(,:<])            # Start after whitespace or specified chars
                     \#\*\*                       # and after hash sign followed by double asterisks
                         (?P<stream_name>[^\*]+)  # stream name can contain anything
                     \*\*                         # ends by double asterisks
                    """

@one_time
def get_compiled_stream_link_regex() -> Pattern:
    return verbose_compile(STREAM_LINK_REGEX)

STREAM_TOPIC_LINK_REGEX = r"""
                     (?<![^\s'"\(,:<])             # Start after whitespace or specified chars
                     \#\*\*                        # and after hash sign followed by double asterisks
                         (?P<stream_name>[^\*>]+)  # stream name can contain anything except >
                         >                         # > acts as separator
                         (?P<topic_name>[^\*]+)     # topic name can contain anything
                     \*\*                          # ends by double asterisks
                   """

@one_time
def get_compiled_stream_topic_link_regex() -> Pattern:
    return verbose_compile(STREAM_TOPIC_LINK_REGEX)

LINK_REGEX: Pattern = None

def get_web_link_regex() -> str:
    # We create this one time, but not at startup.  So the
    # first message rendered in any process will have some
    # extra costs.  It's roughly 75ms to run this code, so
    # caching the value in LINK_REGEX is super important here.
    global LINK_REGEX
    if LINK_REGEX is not None:
        return LINK_REGEX

    tlds = '|'.join(list_of_tlds())

    # A link starts at a word boundary, and ends at space, punctuation, or end-of-input.
    #
    # We detect a url either by the `https?://` or by building around the TLD.

    # In lieu of having a recursive regex (which python doesn't support) to match
    # arbitrary numbers of nested matching parenthesis, we manually build a regexp that
    # can match up to six
    # The inner_paren_contents chunk matches the innermore non-parenthesis-holding text,
    # and the paren_group matches text with, optionally, a matching set of parens
    inner_paren_contents = r"[^\s()\"]*"
    paren_group = r"""
                    [^\s()\"]*?            # Containing characters that won't end the URL
                    (?: \( %s \)           # and more characters in matched parens
                        [^\s()\"]*?        # followed by more characters
                    )*                     # zero-or-more sets of paired parens
                   """
    nested_paren_chunk = paren_group
    for i in range(6):
        nested_paren_chunk = nested_paren_chunk % (paren_group,)
    nested_paren_chunk = nested_paren_chunk % (inner_paren_contents,)

    file_links = r"| (?:file://(/[^/ ]*)+/?)" if settings.ENABLE_FILE_LINKS else r""
    REGEX = fr"""
        (?<![^\s'"\(,:<])    # Start after whitespace or specified chars
                             # (Double-negative lookbehind to allow start-of-string)
        (?P<url>             # Main group
            (?:(?:           # Domain part
                https?://[\w.:@-]+?   # If it has a protocol, anything goes.
               |(?:                   # Or, if not, be more strict to avoid false-positives
                    (?:[\w-]+\.)+     # One or more domain components, separated by dots
                    (?:{tlds})            # TLDs (filled in via format from tlds-alpha-by-domain.txt)
                )
            )
            (?:/             # A path, beginning with /
                {nested_paren_chunk}           # zero-to-6 sets of paired parens
            )?)              # Path is optional
            | (?:[\w.-]+\@[\w.-]+\.[\w]+) # Email is separate, since it can't have a path
            {file_links}               # File path start with file:///, enable by setting ENABLE_FILE_LINKS=True
            | (?:bitcoin:[13][a-km-zA-HJ-NP-Z1-9]{{25,34}})  # Bitcoin address pattern, see https://mokagio.github.io/tech-journal/2014/11/21/regex-bitcoin.html
        )
        (?=                            # URL must be followed by (not included in group)
            [!:;\?\),\.\'\"\>]*         # Optional punctuation characters
            (?:\Z|\s)                  # followed by whitespace or end of string
        )
        """
    LINK_REGEX = verbose_compile(REGEX)
    return LINK_REGEX

def clear_state_for_testing() -> None:
    # The link regex never changes in production, but our tests
    # try out both sides of ENABLE_FILE_LINKS, so we need
    # a way to clear it.
    global LINK_REGEX
    LINK_REGEX = None

bugdown_logger = logging.getLogger()

def rewrite_local_links_to_relative(db_data: Optional[DbData], link: str) -> str:
    """If the link points to a local destination (e.g. #narrow/...),
    generate a relative link that will open it in the current window.
    """

    if db_data:
        realm_uri_prefix = db_data['realm_uri'] + "/"
        if (
            link.startswith(realm_uri_prefix)
            and urllib.parse.urljoin(realm_uri_prefix, link[len(realm_uri_prefix):])
            == link
        ):
            return link[len(realm_uri_prefix):]

    return link

def url_embed_preview_enabled(message: Optional[Message]=None,
                              realm: Optional[Realm]=None,
                              no_previews: bool=False) -> bool:
    if not settings.INLINE_URL_EMBED_PREVIEW:
        return False

    if no_previews:
        return False

    if realm is None:
        if message is not None:
            realm = message.get_realm()

    if realm is None:
        # realm can be None for odd use cases
        # like generating documentation or running
        # test code
        return True

    return realm.inline_url_embed_preview

def image_preview_enabled(message: Optional[Message]=None,
                          realm: Optional[Realm]=None,
                          no_previews: bool=False) -> bool:
    if not settings.INLINE_IMAGE_PREVIEW:
        return False

    if no_previews:
        return False

    if realm is None:
        if message is not None:
            realm = message.get_realm()

    if realm is None:
        # realm can be None for odd use cases
        # like generating documentation or running
        # test code
        return True

    return realm.inline_image_preview

def list_of_tlds() -> List[str]:
    # HACK we manually blacklist a few domains
    blacklist = ['PY\n', "MD\n"]

    # tlds-alpha-by-domain.txt comes from https://data.iana.org/TLD/tlds-alpha-by-domain.txt
    tlds_file = os.path.join(os.path.dirname(__file__), 'tlds-alpha-by-domain.txt')
    tlds = [tld.lower().strip() for tld in open(tlds_file)
            if tld not in blacklist and not tld[0].startswith('#')]
    tlds.sort(key=len, reverse=True)
    return tlds

def walk_tree(root: Element,
              processor: Callable[[Element], Optional[_T]],
              stop_after_first: bool=False) -> List[_T]:
    results = []
    queue = deque([root])

    while queue:
        currElement = queue.popleft()
        for child in currElement:
            if child:
                queue.append(child)

            result = processor(child)
            if result is not None:
                results.append(result)
                if stop_after_first:
                    return results

    return results

@dataclass
class ElementFamily:
    grandparent: Optional[Element]
    parent: Element
    child: Element
    in_blockquote: bool

T = TypeVar("T")

class ResultWithFamily(Generic[T]):
    family: ElementFamily
    result: T

    def __init__(self, family: ElementFamily, result: T):
        self.family = family
        self.result = result

class ElementPair:
    parent: Optional["ElementPair"]
    value: Element

    def __init__(self, parent: Optional["ElementPair"], value: Element):
        self.parent = parent
        self.value = value

def walk_tree_with_family(root: Element,
                          processor: Callable[[Element], Optional[_T]],
                          ) -> List[ResultWithFamily[_T]]:
    results = []

    queue = deque([ElementPair(parent=None, value=root)])
    while queue:
        currElementPair = queue.popleft()
        for child in currElementPair.value:
            if child:
                queue.append(ElementPair(parent=currElementPair, value=child))
            result = processor(child)
            if result is not None:
                if currElementPair.parent is not None:
                    grandparent_element = currElementPair.parent
                    grandparent = grandparent_element.value
                else:
                    grandparent = None
                family = ElementFamily(
                    grandparent=grandparent,
                    parent=currElementPair.value,
                    child=child,
                    in_blockquote=has_blockquote_ancestor(currElementPair),
                )

                results.append(ResultWithFamily(
                    family=family,
                    result=result,
                ))

    return results

def has_blockquote_ancestor(element_pair: Optional[ElementPair]) -> bool:
    if element_pair is None:
        return False
    elif element_pair.value.tag == 'blockquote':
        return True
    else:
        return has_blockquote_ancestor(element_pair.parent)

@cache_with_key(lambda tweet_id: tweet_id, cache_name="database", with_statsd_key="tweet_data")
def fetch_tweet_data(tweet_id: str) -> Optional[Dict[str, Any]]:
    if settings.TEST_SUITE:
        from . import testing_mocks
        res = testing_mocks.twitter(tweet_id)
    else:
        creds = {
            'consumer_key': settings.TWITTER_CONSUMER_KEY,
            'consumer_secret': settings.TWITTER_CONSUMER_SECRET,
            'access_token_key': settings.TWITTER_ACCESS_TOKEN_KEY,
            'access_token_secret': settings.TWITTER_ACCESS_TOKEN_SECRET,
        }
        if not all(creds.values()):
            return None

        # We lazily import twitter here because its import process is
        # surprisingly slow, and doing so has a significant impact on
        # the startup performance of `manage.py` commands.
        import twitter

        try:
            api = twitter.Api(tweet_mode='extended', **creds)
            # Sometimes Twitter hangs on responses.  Timing out here
            # will cause the Tweet to go through as-is with no inline
            # preview, rather than having the message be rejected
            # entirely. This timeout needs to be less than our overall
            # formatting timeout.
            tweet = timeout(3, api.GetStatus, tweet_id)
            res = tweet.AsDict()
        except AttributeError:
            bugdown_logger.error('Unable to load twitter api, you may have the wrong '
                                 'library installed, see https://github.com/zulip/zulip/issues/86')
            return None
        except TimeoutExpired:
            # We'd like to try again later and not cache the bad result,
            # so we need to re-raise the exception (just as though
            # we were being rate-limited)
            raise
        except twitter.TwitterError as e:
            t = e.args[0]
            if len(t) == 1 and ('code' in t[0]) and (t[0]['code'] == 34):
                # Code 34 means that the message doesn't exist; return
                # None so that we will cache the error
                return None
            elif len(t) == 1 and ('code' in t[0]) and (t[0]['code'] == 88 or
                                                       t[0]['code'] == 130):
                # Code 88 means that we were rate-limited and 130
                # means Twitter is having capacity issues; either way
                # just raise the error so we don't cache None and will
                # try again later.
                raise
            else:
                # It's not clear what to do in cases of other errors,
                # but for now it seems reasonable to log at error
                # level (so that we get notified), but then cache the
                # failure to proceed with our usual work
                bugdown_logger.exception("Unknown error fetching tweet data")
                return None
    return res

HEAD_START_RE = re.compile('^head[ >]')
HEAD_END_RE = re.compile('^/head[ >]')
META_START_RE = re.compile('^meta[ >]')
META_END_RE = re.compile('^/meta[ >]')

def fetch_open_graph_image(url: str) -> Optional[Dict[str, Any]]:
    in_head = False
    # HTML will auto close meta tags, when we start the next tag add
    # a closing tag if it has not been closed yet.
    last_closed = True
    head = []
    # TODO: What if response content is huge? Should we get headers first?
    try:
        content = requests.get(url, timeout=1).text
    except Exception:
        return None
    # Extract the head and meta tags
    # All meta tags are self closing, have no children or are closed
    # automatically.
    for part in content.split('<'):
        if not in_head and HEAD_START_RE.match(part):
            # Started the head node output it to have a document root
            in_head = True
            head.append('<head>')
        elif in_head and HEAD_END_RE.match(part):
            # Found the end of the head close any remaining tag then stop
            # processing
            in_head = False
            if not last_closed:
                last_closed = True
                head.append('</meta>')
            head.append('</head>')
            break

        elif in_head and META_START_RE.match(part):
            # Found a meta node copy it
            if not last_closed:
                head.append('</meta>')
                last_closed = True
            head.append('<')
            head.append(part)
            if '/>' not in part:
                last_closed = False

        elif in_head and META_END_RE.match(part):
            # End of a meta node just copy it to close the tag
            head.append('<')
            head.append(part)
            last_closed = True

    try:
        doc = etree.fromstring(''.join(head))
    except etree.ParseError:
        return None
    og_image = doc.find('meta[@property="og:image"]')
    og_title = doc.find('meta[@property="og:title"]')
    og_desc = doc.find('meta[@property="og:description"]')
    title = None
    desc = None
    if og_image is not None:
        image = og_image.get('content')
    else:
        return None
    if og_title is not None:
        title = og_title.get('content')
    if og_desc is not None:
        desc = og_desc.get('content')
    return {'image': image, 'title': title, 'desc': desc}

def get_tweet_id(url: str) -> Optional[str]:
    parsed_url = urllib.parse.urlparse(url)
    if not (parsed_url.netloc == 'twitter.com' or parsed_url.netloc.endswith('.twitter.com')):
        return None
    to_match = parsed_url.path
    # In old-style twitter.com/#!/wdaher/status/1231241234-style URLs,
    # we need to look at the fragment instead
    if parsed_url.path == '/' and len(parsed_url.fragment) > 5:
        to_match = parsed_url.fragment

    tweet_id_match = re.match(r'^!?/.*?/status(es)?/(?P<tweetid>\d{10,30})(/photo/[0-9])?/?$', to_match)
    if not tweet_id_match:
        return None
    return tweet_id_match.group("tweetid")

class InlineHttpsProcessor(markdown.treeprocessors.Treeprocessor):
    def run(self, root: Element) -> None:
        # Get all URLs from the blob
        found_imgs = walk_tree(root, lambda e: e if e.tag == "img" else None)
        for img in found_imgs:
            url = img.get("src")
            if urllib.parse.urlsplit(url).scheme != "http":
                # Don't rewrite images on our own site (e.g. emoji).
                continue
            img.set("src", get_camo_url(url))

class BacktickPattern(markdown.inlinepatterns.Pattern):
    """ Return a `<code>` element containing the matching text. """

    def __init__(self, pattern: str) -> None:
        markdown.inlinepatterns.Pattern.__init__(self, pattern)
        self.ESCAPED_BSLASH = '{}{}{}'.format(markdown.util.STX, ord('\\'), markdown.util.ETX)
        self.tag = 'code'

    def handleMatch(self, m: Match[str]) -> Union[str, Element]:
        if m.group(4):
            el = Element(self.tag)
            # Modified to not strip whitespace
            el.text = markdown.util.AtomicString(m.group(4))
            return el
        else:
            return m.group(2).replace('\\\\', self.ESCAPED_BSLASH)

class InlineInterestingLinkProcessor(markdown.treeprocessors.Treeprocessor):
    TWITTER_MAX_IMAGE_HEIGHT = 400
    TWITTER_MAX_TO_PREVIEW = 3
    INLINE_PREVIEW_LIMIT_PER_MESSAGE = 5

    def __init__(self, md: markdown.Markdown) -> None:
        markdown.treeprocessors.Treeprocessor.__init__(self, md)

    def add_a(
            self,
            root: Element,
            url: str,
            link: str,
            title: Optional[str]=None,
            desc: Optional[str]=None,
            class_attr: str="message_inline_image",
            data_id: Optional[str]=None,
            insertion_index: Optional[int]=None,
            already_thumbnailed: bool=False,
    ) -> None:
        desc = desc if desc is not None else ""

        # Update message.has_image attribute.
        if 'message_inline_image' in class_attr and self.md.zulip_message:
            self.md.zulip_message.has_image = True

        if insertion_index is not None:
            div = Element("div")
            root.insert(insertion_index, div)
        else:
            div = SubElement(root, "div")

        div.set("class", class_attr)
        a = SubElement(div, "a")
        a.set("href", link)
        if title is not None:
            a.set("title", title)
        if data_id is not None:
            a.set("data-id", data_id)
        img = SubElement(a, "img")
        if settings.THUMBNAIL_IMAGES and (not already_thumbnailed) and user_uploads_or_external(url):
            # See docs/thumbnailing.md for some high-level documentation.
            #
            # We strip leading '/' from relative URLs here to ensure
            # consistency in what gets passed to /thumbnail
            url = url.lstrip('/')
            img.set("src", "/thumbnail?url={}&size=thumbnail".format(
                urllib.parse.quote(url, safe=''),
            ))
            img.set('data-src-fullsize', "/thumbnail?url={}&size=full".format(
                urllib.parse.quote(url, safe=''),
            ))
        else:
            img.set("src", url)

        if class_attr == "message_inline_ref":
            summary_div = SubElement(div, "div")
            title_div = SubElement(summary_div, "div")
            title_div.set("class", "message_inline_image_title")
            title_div.text = title
            desc_div = SubElement(summary_div, "desc")
            desc_div.set("class", "message_inline_image_desc")

    def add_oembed_data(self, root: Element, link: str, extracted_data: Dict[str, Any]) -> bool:
        oembed_resource_type = extracted_data.get('type', '')
        title = extracted_data.get('title')

        if oembed_resource_type == 'photo':
            image = extracted_data.get('image')
            if image:
                self.add_a(root, image, link, title=title)
                return True

        elif oembed_resource_type == 'video':
            html = extracted_data['html']
            image = extracted_data['image']
            title = extracted_data.get('title')
            description = extracted_data.get('description')
            self.add_a(root, image, link, title, description,
                       "embed-video message_inline_image",
                       html, already_thumbnailed=True)
            return True

        return False

    def add_embed(self, root: Element, link: str, extracted_data: Dict[str, Any]) -> None:
        oembed = extracted_data.get('oembed', False)
        if oembed and self.add_oembed_data(root, link, extracted_data):
            return

        img_link = extracted_data.get('image')
        if not img_link:
            # Don't add an embed if an image is not found
            return

        container = SubElement(root, "div")
        container.set("class", "message_embed")

        parsed_img_link = urllib.parse.urlparse(img_link)
        # Append domain where relative img_link url is given
        if not parsed_img_link.netloc:
            parsed_url = urllib.parse.urlparse(link)
            domain = '{url.scheme}://{url.netloc}/'.format(url=parsed_url)
            img_link = urllib.parse.urljoin(domain, img_link)
        img = SubElement(container, "a")
        img.set("style", "background-image: url(" + img_link + ")")
        img.set("href", link)
        img.set("class", "message_embed_image")

        data_container = SubElement(container, "div")
        data_container.set("class", "data-container")

        title = extracted_data.get('title')
        if title:
            title_elm = SubElement(data_container, "div")
            title_elm.set("class", "message_embed_title")
            a = SubElement(title_elm, "a")
            a.set("href", link)
            a.set("title", title)
            a.text = title
        description = extracted_data.get('description')
        if description:
            description_elm = SubElement(data_container, "div")
            description_elm.set("class", "message_embed_description")
            description_elm.text = description

    def get_actual_image_url(self, url: str) -> str:
        # Add specific per-site cases to convert image-preview urls to image urls.
        # See https://github.com/zulip/zulip/issues/4658 for more information
        parsed_url = urllib.parse.urlparse(url)
        if (parsed_url.netloc == 'github.com' or parsed_url.netloc.endswith('.github.com')):
            # https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png ->
            # https://raw.githubusercontent.com/zulip/zulip/master/static/images/logo/zulip-icon-128x128.png
            split_path = parsed_url.path.split('/')
            if len(split_path) > 3 and split_path[3] == "blob":
                return urllib.parse.urljoin('https://raw.githubusercontent.com',
                                            '/'.join(split_path[0:3] + split_path[4:]))

        return url

    def is_image(self, url: str) -> bool:
        if not self.md.image_preview_enabled:
            return False
        parsed_url = urllib.parse.urlparse(url)
        # remove html urls which end with img extensions that can not be shorted
        if parsed_url.netloc == 'pasteboard.co':
            return False

        # List from https://support.google.com/chromeos/bin/answer.py?hl=en&answer=183093
        for ext in [".bmp", ".gif", ".jpe", "jpeg", ".jpg", ".png", ".webp"]:
            if parsed_url.path.lower().endswith(ext):
                return True
        return False

    def corrected_image_source(self, url: str) -> str:
        # This function adjusts any urls from linx.li and
        # wikipedia.org to point to the actual image url.  It's
        # structurally very similar to dropbox_image, and possibly
        # should be rewritten to use open graph, but has some value.
        parsed_url = urllib.parse.urlparse(url)
        if parsed_url.netloc.lower().endswith('.wikipedia.org'):
            # Redirecting from "/wiki/File:" to "/wiki/Special:FilePath/File:"
            # A possible alternative, that avoids the redirect after hitting "Special:"
            # is using the first characters of md5($filename) to generate the url
            domain = parsed_url.scheme + "://" + parsed_url.netloc
            correct_url = domain + parsed_url.path[:6] + 'Special:FilePath' + parsed_url.path[5:]
            return correct_url
        if parsed_url.netloc == 'linx.li':
            return 'https://linx.li/s' + parsed_url.path
        return None

    def dropbox_image(self, url: str) -> Optional[Dict[str, Any]]:
        # TODO: The returned Dict could possibly be a TypedDict in future.
        parsed_url = urllib.parse.urlparse(url)
        if (parsed_url.netloc == 'dropbox.com' or parsed_url.netloc.endswith('.dropbox.com')):
            is_album = parsed_url.path.startswith('/sc/') or parsed_url.path.startswith('/photos/')
            # Only allow preview Dropbox shared links
            if not (parsed_url.path.startswith('/s/') or
                    parsed_url.path.startswith('/sh/') or
                    is_album):
                return None

            # Try to retrieve open graph protocol info for a preview
            # This might be redundant right now for shared links for images.
            # However, we might want to make use of title and description
            # in the future. If the actual image is too big, we might also
            # want to use the open graph image.
            image_info = fetch_open_graph_image(url)

            is_image = is_album or self.is_image(url)

            # If it is from an album or not an actual image file,
            # just use open graph image.
            if is_album or not is_image:
                # Failed to follow link to find an image preview so
                # use placeholder image and guess filename
                if image_info is None:
                    return None

                image_info["is_image"] = is_image
                return image_info

            # Otherwise, try to retrieve the actual image.
            # This is because open graph image from Dropbox may have padding
            # and gifs do not work.
            # TODO: What if image is huge? Should we get headers first?
            if image_info is None:
                image_info = dict()
            image_info['is_image'] = True
            parsed_url_list = list(parsed_url)
            parsed_url_list[4] = "dl=1"  # Replaces query
            image_info["image"] = urllib.parse.urlunparse(parsed_url_list)

            return image_info
        return None

    def youtube_id(self, url: str) -> Optional[str]:
        if not self.md.image_preview_enabled:
            return None
        # Youtube video id extraction regular expression from https://pastebin.com/KyKAFv1s
        # Slightly modified to support URLs of the forms
        #   - youtu.be/<id>
        #   - youtube.com/playlist?v=<id>&list=<list-id>
        #   - youtube.com/watch_videos?video_ids=<id1>,<id2>,<id3>
        # If it matches, match.group(2) is the video id.
        schema_re = r'(?:https?://)'
        host_re = r'(?:youtu\.be/|(?:\w+\.)?youtube(?:-nocookie)?\.com/)'
        param_re = r'(?:(?:(?:v|embed)/)|' + \
            r'(?:(?:(?:watch|playlist)(?:_popup|_videos)?(?:\.php)?)?(?:\?|#!?)(?:.+&)?v(?:ideo_ids)?=))'
        id_re = r'([0-9A-Za-z_-]+)'
        youtube_re = r'^({schema_re}?{host_re}{param_re}?)?{id_re}(?(1).+)?$'
        youtube_re = youtube_re.format(schema_re=schema_re, host_re=host_re, id_re=id_re, param_re=param_re)
        match = re.match(youtube_re, url)
        # URLs of the form youtube.com/playlist?list=<list-id> are incorrectly matched
        if match is None or match.group(2) == 'playlist':
            return None
        return match.group(2)

    def youtube_title(self, extracted_data: Dict[str, Any]) -> Optional[str]:
        title = extracted_data.get("title")
        if title is not None:
            return f"YouTube - {title}"
        return None

    def youtube_image(self, url: str) -> Optional[str]:
        yt_id = self.youtube_id(url)

        if yt_id is not None:
            return f"https://i.ytimg.com/vi/{yt_id}/default.jpg"
        return None

    def vimeo_id(self, url: str) -> Optional[str]:
        if not self.md.image_preview_enabled:
            return None
        #(http|https)?:\/\/(www\.)?vimeo.com\/(?:channels\/(?:\w+\/)?|groups\/([^\/]*)\/videos\/|)(\d+)(?:|\/\?)
        # If it matches, match.group('id') is the video id.

        vimeo_re = r'^((http|https)?:\/\/(www\.)?vimeo.com\/' + \
                   r'(?:channels\/(?:\w+\/)?|groups\/' + \
                   r'([^\/]*)\/videos\/|)(\d+)(?:|\/\?))$'
        match = re.match(vimeo_re, url)
        if match is None:
            return None
        return match.group(5)

    def vimeo_title(self, extracted_data: Dict[str, Any]) -> Optional[str]:
        title = extracted_data.get("title")
        if title is not None:
            return f"Vimeo - {title}"
        return None

    def twitter_text(self, text: str,
                     urls: List[Dict[str, str]],
                     user_mentions: List[Dict[str, Any]],
                     media: List[Dict[str, Any]]) -> Element:
        """
        Use data from the twitter API to turn links, mentions and media into A
        tags. Also convert unicode emojis to images.

        This works by using the urls, user_mentions and media data from
        the twitter API and searching for unicode emojis in the text using
        `unicode_emoji_regex`.

        The first step is finding the locations of the URLs, mentions, media and
        emoji in the text. For each match we build a dictionary with type, the start
        location, end location, the URL to link to, and the text(codepoint and title
        in case of emojis) to be used in the link(image in case of emojis).

        Next we sort the matches by start location. And for each we add the
        text from the end of the last link to the start of the current link to
        the output. The text needs to added to the text attribute of the first
        node (the P tag) or the tail the last link created.

        Finally we add any remaining text to the last node.
        """

        to_process: List[Dict[str, Any]] = []
        # Build dicts for URLs
        for url_data in urls:
            short_url = url_data["url"]
            full_url = url_data["expanded_url"]
            for match in re.finditer(re.escape(short_url), text, re.IGNORECASE):
                to_process.append({
                    'type': 'url',
                    'start': match.start(),
                    'end': match.end(),
                    'url': short_url,
                    'text': full_url,
                })
        # Build dicts for mentions
        for user_mention in user_mentions:
            screen_name = user_mention['screen_name']
            mention_string = '@' + screen_name
            for match in re.finditer(re.escape(mention_string), text, re.IGNORECASE):
                to_process.append({
                    'type': 'mention',
                    'start': match.start(),
                    'end': match.end(),
                    'url': 'https://twitter.com/' + urllib.parse.quote(screen_name),
                    'text': mention_string,
                })
        # Build dicts for media
        for media_item in media:
            short_url = media_item['url']
            expanded_url = media_item['expanded_url']
            for match in re.finditer(re.escape(short_url), text, re.IGNORECASE):
                to_process.append({
                    'type': 'media',
                    'start': match.start(),
                    'end': match.end(),
                    'url': short_url,
                    'text': expanded_url,
                })
        # Build dicts for emojis
        for match in re.finditer(unicode_emoji_regex, text, re.IGNORECASE):
            orig_syntax = match.group('syntax')
            codepoint = unicode_emoji_to_codepoint(orig_syntax)
            if codepoint in codepoint_to_name:
                display_string = ':' + codepoint_to_name[codepoint] + ':'
                to_process.append({
                    'type': 'emoji',
                    'start': match.start(),
                    'end': match.end(),
                    'codepoint': codepoint,
                    'title': display_string,
                })

        to_process.sort(key=lambda x: x['start'])
        p = current_node = Element('p')

        def set_text(text: str) -> None:
            """
            Helper to set the text or the tail of the current_node
            """
            if current_node == p:
                current_node.text = text
            else:
                current_node.tail = text

        db_data = self.md.zulip_db_data
        current_index = 0
        for item in to_process:
            # The text we want to link starts in already linked text skip it
            if item['start'] < current_index:
                continue
            # Add text from the end of last link to the start of the current
            # link
            set_text(text[current_index:item['start']])
            current_index = item['end']
            if item['type'] != 'emoji':
                elem = url_to_a(db_data, item['url'], item['text'])
                assert isinstance(elem, Element)
            else:
                elem = make_emoji(item['codepoint'], item['title'])
            current_node = elem
            p.append(elem)

        # Add any unused text
        set_text(text[current_index:])
        return p

    def twitter_link(self, url: str) -> Optional[Element]:
        tweet_id = get_tweet_id(url)

        if tweet_id is None:
            return None

        try:
            res = fetch_tweet_data(tweet_id)
            if res is None:
                return None
            user: Dict[str, Any] = res['user']
            tweet = Element("div")
            tweet.set("class", "twitter-tweet")
            img_a = SubElement(tweet, 'a')
            img_a.set("href", url)
            profile_img = SubElement(img_a, 'img')
            profile_img.set('class', 'twitter-avatar')
            # For some reason, for, e.g. tweet 285072525413724161,
            # python-twitter does not give us a
            # profile_image_url_https, but instead puts that URL in
            # profile_image_url. So use _https if available, but fall
            # back gracefully.
            image_url = user.get('profile_image_url_https', user['profile_image_url'])
            profile_img.set('src', image_url)

            text = html.unescape(res['full_text'])
            urls = res.get('urls', [])
            user_mentions = res.get('user_mentions', [])
            media: List[Dict[str, Any]] = res.get('media', [])
            p = self.twitter_text(text, urls, user_mentions, media)
            tweet.append(p)

            span = SubElement(tweet, 'span')
            span.text = "- {} (@{})".format(user['name'], user['screen_name'])

            # Add image previews
            for media_item in media:
                # Only photos have a preview image
                if media_item['type'] != 'photo':
                    continue

                # Find the image size that is smaller than
                # TWITTER_MAX_IMAGE_HEIGHT px tall or the smallest
                size_name_tuples = list(media_item['sizes'].items())
                size_name_tuples.sort(reverse=True,
                                      key=lambda x: x[1]['h'])
                for size_name, size in size_name_tuples:
                    if size['h'] < self.TWITTER_MAX_IMAGE_HEIGHT:
                        break

                media_url = '{}:{}'.format(media_item['media_url_https'], size_name)
                img_div = SubElement(tweet, 'div')
                img_div.set('class', 'twitter-image')
                img_a = SubElement(img_div, 'a')
                img_a.set('href', media_item['url'])
                img = SubElement(img_a, 'img')
                img.set('src', media_url)

            return tweet
        except Exception:
            # We put this in its own try-except because it requires external
            # connectivity. If Twitter flakes out, we don't want to not-render
            # the entire message; we just want to not show the Twitter preview.
            bugdown_logger.warning("Error building Twitter link", exc_info=True)
            return None

    def get_url_data(self, e: Element) -> Optional[Tuple[str, Optional[str]]]:
        if e.tag == "a":
            return (e.get("href"), e.text)
        return None

    def handle_image_inlining(
        self,
        root: Element,
        found_url: ResultWithFamily[Tuple[str, Optional[str]]],
    ) -> None:
        grandparent = found_url.family.grandparent
        parent = found_url.family.parent
        ahref_element = found_url.family.child
        (url, text) = found_url.result
        actual_url = self.get_actual_image_url(url)

        # url != text usually implies a named link, which we opt not to remove
        url_eq_text = text is None or url == text
        title = None if url_eq_text else text

        if parent.tag == 'li':
            self.add_a(parent, self.get_actual_image_url(url), url, title=title)
            if not parent.text and not ahref_element.tail and url_eq_text:
                parent.remove(ahref_element)

        elif parent.tag == 'p':
            parent_index = None
            for index, uncle in enumerate(grandparent):
                if uncle is parent:
                    parent_index = index
                    break

            if parent_index is not None:
                ins_index = self.find_proper_insertion_index(grandparent, parent, parent_index)
                self.add_a(grandparent, actual_url, url, title=title, insertion_index=ins_index)

            else:
                # We're not inserting after parent, since parent not found.
                # Append to end of list of grandparent's children as normal
                self.add_a(grandparent, actual_url, url, title=title)

            # If link is alone in a paragraph, delete paragraph containing it
            if (len(parent) == 1 and
                    (not parent.text or parent.text == "\n") and
                    not ahref_element.tail and
                    url_eq_text):
                grandparent.remove(parent)

        else:
            # If none of the above criteria match, fall back to old behavior
            self.add_a(root, actual_url, url, title=title)

    def find_proper_insertion_index(self, grandparent: Element, parent: Element,
                                    parent_index_in_grandparent: int) -> int:
        # If there are several inline images from same paragraph, ensure that
        # they are in correct (and not opposite) order by inserting after last
        # inline image from paragraph 'parent'

        parent_links = [ele.attrib['href'] for ele in parent.iter(tag="a")]
        insertion_index = parent_index_in_grandparent

        while True:
            insertion_index += 1
            if insertion_index >= len(grandparent):
                return insertion_index

            uncle = grandparent[insertion_index]
            inline_image_classes = ['message_inline_image', 'message_inline_ref']
            if (
                uncle.tag != 'div' or
                'class' not in uncle.keys() or
                uncle.attrib['class'] not in inline_image_classes
            ):
                return insertion_index

            uncle_link = list(uncle.iter(tag="a"))[0].attrib['href']
            if uncle_link not in parent_links:
                return insertion_index

    def is_absolute_url(self, url: str) -> bool:
        return bool(urllib.parse.urlparse(url).netloc)

    def run(self, root: Element) -> None:
        # Get all URLs from the blob
        found_urls = walk_tree_with_family(root, self.get_url_data)
        unique_urls = {found_url.result[0] for found_url in found_urls}
        # Collect unique URLs which are not quoted as we don't do
        # inline previews for links inside blockquotes.
        unique_previewable_urls = {found_url.result[0] for found_url in found_urls
                                   if not found_url.family.in_blockquote}

        # Set has_link and similar flags whenever a message is processed by bugdown
        if self.md.zulip_message:
            self.md.zulip_message.has_link = len(found_urls) > 0
            self.md.zulip_message.has_image = False  # This is updated in self.add_a
            self.md.zulip_message.potential_attachment_path_ids = []

            for url in unique_urls:
                # Due to rewrite_local_links_to_relative, we need to
                # handle both relative URLs beginning with
                # `/user_uploads` and beginning with `user_uploads`.
                # This urllib construction converts the latter into
                # the former.
                parsed_url = urllib.parse.urlsplit(urllib.parse.urljoin("/", url))
                host = parsed_url.netloc

                if host != '' and host != self.md.zulip_realm.host:
                    continue

                if not parsed_url.path.startswith("/user_uploads/"):
                    continue

                path_id = parsed_url.path[len("/user_uploads/"):]
                self.md.zulip_message.potential_attachment_path_ids.append(path_id)

        if len(found_urls) == 0:
            return

        if len(unique_previewable_urls) > self.INLINE_PREVIEW_LIMIT_PER_MESSAGE:
            return

        processed_urls: Set[str] = set()
        rendered_tweet_count = 0

        for found_url in found_urls:
            (url, text) = found_url.result

            if url in unique_previewable_urls and url not in processed_urls:
                processed_urls.add(url)
            else:
                continue

            if not self.is_absolute_url(url):
                if self.is_image(url):
                    self.handle_image_inlining(root, found_url)
                # We don't have a strong use case for doing url preview for relative links.
                continue

            dropbox_image = self.dropbox_image(url)
            if dropbox_image is not None:
                class_attr = "message_inline_ref"
                is_image = dropbox_image["is_image"]
                if is_image:
                    class_attr = "message_inline_image"
                    # Not making use of title and description of images
                self.add_a(root, dropbox_image['image'], url,
                           title=dropbox_image.get('title'),
                           desc=dropbox_image.get('desc', ""),
                           class_attr=class_attr,
                           already_thumbnailed=True)
                continue

            if self.is_image(url):
                image_source = self.corrected_image_source(url)
                if image_source is not None:
                    found_url = ResultWithFamily(
                        family=found_url.family,
                        result=(image_source, image_source),
                    )
                self.handle_image_inlining(root, found_url)
                continue

            if get_tweet_id(url) is not None:
                if rendered_tweet_count >= self.TWITTER_MAX_TO_PREVIEW:
                    # Only render at most one tweet per message
                    continue
                twitter_data = self.twitter_link(url)
                if twitter_data is None:
                    # This link is not actually a tweet known to twitter
                    continue
                rendered_tweet_count += 1
                div = SubElement(root, "div")
                div.set("class", "inline-preview-twitter")
                div.insert(0, twitter_data)
                continue
            youtube = self.youtube_image(url)
            if youtube is not None:
                yt_id = self.youtube_id(url)
                self.add_a(root, youtube, url, None, None,
                           "youtube-video message_inline_image",
                           yt_id, already_thumbnailed=True)
                # NOTE: We don't `continue` here, to allow replacing the URL with
                # the title, if INLINE_URL_EMBED_PREVIEW feature is enabled.
                # The entire preview would ideally be shown only if the feature
                # is enabled, but URL previews are a beta feature and YouTube
                # previews are pretty stable.

            db_data = self.md.zulip_db_data
            if db_data and db_data['sent_by_bot']:
                continue

            if not self.md.url_embed_preview_enabled:
                continue

            try:
                extracted_data = link_preview.link_embed_data_from_cache(url)
            except NotFoundInCache:
                self.md.zulip_message.links_for_preview.add(url)
                continue

            if extracted_data:
                if youtube is not None:
                    title = self.youtube_title(extracted_data)
                    if title is not None:
                        found_url.family.child.text = title
                    continue
                self.add_embed(root, url, extracted_data)
                if self.vimeo_id(url):
                    title = self.vimeo_title(extracted_data)
                    if title:
                        found_url.family.child.text = title

class Avatar(markdown.inlinepatterns.Pattern):
    def handleMatch(self, match: Match[str]) -> Optional[Element]:
        img = Element('img')
        email_address = match.group('email')
        email = email_address.strip().lower()
        profile_id = None

        db_data = self.md.zulip_db_data
        if db_data is not None:
            user_dict = db_data['email_info'].get(email)
            if user_dict is not None:
                profile_id = user_dict['id']

        img.set('class', 'message_body_gravatar')
        img.set('src', f'/avatar/{profile_id or email}?s=30')
        img.set('title', email)
        img.set('alt', email)
        return img

def possible_avatar_emails(content: str) -> Set[str]:
    emails = set()
    for REGEX in [AVATAR_REGEX, GRAVATAR_REGEX]:
        matches = re.findall(REGEX, content)
        for email in matches:
            if email:
                emails.add(email)

    return emails

class Timestamp(markdown.inlinepatterns.Pattern):
    def handleMatch(self, match: Match[str]) -> Optional[Element]:
        time_input_string = match.group('time')
        timestamp = None
        try:
            timestamp = dateutil.parser.parse(time_input_string, tzinfos=get_common_timezones())
        except ValueError:
            try:
                timestamp = datetime.datetime.fromtimestamp(float(time_input_string))
            except ValueError:
                pass

        if not timestamp:
            error_element = Element('span')
            error_element.set('class', 'timestamp-error')
            error_element.text = markdown.util.AtomicString(
                f"Invalid time format: {time_input_string}")
            return error_element

        # Use HTML5 <time> element for valid timestamps.
        time_element = Element('time')
        if timestamp.tzinfo:
            timestamp = timestamp.astimezone(pytz.utc)
        else:
            timestamp = pytz.utc.localize(timestamp)
        time_element.set('datetime', timestamp.isoformat().replace('+00:00', 'Z'))
        # Set text to initial input, so simple clients translating
        # HTML to text will at least display something.
        time_element.text = markdown.util.AtomicString(time_input_string)
        return time_element

# All of our emojis(non ZWJ sequences) belong to one of these unicode blocks:
# \U0001f100-\U0001f1ff - Enclosed Alphanumeric Supplement
# \U0001f200-\U0001f2ff - Enclosed Ideographic Supplement
# \U0001f300-\U0001f5ff - Miscellaneous Symbols and Pictographs
# \U0001f600-\U0001f64f - Emoticons (Emoji)
# \U0001f680-\U0001f6ff - Transport and Map Symbols
# \U0001f900-\U0001f9ff - Supplemental Symbols and Pictographs
# \u2000-\u206f         - General Punctuation
# \u2300-\u23ff         - Miscellaneous Technical
# \u2400-\u243f         - Control Pictures
# \u2440-\u245f         - Optical Character Recognition
# \u2460-\u24ff         - Enclosed Alphanumerics
# \u2500-\u257f         - Box Drawing
# \u2580-\u259f         - Block Elements
# \u25a0-\u25ff         - Geometric Shapes
# \u2600-\u26ff         - Miscellaneous Symbols
# \u2700-\u27bf         - Dingbats
# \u2900-\u297f         - Supplemental Arrows-B
# \u2b00-\u2bff         - Miscellaneous Symbols and Arrows
# \u3000-\u303f         - CJK Symbols and Punctuation
# \u3200-\u32ff         - Enclosed CJK Letters and Months
unicode_emoji_regex = '(?P<syntax>['\
    '\U0001F100-\U0001F64F'    \
    '\U0001F680-\U0001F6FF'    \
    '\U0001F900-\U0001F9FF'    \
    '\u2000-\u206F'            \
    '\u2300-\u27BF'            \
    '\u2900-\u297F'            \
    '\u2B00-\u2BFF'            \
    '\u3000-\u303F'            \
    '\u3200-\u32FF'            \
    '])'
# The equivalent JS regex is \ud83c[\udd00-\udfff]|\ud83d[\udc00-\ude4f]|\ud83d[\ude80-\udeff]|
# \ud83e[\udd00-\uddff]|[\u2000-\u206f]|[\u2300-\u27bf]|[\u2b00-\u2bff]|[\u3000-\u303f]|
# [\u3200-\u32ff]. See below comments for explanation. The JS regex is used by marked.js for
# frontend unicode emoji processing.
# The JS regex \ud83c[\udd00-\udfff]|\ud83d[\udc00-\ude4f] represents U0001f100-\U0001f64f
# The JS regex \ud83d[\ude80-\udeff] represents \U0001f680-\U0001f6ff
# The JS regex \ud83e[\udd00-\uddff] represents \U0001f900-\U0001f9ff
# The JS regex [\u2000-\u206f] represents \u2000-\u206f
# The JS regex [\u2300-\u27bf] represents \u2300-\u27bf
# Similarly other JS regexes can be mapped to the respective unicode blocks.
# For more information, please refer to the following article:
# http://crocodillon.com/blog/parsing-emoji-unicode-in-javascript

def make_emoji(codepoint: str, display_string: str) -> Element:
    # Replace underscore in emoji's title with space
    title = display_string[1:-1].replace("_", " ")
    span = Element('span')
    span.set('class', f'emoji emoji-{codepoint}')
    span.set('title', title)
    span.set('role', 'img')
    span.set('aria-label', title)
    span.text = markdown.util.AtomicString(display_string)
    return span

def make_realm_emoji(src: str, display_string: str) -> Element:
    elt = Element('img')
    elt.set('src', src)
    elt.set('class', 'emoji')
    elt.set("alt", display_string)
    elt.set("title", display_string[1:-1].replace("_", " "))
    return elt

def unicode_emoji_to_codepoint(unicode_emoji: str) -> str:
    codepoint = hex(ord(unicode_emoji))[2:]
    # Unicode codepoints are minimum of length 4, padded
    # with zeroes if the length is less than zero.
    while len(codepoint) < 4:
        codepoint = '0' + codepoint
    return codepoint

class EmoticonTranslation(markdown.inlinepatterns.Pattern):
    """ Translates emoticons like `:)` into emoji like `:smile:`. """

    def handleMatch(self, match: Match[str]) -> Optional[Element]:
        db_data = self.md.zulip_db_data
        if db_data is None or not db_data['translate_emoticons']:
            return None

        emoticon = match.group('emoticon')
        translated = translate_emoticons(emoticon)
        name = translated[1:-1]
        return make_emoji(name_to_codepoint[name], translated)

class UnicodeEmoji(markdown.inlinepatterns.Pattern):
    def handleMatch(self, match: Match[str]) -> Optional[Element]:
        orig_syntax = match.group('syntax')
        codepoint = unicode_emoji_to_codepoint(orig_syntax)
        if codepoint in codepoint_to_name:
            display_string = ':' + codepoint_to_name[codepoint] + ':'
            return make_emoji(codepoint, display_string)
        else:
            return None

class Emoji(markdown.inlinepatterns.Pattern):
    def handleMatch(self, match: Match[str]) -> Optional[Element]:
        orig_syntax = match.group("syntax")
        name = orig_syntax[1:-1]

        active_realm_emoji: Dict[str, Dict[str, str]] = {}
        db_data = self.md.zulip_db_data
        if db_data is not None:
            active_realm_emoji = db_data['active_realm_emoji']

        if self.md.zulip_message and name in active_realm_emoji:
            return make_realm_emoji(active_realm_emoji[name]['source_url'], orig_syntax)
        elif name == 'zulip':
            return make_realm_emoji('/static/generated/emoji/images/emoji/unicode/zulip.png', orig_syntax)
        elif name in name_to_codepoint:
            return make_emoji(name_to_codepoint[name], orig_syntax)
        else:
            return orig_syntax

def content_has_emoji_syntax(content: str) -> bool:
    return re.search(EMOJI_REGEX, content) is not None

class Tex(markdown.inlinepatterns.Pattern):
    def handleMatch(self, match: Match[str]) -> Element:
        rendered = render_tex(match.group('body'), is_inline=True)
        if rendered is not None:
            # We need to give Python-Markdown an ElementTree object, but if we
            # give it one with correctly stored XML namespaces, it will mangle
            # everything when serializing it.  So we play this stupid game to
            # store xmlns as a normal attribute.  :-[
            assert ' zulip-xmlns="' not in rendered
            rendered = rendered.replace(' xmlns="', ' zulip-xmlns="')
            parsed = etree.iterparse(StringIO(rendered))
            for event, elem in parsed:
                if 'zulip-xmlns' in elem.attrib:
                    elem.attrib['xmlns'] = elem.attrib.pop('zulip-xmlns')
                root = elem
            return root
        else:  # Something went wrong while rendering
            span = Element('span')
            span.set('class', 'tex-error')
            span.text = '$$' + match.group('body') + '$$'
            return span


def sanitize_url(url: str) -> Optional[str]:
    """
    Sanitize a url against xss attacks.
    See the docstring on markdown.inlinepatterns.LinkPattern.sanitize_url.
    """
    try:
        parts = urllib.parse.urlparse(url.replace(' ', '%20'))
        scheme, netloc, path, params, query, fragment = parts
    except ValueError:
        # Bad url - so bad it couldn't be parsed.
        return ''

    # If there is no scheme or netloc and there is a '@' in the path,
    # treat it as a mailto: and set the appropriate scheme
    if scheme == '' and netloc == '' and '@' in path:
        scheme = 'mailto'
    elif scheme == '' and netloc == '' and len(path) > 0 and path[0] == '/':
        # Allow domain-relative links
        return urllib.parse.urlunparse(('', '', path, params, query, fragment))
    elif (scheme, netloc, path, params, query) == ('', '', '', '', '') and len(fragment) > 0:
        # Allow fragment links
        return urllib.parse.urlunparse(('', '', '', '', '', fragment))

    # Zulip modification: If scheme is not specified, assume http://
    # We re-enter sanitize_url because netloc etc. need to be re-parsed.
    if not scheme:
        return sanitize_url('http://' + url)

    locless_schemes = ['mailto', 'news', 'file', 'bitcoin']
    if netloc == '' and scheme not in locless_schemes:
        # This fails regardless of anything else.
        # Return immediately to save additional processing
        return None

    # Upstream code will accept a URL like javascript://foo because it
    # appears to have a netloc.  Additionally there are plenty of other
    # schemes that do weird things like launch external programs.  To be
    # on the safe side, we whitelist the scheme.
    if scheme not in ('http', 'https', 'ftp', 'mailto', 'file', 'bitcoin'):
        return None

    # Upstream code scans path, parameters, and query for colon characters
    # because
    #
    #    some aliases [for javascript:] will appear to urllib.parse to have
    #    no scheme. On top of that relative links (i.e.: "foo/bar.html")
    #    have no scheme.
    #
    # We already converted an empty scheme to http:// above, so we skip
    # the colon check, which would also forbid a lot of legitimate URLs.

    # Url passes all tests. Return url as-is.
    return urllib.parse.urlunparse((scheme, netloc, path, params, query, fragment))

def url_to_a(db_data: Optional[DbData], url: str, text: Optional[str]=None) -> Union[Element, str]:
    a = Element('a')

    href = sanitize_url(url)
    if href is None:
        # Rejected by sanitize_url; render it as plain text.
        return url
    if text is None:
        text = markdown.util.AtomicString(url)

    href = rewrite_local_links_to_relative(db_data, href)

    a.set('href', href)
    a.text = text
    return a

class CompiledPattern(markdown.inlinepatterns.Pattern):
    def __init__(self, compiled_re: Pattern, md: markdown.Markdown) -> None:
        # This is similar to the superclass's small __init__ function,
        # but we skip the compilation step and let the caller give us
        # a compiled regex.
        self.compiled_re = compiled_re
        self.md = md

class AutoLink(CompiledPattern):
    def handleMatch(self, match: Match[str]) -> ElementStringNone:
        url = match.group('url')
        db_data = self.md.zulip_db_data
        return url_to_a(db_data, url)

class OListProcessor(sane_lists.SaneOListProcessor):
    def __init__(self, parser: Any) -> None:
        parser.md.tab_length = 2
        super().__init__(parser)
        parser.md.tab_length = 4

class UListProcessor(sane_lists.SaneUListProcessor):
    """ Unordered lists, but with 2-space indent """

    def __init__(self, parser: Any) -> None:
        parser.md.tab_length = 2
        super().__init__(parser)
        parser.md.tab_length = 4

class ListIndentProcessor(markdown.blockprocessors.ListIndentProcessor):
    """ Process unordered list blocks.

        Based on markdown.blockprocessors.ListIndentProcessor, but with 2-space indent
    """

    def __init__(self, parser: Any) -> None:

        # HACK: Set the tab length to 2 just for the initialization of
        # this class, so that bulleted lists (and only bulleted lists)
        # work off 2-space indentation.
        parser.md.tab_length = 2
        super().__init__(parser)
        parser.md.tab_length = 4

class HashHeaderProcessor(markdown.blockprocessors.HashHeaderProcessor):
    """ Process Hash Headers.

        Based on markdown.blockprocessors.HashHeaderProcessor, but requires space for heading.
    """

    # Original regex for hashheader is
    # RE = re.compile(r'(?:^|\n)(?P<level>#{1,6})(?P<header>(?:\\.|[^\\])*?)#*(?:\n|$)')
    RE = re.compile(r'(?:^|\n)(?P<level>#{1,6})\s(?P<header>(?:\\.|[^\\])*?)#*(?:\n|$)')

class BlockQuoteProcessor(markdown.blockprocessors.BlockQuoteProcessor):
    """ Process BlockQuotes.

        Based on markdown.blockprocessors.BlockQuoteProcessor, but with 2-space indent
    """

    # Original regex for blockquote is RE = re.compile(r'(^|\n)[ ]{0,3}>[ ]?(.*)')
    RE = re.compile(r'(^|\n)(?!(?:[ ]{0,3}>\s*(?:$|\n))*(?:$|\n))'
                    r'[ ]{0,3}>[ ]?(.*)')
    mention_re = re.compile(mention.find_mentions)

    def clean(self, line: str) -> str:
        # Silence all the mentions inside blockquotes
        line = re.sub(self.mention_re, lambda m: "@_{}".format(m.group('match')), line)

        # And then run the upstream processor's code for removing the '>'
        return super().clean(line)

@dataclass
class Fence:
    fence_str: str
    is_code: bool

class BugdownListPreprocessor(markdown.preprocessors.Preprocessor):
    """ Allows list blocks that come directly after another block
        to be rendered as a list.

        Detects paragraphs that have a matching list item that comes
        directly after a line of text, and inserts a newline between
        to satisfy Markdown"""

    LI_RE = re.compile(r'^[ ]*([*+-]|\d\.)[ ]+(.*)', re.MULTILINE)

    def run(self, lines: List[str]) -> List[str]:
        """ Insert a newline between a paragraph and ulist if missing """
        inserts = 0
        in_code_fence: bool = False
        open_fences: List[Fence] = []
        copy = lines[:]
        for i in range(len(lines) - 1):
            # Ignore anything that is inside a fenced code block but not quoted.
            # We ignore all lines where some parent is a non quote code block.
            m = FENCE_RE.match(lines[i])
            if m:
                fence_str = m.group('fence')
                is_code = not m.group('lang') in ('quote', 'quoted')
                has_open_fences = not len(open_fences) == 0
                matches_last_fence = fence_str == open_fences[-1].fence_str if has_open_fences else False
                closes_last_fence = not m.group('lang') and matches_last_fence

                if closes_last_fence:
                    open_fences.pop()
                else:
                    open_fences.append(Fence(fence_str, is_code))

                in_code_fence = any([fence.is_code for fence in open_fences])

            # If we're not in a fenced block and we detect an upcoming list
            # hanging off any block (including a list of another type), add
            # a newline.
            li1 = self.LI_RE.match(lines[i])
            li2 = self.LI_RE.match(lines[i+1])
            if not in_code_fence and lines[i]:
                if (li2 and not li1) or (li1 and li2 and
                                         (len(li1.group(1)) == 1) != (len(li2.group(1)) == 1)):
                    copy.insert(i+inserts+1, '')
                    inserts += 1
        return copy

# Name for the outer capture group we use to separate whitespace and
# other delimiters from the actual content.  This value won't be an
# option in user-entered capture groups.
OUTER_CAPTURE_GROUP = "linkifier_actual_match"
def prepare_realm_pattern(source: str) -> str:
    """Augment a realm filter so it only matches after start-of-string,
    whitespace, or opening delimiters, won't match if there are word
    characters directly after, and saves what was matched as
    OUTER_CAPTURE_GROUP."""
    return fr"""(?<![^\s'"\(,:<])(?P<{OUTER_CAPTURE_GROUP}>{source})(?!\w)"""

# Given a regular expression pattern, linkifies groups that match it
# using the provided format string to construct the URL.
class RealmFilterPattern(markdown.inlinepatterns.Pattern):
    """ Applied a given realm filter to the input """

    def __init__(self, source_pattern: str,
                 format_string: str,
                 markdown_instance: Optional[markdown.Markdown]=None) -> None:
        self.pattern = prepare_realm_pattern(source_pattern)
        self.format_string = format_string
        markdown.inlinepatterns.Pattern.__init__(self, self.pattern, markdown_instance)

    def handleMatch(self, m: Match[str]) -> Union[Element, str]:
        db_data = self.md.zulip_db_data
        return url_to_a(db_data,
                        self.format_string % m.groupdict(),
                        m.group(OUTER_CAPTURE_GROUP))

class UserMentionPattern(markdown.inlinepatterns.Pattern):
    def handleMatch(self, m: Match[str]) -> Optional[Element]:
        match = m.group('match')
        silent = m.group('silent') == '_'

        db_data = self.md.zulip_db_data
        if self.md.zulip_message and db_data is not None:
            if match.startswith("**") and match.endswith("**"):
                name = match[2:-2]
            else:
                return None

            wildcard = mention.user_mention_matches_wildcard(name)

            id_syntax_match = re.match(r'.+\|(?P<user_id>\d+)$', name)
            if id_syntax_match:
                id = id_syntax_match.group("user_id")
                user = db_data['mention_data'].get_user_by_id(id)
            else:
                user = db_data['mention_data'].get_user_by_name(name)

            if wildcard:
                self.md.zulip_message.mentions_wildcard = True
                user_id = "*"
            elif user:
                if not silent:
                    self.md.zulip_message.mentions_user_ids.add(user['id'])
                name = user['full_name']
                user_id = str(user['id'])
            else:
                # Don't highlight @mentions that don't refer to a valid user
                return None

            el = Element("span")
            el.set('data-user-id', user_id)
            text = f"{name}"
            if silent:
                el.set('class', 'user-mention silent')
            else:
                el.set('class', 'user-mention')
                text = f"@{text}"
            el.text = markdown.util.AtomicString(text)
            return el
        return None

class UserGroupMentionPattern(markdown.inlinepatterns.Pattern):
    def handleMatch(self, m: Match[str]) -> Optional[Element]:
        match = m.group(2)

        db_data = self.md.zulip_db_data
        if self.md.zulip_message and db_data is not None:
            name = extract_user_group(match)
            user_group = db_data['mention_data'].get_user_group(name)
            if user_group:
                self.md.zulip_message.mentions_user_group_ids.add(user_group.id)
                name = user_group.name
                user_group_id = str(user_group.id)
            else:
                # Don't highlight @-mentions that don't refer to a valid user
                # group.
                return None

            el = Element("span")
            el.set('class', 'user-group-mention')
            el.set('data-user-group-id', user_group_id)
            text = f"@{name}"
            el.text = markdown.util.AtomicString(text)
            return el
        return None

class StreamPattern(CompiledPattern):
    def find_stream_by_name(self, name: Match[str]) -> Optional[Dict[str, Any]]:
        db_data = self.md.zulip_db_data
        if db_data is None:
            return None
        stream = db_data['stream_names'].get(name)
        return stream

    def handleMatch(self, m: Match[str]) -> Optional[Element]:
        name = m.group('stream_name')

        if self.md.zulip_message:
            stream = self.find_stream_by_name(name)
            if stream is None:
                return None
            el = Element('a')
            el.set('class', 'stream')
            el.set('data-stream-id', str(stream['id']))
            # TODO: We should quite possibly not be specifying the
            # href here and instead having the browser auto-add the
            # href when it processes a message with one of these, to
            # provide more clarity to API clients.
            # Also do the same for StreamTopicPattern.
            stream_url = encode_stream(stream['id'], name)
            el.set('href', f'/#narrow/stream/{stream_url}')
            text = f'#{name}'
            el.text = markdown.util.AtomicString(text)
            return el
        return None

class StreamTopicPattern(CompiledPattern):
    def find_stream_by_name(self, name: Match[str]) -> Optional[Dict[str, Any]]:
        db_data = self.md.zulip_db_data
        if db_data is None:
            return None
        stream = db_data['stream_names'].get(name)
        return stream

    def handleMatch(self, m: Match[str]) -> Optional[Element]:
        stream_name = m.group('stream_name')
        topic_name = m.group('topic_name')

        if self.md.zulip_message:
            stream = self.find_stream_by_name(stream_name)
            if stream is None or topic_name is None:
                return None
            el = Element('a')
            el.set('class', 'stream-topic')
            el.set('data-stream-id', str(stream['id']))
            stream_url = encode_stream(stream['id'], stream_name)
            topic_url = hash_util_encode(topic_name)
            link = f'/#narrow/stream/{stream_url}/topic/{topic_url}'
            el.set('href', link)
            text = f'#{stream_name} > {topic_name}'
            el.text = markdown.util.AtomicString(text)
            return el
        return None

def possible_linked_stream_names(content: str) -> Set[str]:
    matches = re.findall(STREAM_LINK_REGEX, content, re.VERBOSE)
    for match in re.finditer(STREAM_TOPIC_LINK_REGEX, content, re.VERBOSE):
        matches.append(match.group('stream_name'))
    return set(matches)

class AlertWordNotificationProcessor(markdown.preprocessors.Preprocessor):

    allowed_before_punctuation = {' ', '\n', '(', '"', '.', ',', '\'', ';', '[', '*', '`', '>'}
    allowed_after_punctuation = {' ', '\n', ')', '",', '?', ':', '.', ',', '\'', ';', ']', '!',
                                 '*', '`'}

    def check_valid_start_position(self, content: str, index: int) -> bool:
        if index <= 0 or content[index] in self.allowed_before_punctuation:
            return True
        return False

    def check_valid_end_position(self, content: str, index: int) -> bool:
        if index >= len(content) or content[index] in self.allowed_after_punctuation:
            return True
        return False

    def run(self, lines: Iterable[str]) -> Iterable[str]:
        db_data = self.md.zulip_db_data
        if self.md.zulip_message and db_data is not None:
            # We check for alert words here, the set of which are
            # dependent on which users may see this message.
            #
            # Our caller passes in the list of possible_words.  We
            # don't do any special rendering; we just append the alert words
            # we find to the set self.md.zulip_message.alert_words.

            realm_alert_words_automaton = db_data['realm_alert_words_automaton']

            if realm_alert_words_automaton is not None:
                content = '\n'.join(lines).lower()
                for end_index, (original_value, user_ids) in realm_alert_words_automaton.iter(content):
                    if self.check_valid_start_position(content, end_index - len(original_value)) and \
                       self.check_valid_end_position(content, end_index + 1):
                        self.md.zulip_message.user_ids_with_alert_words.update(user_ids)
        return lines

class LinkInlineProcessor(markdown.inlinepatterns.LinkInlineProcessor):
    def zulip_specific_link_changes(self, el: Element) -> Union[None, Element]:
        href = el.get('href')

        # Sanitize url or don't parse link. See linkify_tests in markdown_test_cases for banned syntax.
        href = sanitize_url(self.unescape(href.strip()))
        if href is None:
            return None  # no-op; the link is not processed.

        # Rewrite local links to be relative
        db_data = self.md.zulip_db_data
        href = rewrite_local_links_to_relative(db_data, href)

        # Make changes to <a> tag attributes
        el.set("href", href)

        # Show link href if title is empty
        if not el.text.strip():
            el.text = href

        # Prevent realm_filters from running on the content of a Markdown link, breaking up the link.
        # This is a monkey-patch, but it might be worth sending a version of this change upstream.
        el.text = markdown.util.AtomicString(el.text)

        return el

    def handleMatch(self, m: Match[str], data: str) -> Tuple[Union[None, Element], int, int]:
        el, match_start, index = super().handleMatch(m, data)
        if el is not None:
            el = self.zulip_specific_link_changes(el)
        return el, match_start, index

def get_sub_registry(r: markdown.util.Registry, keys: List[str]) -> markdown.util.Registry:
    # Registry is a new class added by py-markdown to replace Ordered List.
    # Since Registry doesn't support .keys(), it is easier to make a new
    # object instead of removing keys from the existing object.
    new_r = markdown.util.Registry()
    for k in keys:
        new_r.register(r[k], k, r.get_index_for_name(k))
    return new_r

# These are used as keys ("realm_filters_keys") to md_engines and the respective
# realm filter caches
DEFAULT_BUGDOWN_KEY = -1
ZEPHYR_MIRROR_BUGDOWN_KEY = -2

class Bugdown(markdown.Markdown):
    def __init__(self, *args: Any, **kwargs: Union[bool, int, List[Any]]) -> None:
        # define default configs
        self.config = {
            "realm_filters": [kwargs['realm_filters'],
                              "Realm-specific filters for realm_filters_key {}".format(kwargs['realm'])],
            "realm": [kwargs['realm'], "Realm id"],
            "code_block_processor_disabled": [kwargs['code_block_processor_disabled'],
                                              "Disabled for email gateway"],
        }

        super().__init__(*args, **kwargs)
        self.set_output_format('html')

    def build_parser(self) -> markdown.Markdown:
        # Build the parser using selected default features from py-markdown.
        # The complete list of all available processors can be found in the
        # super().build_parser() function.
        #
        # Note: for any py-markdown updates, manually check if we want any
        # of the new features added upstream or not; they wouldn't get
        # included by default.
        self.preprocessors = self.build_preprocessors()
        self.parser = self.build_block_parser()
        self.inlinePatterns = self.build_inlinepatterns()
        self.treeprocessors = self.build_treeprocessors()
        self.postprocessors = self.build_postprocessors()
        self.handle_zephyr_mirror()
        return self

    def build_preprocessors(self) -> markdown.util.Registry:
        # We disable the following preprocessors from upstream:
        #
        # html_block - insecure
        # reference - references don't make sense in a chat context.
        preprocessors = markdown.util.Registry()
        preprocessors.register(BugdownListPreprocessor(self), 'hanging_lists', 35)
        preprocessors.register(markdown.preprocessors.NormalizeWhitespace(self), 'normalize_whitespace', 30)
        preprocessors.register(fenced_code.FencedBlockPreprocessor(self), 'fenced_code_block', 25)
        preprocessors.register(AlertWordNotificationProcessor(self), 'custom_text_notifications', 20)
        return preprocessors

    def build_block_parser(self) -> markdown.util.Registry:
        # We disable the following blockparsers from upstream:
        #
        # indent - replaced by ours
        # setextheader - disabled; we only support hashheaders for headings
        # olist - replaced by ours
        # ulist - replaced by ours
        # quote - replaced by ours
        parser = markdown.blockprocessors.BlockParser(self)
        parser.blockprocessors.register(markdown.blockprocessors.EmptyBlockProcessor(parser), 'empty', 95)
        parser.blockprocessors.register(ListIndentProcessor(parser), 'indent', 90)
        if not self.getConfig('code_block_processor_disabled'):
            parser.blockprocessors.register(markdown.blockprocessors.CodeBlockProcessor(parser), 'code', 85)
        parser.blockprocessors.register(HashHeaderProcessor(parser), 'hashheader', 80)
        # We get priority 75 from 'table' extension
        parser.blockprocessors.register(markdown.blockprocessors.HRProcessor(parser), 'hr', 70)
        parser.blockprocessors.register(OListProcessor(parser), 'olist', 65)
        parser.blockprocessors.register(UListProcessor(parser), 'ulist', 60)
        parser.blockprocessors.register(BlockQuoteProcessor(parser), 'quote', 55)
        parser.blockprocessors.register(markdown.blockprocessors.ParagraphProcessor(parser), 'paragraph', 50)
        return parser

    def build_inlinepatterns(self) -> markdown.util.Registry:
        # We disable the following upstream inline patterns:
        #
        # backtick -        replaced by ours
        # escape -          probably will re-add at some point.
        # link -            replaced by ours
        # image_link -      replaced by ours
        # autolink -        replaced by ours
        # automail -        replaced by ours
        # linebreak -       we use nl2br and consider that good enough
        # html -            insecure
        # reference -       references not useful
        # image_reference - references not useful
        # short_reference - references not useful
        # ---------------------------------------------------
        # strong_em -       for these three patterns,
        # strong2 -         we have our own versions where
        # emphasis2 -       we disable _ for bold and emphasis

        # Declare regexes for clean single line calls to .register().
        NOT_STRONG_RE = markdown.inlinepatterns.NOT_STRONG_RE
        # Custom strikethrough syntax: ~~foo~~
        DEL_RE = r'(?<!~)(\~\~)([^~\n]+?)(\~\~)(?!~)'
        # Custom bold syntax: **foo** but not __foo__
        # str inside ** must start and end with a word character
        # it need for things like "const char *x = (char *)y"
        EMPHASIS_RE = r'(\*)(?!\s+)([^\*^\n]+)(?<!\s)\*'
        ENTITY_RE = markdown.inlinepatterns.ENTITY_RE
        STRONG_EM_RE = r'(\*\*\*)(?!\s+)([^\*^\n]+)(?<!\s)\*\*\*'
        # Inline code block without whitespace stripping
        BACKTICK_RE = r'(?:(?<!\\)((?:\\{2})+)(?=`+)|(?<!\\)(`+)(.+?)(?<!`)\3(?!`))'

        # Add Inline Patterns.  We use a custom numbering of the
        # rules, that preserves the order from upstream but leaves
        # space for us to add our own.
        reg = markdown.util.Registry()
        reg.register(BacktickPattern(BACKTICK_RE), 'backtick', 105)
        reg.register(markdown.inlinepatterns.DoubleTagPattern(STRONG_EM_RE, 'strong,em'), 'strong_em', 100)
        reg.register(UserMentionPattern(mention.find_mentions, self), 'usermention', 95)
        reg.register(Tex(r'\B(?<!\$)\$\$(?P<body>[^\n_$](\\\$|[^$\n])*)\$\$(?!\$)\B'), 'tex', 90)
        reg.register(StreamTopicPattern(get_compiled_stream_topic_link_regex(), self), 'topic', 87)
        reg.register(StreamPattern(get_compiled_stream_link_regex(), self), 'stream', 85)
        reg.register(Avatar(AVATAR_REGEX, self), 'avatar', 80)
        reg.register(Timestamp(r'!time\((?P<time>[^)]*)\)'), 'timestamp', 75)
        # Note that !gravatar syntax should be deprecated long term.
        reg.register(Avatar(GRAVATAR_REGEX, self), 'gravatar', 70)
        reg.register(UserGroupMentionPattern(mention.user_group_mentions, self), 'usergroupmention', 65)
        reg.register(LinkInlineProcessor(markdown.inlinepatterns.LINK_RE, self), 'link', 60)
        reg.register(AutoLink(get_web_link_regex(), self), 'autolink', 55)
        # Reserve priority 45-54 for Realm Filters
        reg = self.register_realm_filters(reg)
        reg.register(markdown.inlinepatterns.HtmlInlineProcessor(ENTITY_RE, self), 'entity', 40)
        reg.register(markdown.inlinepatterns.SimpleTagPattern(r'(\*\*)([^\n]+?)\2', 'strong'), 'strong', 35)
        reg.register(markdown.inlinepatterns.SimpleTagPattern(EMPHASIS_RE, 'em'), 'emphasis', 30)
        reg.register(markdown.inlinepatterns.SimpleTagPattern(DEL_RE, 'del'), 'del', 25)
        reg.register(markdown.inlinepatterns.SimpleTextInlineProcessor(NOT_STRONG_RE), 'not_strong', 20)
        reg.register(Emoji(EMOJI_REGEX, self), 'emoji', 15)
        reg.register(EmoticonTranslation(emoticon_regex, self), 'translate_emoticons', 10)
        # We get priority 5 from 'nl2br' extension
        reg.register(UnicodeEmoji(unicode_emoji_regex), 'unicodeemoji', 0)
        return reg

    def register_realm_filters(self, inlinePatterns: markdown.util.Registry) -> markdown.util.Registry:
        for (pattern, format_string, id) in self.getConfig("realm_filters"):
            inlinePatterns.register(RealmFilterPattern(pattern, format_string, self),
                                    f'realm_filters/{pattern}', 45)
        return inlinePatterns

    def build_treeprocessors(self) -> markdown.util.Registry:
        # Here we build all the processors from upstream, plus a few of our own.
        treeprocessors = markdown.util.Registry()
        # We get priority 30 from 'hilite' extension
        treeprocessors.register(markdown.treeprocessors.InlineProcessor(self), 'inline', 25)
        treeprocessors.register(markdown.treeprocessors.PrettifyTreeprocessor(self), 'prettify', 20)
        treeprocessors.register(InlineInterestingLinkProcessor(self), 'inline_interesting_links', 15)
        if settings.CAMO_URI:
            treeprocessors.register(InlineHttpsProcessor(self), 'rewrite_to_https', 10)
        return treeprocessors

    def build_postprocessors(self) -> markdown.util.Registry:
        # These are the default python-markdown processors, unmodified.
        postprocessors = markdown.util.Registry()
        postprocessors.register(markdown.postprocessors.RawHtmlPostprocessor(self), 'raw_html', 20)
        postprocessors.register(markdown.postprocessors.AndSubstitutePostprocessor(), 'amp_substitute', 15)
        postprocessors.register(markdown.postprocessors.UnescapePostprocessor(), 'unescape', 10)
        return postprocessors

    def getConfig(self, key: str, default: str='') -> Any:
        """ Return a setting for the given key or an empty string. """
        if key in self.config:
            return self.config[key][0]
        else:
            return default

    def handle_zephyr_mirror(self) -> None:
        if self.getConfig("realm") == ZEPHYR_MIRROR_BUGDOWN_KEY:
            # Disable almost all inline patterns for zephyr mirror
            # users' traffic that is mirrored.  Note that
            # inline_interesting_links is a treeprocessor and thus is
            # not removed
            self.inlinePatterns = get_sub_registry(self.inlinePatterns, ['autolink'])
            self.treeprocessors = get_sub_registry(self.treeprocessors, ['inline_interesting_links',
                                                                         'rewrite_to_https'])
            # insert new 'inline' processor because we have changed self.inlinePatterns
            # but InlineProcessor copies md as self.md in __init__.
            self.treeprocessors.register(markdown.treeprocessors.InlineProcessor(self), 'inline', 25)
            self.preprocessors = get_sub_registry(self.preprocessors, ['custom_text_notifications'])
            self.parser.blockprocessors = get_sub_registry(self.parser.blockprocessors, ['paragraph'])

md_engines: Dict[Tuple[int, bool], markdown.Markdown] = {}
realm_filter_data: Dict[int, List[Tuple[str, str, int]]] = {}

def make_md_engine(realm_filters_key: int, email_gateway: bool) -> None:
    md_engine_key = (realm_filters_key, email_gateway)
    if md_engine_key in md_engines:
        del md_engines[md_engine_key]

    realm_filters = realm_filter_data[realm_filters_key]
    md_engines[md_engine_key] = build_engine(
        realm_filters=realm_filters,
        realm_filters_key=realm_filters_key,
        email_gateway=email_gateway,
    )

def build_engine(realm_filters: List[Tuple[str, str, int]],
                 realm_filters_key: int,
                 email_gateway: bool) -> markdown.Markdown:
    engine = Bugdown(
        realm_filters=realm_filters,
        realm=realm_filters_key,
        code_block_processor_disabled=email_gateway,
        extensions = [
            nl2br.makeExtension(),
            tables.makeExtension(),
            codehilite.makeExtension(
                linenums=False,
                guess_lang=False,
            ),
        ])
    return engine

# Split the topic name into multiple sections so that we can easily use
# our common single link matching regex on it.
basic_link_splitter = re.compile(r'[ !;\?\),\'\"]')

# Security note: We don't do any HTML escaping in this
# function on the URLs; they are expected to be HTML-escaped when
# rendered by clients (just as links rendered into message bodies
# are validated and escaped inside `url_to_a`).
def topic_links(realm_filters_key: int, topic_name: str) -> List[str]:
    matches: List[str] = []

    realm_filters = realm_filters_for_realm(realm_filters_key)

    for realm_filter in realm_filters:
        pattern = prepare_realm_pattern(realm_filter[0])
        for m in re.finditer(pattern, topic_name):
            matches += [realm_filter[1] % m.groupdict()]

    # Also make raw urls navigable.
    for sub_string in basic_link_splitter.split(topic_name):
        link_match = re.match(get_web_link_regex(), sub_string)
        if link_match:
            url = link_match.group('url')
            url_object = parse(url)
            if not url_object.scheme:
                url = url_object.replace(scheme='https').to_text()
            matches.append(url)

    return matches

def maybe_update_markdown_engines(realm_filters_key: Optional[int], email_gateway: bool) -> None:
    # If realm_filters_key is None, load all filters
    global realm_filter_data
    if realm_filters_key is None:
        all_filters = all_realm_filters()
        all_filters[DEFAULT_BUGDOWN_KEY] = []
        for realm_filters_key, filters in all_filters.items():
            realm_filter_data[realm_filters_key] = filters
            make_md_engine(realm_filters_key, email_gateway)
        # Hack to ensure that getConfig("realm") is right for mirrored Zephyrs
        realm_filter_data[ZEPHYR_MIRROR_BUGDOWN_KEY] = []
        make_md_engine(ZEPHYR_MIRROR_BUGDOWN_KEY, False)
    else:
        realm_filters = realm_filters_for_realm(realm_filters_key)
        if realm_filters_key not in realm_filter_data or    \
                realm_filter_data[realm_filters_key] != realm_filters:
            # Realm filters data has changed, update `realm_filter_data` and any
            # of the existing markdown engines using this set of realm filters.
            realm_filter_data[realm_filters_key] = realm_filters
            for email_gateway_flag in [True, False]:
                if (realm_filters_key, email_gateway_flag) in md_engines:
                    # Update only existing engines(if any), don't create new one.
                    make_md_engine(realm_filters_key, email_gateway_flag)

        if (realm_filters_key, email_gateway) not in md_engines:
            # Markdown engine corresponding to this key doesn't exists so create one.
            make_md_engine(realm_filters_key, email_gateway)

# We want to log Markdown parser failures, but shouldn't log the actual input
# message for privacy reasons.  The compromise is to replace all alphanumeric
# characters with 'x'.
#
# We also use repr() to improve reproducibility, and to escape terminal control
# codes, which can do surprisingly nasty things.
_privacy_re = re.compile('\\w', flags=re.UNICODE)
def privacy_clean_markdown(content: str) -> str:
    return repr(_privacy_re.sub('x', content))

def log_bugdown_error(msg: str) -> None:
    """We use this unusual logging approach to log the bugdown error, in
    order to prevent AdminNotifyHandler from sending the sanitized
    original markdown formatting into another Zulip message, which
    could cause an infinite exception loop."""
    bugdown_logger.error(msg)

def get_email_info(realm_id: int, emails: Set[str]) -> Dict[str, FullNameInfo]:
    if not emails:
        return dict()

    q_list = {
        Q(email__iexact=email.strip().lower())
        for email in emails
    }

    rows = UserProfile.objects.filter(
        realm_id=realm_id,
    ).filter(
        functools.reduce(lambda a, b: a | b, q_list),
    ).values(
        'id',
        'email',
    )

    dct = {
        row['email'].strip().lower(): row
        for row in rows
    }
    return dct

def get_possible_mentions_info(realm_id: int, mention_texts: Set[str]) -> List[FullNameInfo]:
    if not mention_texts:
        return list()

    # Remove the trailing part of the `name|id` mention syntax,
    # thus storing only full names in full_names.
    full_names = set()
    name_re = r'(?P<full_name>.+)\|\d+$'
    for mention_text in mention_texts:
        name_syntax_match = re.match(name_re, mention_text)
        if name_syntax_match:
            full_names.add(name_syntax_match.group("full_name"))
        else:
            full_names.add(mention_text)

    q_list = {
        Q(full_name__iexact=full_name)
        for full_name in full_names
    }

    rows = UserProfile.objects.filter(
        realm_id=realm_id,
        is_active=True,
    ).filter(
        functools.reduce(lambda a, b: a | b, q_list),
    ).values(
        'id',
        'full_name',
        'email',
    )
    return list(rows)

class MentionData:
    def __init__(self, realm_id: int, content: str) -> None:
        mention_texts, has_wildcards = possible_mentions(content)
        possible_mentions_info = get_possible_mentions_info(realm_id, mention_texts)
        self.full_name_info = {
            row['full_name'].lower(): row
            for row in possible_mentions_info
        }
        self.user_id_info = {
            row['id']: row
            for row in possible_mentions_info
        }
        self.init_user_group_data(realm_id=realm_id, content=content)
        self.has_wildcards = has_wildcards

    def message_has_wildcards(self) -> bool:
        return self.has_wildcards

    def init_user_group_data(self,
                             realm_id: int,
                             content: str) -> None:
        user_group_names = possible_user_group_mentions(content)
        self.user_group_name_info = get_user_group_name_info(realm_id, user_group_names)
        self.user_group_members: Dict[int, List[int]] = defaultdict(list)
        group_ids = [group.id for group in self.user_group_name_info.values()]

        if not group_ids:
            # Early-return to avoid the cost of hitting the ORM,
            # which shows up in profiles.
            return

        membership = UserGroupMembership.objects.filter(user_group_id__in=group_ids)
        for info in membership.values('user_group_id', 'user_profile_id'):
            group_id = info['user_group_id']
            user_profile_id = info['user_profile_id']
            self.user_group_members[group_id].append(user_profile_id)

    def get_user_by_name(self, name: str) -> Optional[FullNameInfo]:
        # warning: get_user_by_name is not dependable if two
        # users of the same full name are mentioned. Use
        # get_user_by_id where possible.
        return self.full_name_info.get(name.lower(), None)

    def get_user_by_id(self, id: str) -> Optional[FullNameInfo]:
        return self.user_id_info.get(int(id), None)

    def get_user_ids(self) -> Set[int]:
        """
        Returns the user IDs that might have been mentioned by this
        content.  Note that because this data structure has not parsed
        the message and does not know about escaping/code blocks, this
        will overestimate the list of user ids.
        """
        return set(self.user_id_info.keys())

    def get_user_group(self, name: str) -> Optional[UserGroup]:
        return self.user_group_name_info.get(name.lower(), None)

    def get_group_members(self, user_group_id: int) -> List[int]:
        return self.user_group_members.get(user_group_id, [])

def get_user_group_name_info(realm_id: int, user_group_names: Set[str]) -> Dict[str, UserGroup]:
    if not user_group_names:
        return dict()

    rows = UserGroup.objects.filter(realm_id=realm_id,
                                    name__in=user_group_names)
    dct = {row.name.lower(): row for row in rows}
    return dct

def get_stream_name_info(realm: Realm, stream_names: Set[str]) -> Dict[str, FullNameInfo]:
    if not stream_names:
        return dict()

    q_list = {
        Q(name=name)
        for name in stream_names
    }

    rows = get_active_streams(
        realm=realm,
    ).filter(
        functools.reduce(lambda a, b: a | b, q_list),
    ).values(
        'id',
        'name',
    )

    dct = {
        row['name']: row
        for row in rows
    }
    return dct


def do_convert(content: str,
               realm_alert_words_automaton: Optional[ahocorasick.Automaton] = None,
               message: Optional[Message]=None,
               message_realm: Optional[Realm]=None,
               sent_by_bot: bool=False,
               translate_emoticons: bool=False,
               mention_data: Optional[MentionData]=None,
               email_gateway: bool=False,
               no_previews: bool=False) -> str:
    """Convert Markdown to HTML, with Zulip-specific settings and hacks."""
    # This logic is a bit convoluted, but the overall goal is to support a range of use cases:
    # * Nothing is passed in other than content -> just run default options (e.g. for docs)
    # * message is passed, but no realm is -> look up realm from message
    # * message_realm is passed -> use that realm for bugdown purposes
    if message is not None:
        if message_realm is None:
            message_realm = message.get_realm()
    if message_realm is None:
        realm_filters_key = DEFAULT_BUGDOWN_KEY
    else:
        realm_filters_key = message_realm.id

    if message and hasattr(message, 'id') and message.id:
        logging_message_id = 'id# ' + str(message.id)
    else:
        logging_message_id = 'unknown'

    if message is not None and message_realm is not None:
        if message_realm.is_zephyr_mirror_realm:
            if message.sending_client.name == "zephyr_mirror":
                # Use slightly customized Markdown processor for content
                # delivered via zephyr_mirror
                realm_filters_key = ZEPHYR_MIRROR_BUGDOWN_KEY

    maybe_update_markdown_engines(realm_filters_key, email_gateway)
    md_engine_key = (realm_filters_key, email_gateway)

    if md_engine_key in md_engines:
        _md_engine = md_engines[md_engine_key]
    else:
        if DEFAULT_BUGDOWN_KEY not in md_engines:
            maybe_update_markdown_engines(realm_filters_key=None, email_gateway=False)

        _md_engine = md_engines[(DEFAULT_BUGDOWN_KEY, email_gateway)]
    # Reset the parser; otherwise it will get slower over time.
    _md_engine.reset()

    # Filters such as UserMentionPattern need a message.
    _md_engine.zulip_message = message
    _md_engine.zulip_realm = message_realm
    _md_engine.zulip_db_data = None  # for now
    _md_engine.image_preview_enabled = image_preview_enabled(
        message, message_realm, no_previews)
    _md_engine.url_embed_preview_enabled = url_embed_preview_enabled(
        message, message_realm, no_previews)

    # Pre-fetch data from the DB that is used in the bugdown thread
    if message is not None:
        assert message_realm is not None  # ensured above if message is not None

        # Here we fetch the data structures needed to render
        # mentions/avatars/stream mentions from the database, but only
        # if there is syntax in the message that might use them, since
        # the fetches are somewhat expensive and these types of syntax
        # are uncommon enough that it's a useful optimization.

        if mention_data is None:
            mention_data = MentionData(message_realm.id, content)

        emails = possible_avatar_emails(content)
        email_info = get_email_info(message_realm.id, emails)

        stream_names = possible_linked_stream_names(content)
        stream_name_info = get_stream_name_info(message_realm, stream_names)

        if content_has_emoji_syntax(content):
            active_realm_emoji = message_realm.get_active_emoji()
        else:
            active_realm_emoji = dict()

        _md_engine.zulip_db_data = {
            'realm_alert_words_automaton': realm_alert_words_automaton,
            'email_info': email_info,
            'mention_data': mention_data,
            'active_realm_emoji': active_realm_emoji,
            'realm_uri': message_realm.uri,
            'sent_by_bot': sent_by_bot,
            'stream_names': stream_name_info,
            'translate_emoticons': translate_emoticons,
        }

    try:
        # Spend at most 5 seconds rendering; this protects the backend
        # from being overloaded by bugs (e.g. markdown logic that is
        # extremely inefficient in corner cases) as well as user
        # errors (e.g. a realm filter that makes some syntax
        # infinite-loop).
        rendered_content = timeout(5, _md_engine.convert, content)

        # Throw an exception if the content is huge; this protects the
        # rest of the codebase from any bugs where we end up rendering
        # something huge.
        if len(rendered_content) > MAX_MESSAGE_LENGTH * 10:
            raise BugdownRenderingException(
                f'Rendered content exceeds {MAX_MESSAGE_LENGTH * 10} characters (message {logging_message_id})'
            )
        return rendered_content
    except Exception:
        cleaned = privacy_clean_markdown(content)
        # NOTE: Don't change this message without also changing the
        # logic in logging_handlers.py or we can create recursive
        # exceptions.
        bugdown_logger.exception(
            'Exception in Markdown parser; input (sanitized) was: %s\n (message %s)',
            cleaned,
            logging_message_id,
        )

        raise BugdownRenderingException()
    finally:
        # These next three lines are slightly paranoid, since
        # we always set these right before actually using the
        # engine, but better safe then sorry.
        _md_engine.zulip_message = None
        _md_engine.zulip_realm = None
        _md_engine.zulip_db_data = None

bugdown_time_start = 0.0
bugdown_total_time = 0.0
bugdown_total_requests = 0

def get_bugdown_time() -> float:
    return bugdown_total_time

def get_bugdown_requests() -> int:
    return bugdown_total_requests

def bugdown_stats_start() -> None:
    global bugdown_time_start
    bugdown_time_start = time.time()

def bugdown_stats_finish() -> None:
    global bugdown_total_time
    global bugdown_total_requests
    global bugdown_time_start
    bugdown_total_requests += 1
    bugdown_total_time += (time.time() - bugdown_time_start)

def convert(content: str,
            realm_alert_words_automaton: Optional[ahocorasick.Automaton] = None,
            message: Optional[Message]=None,
            message_realm: Optional[Realm]=None,
            sent_by_bot: bool=False,
            translate_emoticons: bool=False,
            mention_data: Optional[MentionData]=None,
            email_gateway: bool=False,
            no_previews: bool=False) -> str:
    bugdown_stats_start()
    ret = do_convert(content, realm_alert_words_automaton,
                     message, message_realm, sent_by_bot,
                     translate_emoticons, mention_data, email_gateway,
                     no_previews=no_previews)
    bugdown_stats_finish()
    return ret