synapse/synapse/rest/media/v1/preview_url_resource.py

# Copyright 2016 OpenMarket Ltd
# Copyright 2020-2021 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import datetime
import errno
import fnmatch
import logging
import os
import re
import shutil
import sys
import traceback
from typing import TYPE_CHECKING, BinaryIO, Iterable, Optional, Tuple
from urllib.parse import urljoin, urlparse, urlsplit
from urllib.request import urlopen

import attr

from twisted.internet.defer import Deferred
from twisted.internet.error import DNSLookupError

from synapse.api.errors import Codes, SynapseError
from synapse.http.client import SimpleHttpClient
from synapse.http.server import (
    DirectServeJsonResource,
    respond_with_json,
    respond_with_json_bytes,
)
from synapse.http.servlet import parse_integer, parse_string
from synapse.http.site import SynapseRequest
from synapse.logging.context import make_deferred_yieldable, run_in_background
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.rest.media.v1._base import get_filename_from_headers
from synapse.rest.media.v1.media_storage import MediaStorage
from synapse.rest.media.v1.oembed import OEmbedProvider
from synapse.rest.media.v1.preview_html import decode_body, parse_html_to_open_graph
from synapse.types import JsonDict, UserID
from synapse.util import json_encoder
from synapse.util.async_helpers import ObservableDeferred
from synapse.util.caches.expiringcache import ExpiringCache
from synapse.util.stringutils import random_string

from ._base import FileInfo

if TYPE_CHECKING:
    from synapse.rest.media.v1.media_repository import MediaRepository
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

OG_TAG_NAME_MAXLEN = 50
OG_TAG_VALUE_MAXLEN = 1000

ONE_HOUR = 60 * 60 * 1000
ONE_DAY = 24 * ONE_HOUR
IMAGE_CACHE_EXPIRY_MS = 2 * ONE_DAY


@attr.s(slots=True, frozen=True, auto_attribs=True)
class DownloadResult:
    length: int
    uri: str
    response_code: int
    media_type: str
    download_name: Optional[str]
    expires: int
    etag: Optional[str]


@attr.s(slots=True, frozen=True, auto_attribs=True)
class MediaInfo:
    """
    Information parsed from downloading media being previewed.
    """

    # The Content-Type header of the response.
    media_type: str
    # The length (in bytes) of the downloaded media.
    media_length: int
    # The media filename, according to the server. This is parsed from the
    # returned headers, if possible.
    download_name: Optional[str]
    # The time of the preview.
    created_ts_ms: int
    # Information from the media storage provider about where the file is stored
    # on disk.
    filesystem_id: str
    filename: str
    # The URI being previewed.
    uri: str
    # The HTTP response code.
    response_code: int
    # The timestamp (in milliseconds) of when this preview expires.
    expires: int
    # The ETag header of the response.
    etag: Optional[str]


class PreviewUrlResource(DirectServeJsonResource):
    """
    The `GET /_matrix/media/r0/preview_url` endpoint provides a generic preview API
    for URLs which outputs Open Graph (https://ogp.me/) responses (with some Matrix
    specific additions).

    This does have trade-offs compared to other designs:

    * Pros:
      * Simple and flexible; can be used by any clients at any point
    * Cons:
      * If each homeserver provides one of these independently, all the homeservers in a
        room may needlessly DoS the target URI
      * The URL metadata must be stored somewhere, rather than just using Matrix
        itself to store the media.
      * Matrix cannot be used to distribute the metadata between homeservers.

    When Synapse is asked to preview a URL it does the following:

    1. Checks against a URL blacklist (defined as `url_preview_url_blacklist` in the
       config).
    2. Checks the URL against an in-memory cache and returns the result if it exists. (This
       is also used to de-duplicate processing of multiple in-flight requests at once.)
    3. Kicks off a background process to generate a preview:
       1. Checks URL and timestamp against the database cache and returns the result if it
          has not expired and was successful (a 2xx return code).
       2. Checks if the URL matches an oEmbed (https://oembed.com/) pattern. If it
          does, update the URL to download.
       3. Downloads the URL and stores it into a file via the media storage provider
          and saves the local media metadata.
       4. If the media is an image:
          1. Generates thumbnails.
          2. Generates an Open Graph response based on image properties.
       5. If the media is HTML:
          1. Decodes the HTML via the stored file.
          2. Generates an Open Graph response from the HTML.
          3. If a JSON oEmbed URL was found in the HTML via autodiscovery:
             1. Downloads the URL and stores it into a file via the media storage provider
                and saves the local media metadata.
             2. Convert the oEmbed response to an Open Graph response.
             3. Override any Open Graph data from the HTML with data from oEmbed.
          4. If an image exists in the Open Graph response:
             1. Downloads the URL and stores it into a file via the media storage
                provider and saves the local media metadata.
             2. Generates thumbnails.
             3. Updates the Open Graph response based on image properties.
       6. If the media is JSON and an oEmbed URL was found:
          1. Convert the oEmbed response to an Open Graph response.
          2. If a thumbnail or image is in the oEmbed response:
             1. Downloads the URL and stores it into a file via the media storage
                provider and saves the local media metadata.
             2. Generates thumbnails.
             3. Updates the Open Graph response based on image properties.
       7. Stores the result in the database cache.
    4. Returns the result.

    The in-memory cache expires after 1 hour.

    Expired entries in the database cache (and their associated media files) are
    deleted every 10 seconds. The default expiration time is 1 hour from download.
    """

    isLeaf = True

    def __init__(
        self,
        hs: "HomeServer",
        media_repo: "MediaRepository",
        media_storage: MediaStorage,
    ):
        super().__init__()

        self.auth = hs.get_auth()
        self.clock = hs.get_clock()
        self.filepaths = media_repo.filepaths
        self.max_spider_size = hs.config.media.max_spider_size
        self.server_name = hs.hostname
        self.store = hs.get_datastores().main
        self.client = SimpleHttpClient(
            hs,
            treq_args={"browser_like_redirects": True},
            ip_whitelist=hs.config.media.url_preview_ip_range_whitelist,
            ip_blacklist=hs.config.media.url_preview_ip_range_blacklist,
            use_proxy=True,
        )
        self.media_repo = media_repo
        self.primary_base_path = media_repo.primary_base_path
        self.media_storage = media_storage

        self._oembed = OEmbedProvider(hs)

        # We run the background jobs if we're the instance specified (or no
        # instance is specified, where we assume there is only one instance
        # serving media).
        instance_running_jobs = hs.config.media.media_instance_running_background_jobs
        self._worker_run_media_background_jobs = (
            instance_running_jobs is None
            or instance_running_jobs == hs.get_instance_name()
        )

        self.url_preview_url_blacklist = hs.config.media.url_preview_url_blacklist
        self.url_preview_accept_language = hs.config.media.url_preview_accept_language

        # memory cache mapping urls to an ObservableDeferred returning
        # JSON-encoded OG metadata
        self._cache: ExpiringCache[str, ObservableDeferred] = ExpiringCache(
            cache_name="url_previews",
            clock=self.clock,
            # don't spider URLs more often than once an hour
            expiry_ms=ONE_HOUR,
        )

        if self._worker_run_media_background_jobs:
            self._cleaner_loop = self.clock.looping_call(
                self._start_expire_url_cache_data, 10 * 1000
            )

    async def _async_render_OPTIONS(self, request: SynapseRequest) -> None:
        request.setHeader(b"Allow", b"OPTIONS, GET")
        respond_with_json(request, 200, {}, send_cors=True)

    async def _async_render_GET(self, request: SynapseRequest) -> None:
        # XXX: if get_user_by_req fails, what should we do in an async render?
        requester = await self.auth.get_user_by_req(request)
        url = parse_string(request, "url", required=True)
        ts = parse_integer(request, "ts")
        if ts is None:
            ts = self.clock.time_msec()

        # XXX: we could move this into _do_preview if we wanted.
        url_tuple = urlsplit(url)
        for entry in self.url_preview_url_blacklist:
            match = True
            for attrib in entry:
                pattern = entry[attrib]
                value = getattr(url_tuple, attrib)
                logger.debug(
                    "Matching attrib '%s' with value '%s' against pattern '%s'",
                    attrib,
                    value,
                    pattern,
                )

                if value is None:
                    match = False
                    continue

                # Some attributes might not be parsed as strings by urlsplit (such as the
                # port, which is parsed as an int). Because we use match functions that
                # expect strings, we want to make sure that's what we give them.
                value_str = str(value)

                if pattern.startswith("^"):
                    if not re.match(pattern, value_str):
                        match = False
                        continue
                else:
                    if not fnmatch.fnmatch(value_str, pattern):
                        match = False
                        continue
            if match:
                logger.warning("URL %s blocked by url_blacklist entry %s", url, entry)
                raise SynapseError(
                    403, "URL blocked by url pattern blacklist entry", Codes.UNKNOWN
                )

        # the in-memory cache:
        # * ensures that only one request is active at a time
        # * takes load off the DB for the thundering herds
        # * also caches any failures (unlike the DB) so we don't keep
        #    requesting the same endpoint

        observable = self._cache.get(url)

        if not observable:
            download = run_in_background(self._do_preview, url, requester.user, ts)
            observable = ObservableDeferred(download, consumeErrors=True)
            self._cache[url] = observable
        else:
            logger.info("Returning cached response")

        og = await make_deferred_yieldable(observable.observe())
        respond_with_json_bytes(request, 200, og, send_cors=True)

    async def _do_preview(self, url: str, user: UserID, ts: int) -> bytes:
        """Check the db, and download the URL and build a preview

        Args:
            url: The URL to preview.
            user: The user requesting the preview.
            ts: The timestamp requested for the preview.

        Returns:
            json-encoded og data
        """
        # check the URL cache in the DB (which will also provide us with
        # historical previews, if we have any)
        cache_result = await self.store.get_url_cache(url, ts)
        if (
            cache_result
            and cache_result["expires_ts"] > ts
            and cache_result["response_code"] / 100 == 2
        ):
            # It may be stored as text in the database, not as bytes (such as
            # PostgreSQL). If so, encode it back before handing it on.
            og = cache_result["og"]
            if isinstance(og, str):
                og = og.encode("utf8")
            return og

        # If this URL can be accessed via oEmbed, use that instead.
        url_to_download = url
        oembed_url = self._oembed.get_oembed_url(url)
        if oembed_url:
            url_to_download = oembed_url

        media_info = await self._handle_url(url_to_download, user)

        logger.debug("got media_info of '%s'", media_info)

        # The number of milliseconds that the response should be considered valid.
        expiration_ms = media_info.expires
        author_name: Optional[str] = None

        if _is_media(media_info.media_type):
            file_id = media_info.filesystem_id
            dims = await self.media_repo._generate_thumbnails(
                None, file_id, file_id, media_info.media_type, url_cache=True
            )

            og = {
                "og:description": media_info.download_name,
                "og:image": f"mxc://{self.server_name}/{media_info.filesystem_id}",
                "og:image:type": media_info.media_type,
                "matrix:image:size": media_info.media_length,
            }

            if dims:
                og["og:image:width"] = dims["width"]
                og["og:image:height"] = dims["height"]
            else:
                logger.warning("Couldn't get dims for %s" % url)

            # define our OG response for this media
        elif _is_html(media_info.media_type):
            # TODO: somehow stop a big HTML tree from exploding synapse's RAM

            with open(media_info.filename, "rb") as file:
                body = file.read()

            tree = decode_body(body, media_info.uri, media_info.media_type)
            if tree is not None:
                # Check if this HTML document points to oEmbed information and
                # defer to that.
                oembed_url = self._oembed.autodiscover_from_html(tree)
                og_from_oembed: JsonDict = {}
                if oembed_url:
                    oembed_info = await self._handle_url(
                        oembed_url, user, allow_data_urls=True
                    )
                    (
                        og_from_oembed,
                        author_name,
                        expiration_ms,
                    ) = await self._handle_oembed_response(
                        url, oembed_info, expiration_ms
                    )

                # Parse Open Graph information from the HTML in case the oEmbed
                # response failed or is incomplete.
                og_from_html = parse_html_to_open_graph(tree)

                # Compile the Open Graph response by using the scraped
                # information from the HTML and overlaying any information
                # from the oEmbed response.
                og = {**og_from_html, **og_from_oembed}

                await self._precache_image_url(user, media_info, og)
            else:
                og = {}

        elif oembed_url:
            # Handle the oEmbed information.
            og, author_name, expiration_ms = await self._handle_oembed_response(
                url, media_info, expiration_ms
            )
            await self._precache_image_url(user, media_info, og)

        else:
            logger.warning("Failed to find any OG data in %s", url)
            og = {}

        # If we don't have a title but we have author_name, copy it as
        # title
        if not og.get("og:title") and author_name:
            og["og:title"] = author_name

        # filter out any stupidly long values
        keys_to_remove = []
        for k, v in og.items():
            # values can be numeric as well as strings, hence the cast to str
            if len(k) > OG_TAG_NAME_MAXLEN or len(str(v)) > OG_TAG_VALUE_MAXLEN:
                logger.warning(
                    "Pruning overlong tag %s from OG data", k[:OG_TAG_NAME_MAXLEN]
                )
                keys_to_remove.append(k)
        for k in keys_to_remove:
            del og[k]

        logger.debug("Calculated OG for %s as %s", url, og)

        jsonog = json_encoder.encode(og)

        # Cap the amount of time to consider a response valid.
        expiration_ms = min(expiration_ms, ONE_DAY)

        # store OG in history-aware DB cache
        await self.store.store_url_cache(
            url,
            media_info.response_code,
            media_info.etag,
            media_info.created_ts_ms + expiration_ms,
            jsonog,
            media_info.filesystem_id,
            media_info.created_ts_ms,
        )

        return jsonog.encode("utf8")

    async def _download_url(self, url: str, output_stream: BinaryIO) -> DownloadResult:
        """
        Fetches a remote URL and parses the headers.

        Args:
             url: The URL to fetch.
             output_stream: The stream to write the content to.

        Returns:
            A tuple of:
                Media length, URL downloaded, the HTTP response code,
                the media type, the downloaded file name, the number of
                milliseconds the result is valid for, the etag header.
        """

        try:
            logger.debug("Trying to get preview for url '%s'", url)
            length, headers, uri, code = await self.client.get_file(
                url,
                output_stream=output_stream,
                max_size=self.max_spider_size,
                headers={
                    b"Accept-Language": self.url_preview_accept_language,
                    # Use a custom user agent for the preview because some sites will only return
                    # Open Graph metadata to crawler user agents. Omit the Synapse version
                    # string to avoid leaking information.
                    b"User-Agent": [
                        "Synapse (bot; +https://github.com/matrix-org/synapse)"
                    ],
                },
                is_allowed_content_type=_is_previewable,
            )
        except SynapseError:
            # Pass SynapseErrors through directly, so that the servlet
            # handler will return a SynapseError to the client instead of
            # blank data or a 500.
            raise
        except DNSLookupError:
            # DNS lookup returned no results
            # Note: This will also be the case if one of the resolved IP
            # addresses is blacklisted
            raise SynapseError(
                502,
                "DNS resolution failure during URL preview generation",
                Codes.UNKNOWN,
            )
        except Exception as e:
            # FIXME: pass through 404s and other error messages nicely
            logger.warning("Error downloading %s: %r", url, e)

            raise SynapseError(
                500,
                "Failed to download content: %s"
                % (traceback.format_exception_only(sys.exc_info()[0], e),),
                Codes.UNKNOWN,
            )

        if b"Content-Type" in headers:
            media_type = headers[b"Content-Type"][0].decode("ascii")
        else:
            media_type = "application/octet-stream"

        download_name = get_filename_from_headers(headers)

        # FIXME: we should calculate a proper expiration based on the
        # Cache-Control and Expire headers.  But for now, assume 1 hour.
        expires = ONE_HOUR
        etag = headers[b"ETag"][0].decode("ascii") if b"ETag" in headers else None

        return DownloadResult(
            length, uri, code, media_type, download_name, expires, etag
        )

    async def _parse_data_url(
        self, url: str, output_stream: BinaryIO
    ) -> DownloadResult:
        """
        Parses a data: URL.

        Args:
             url: The URL to parse.
             output_stream: The stream to write the content to.

        Returns:
            A tuple of:
                Media length, URL downloaded, the HTTP response code,
                the media type, the downloaded file name, the number of
                milliseconds the result is valid for, the etag header.
        """

        try:
            logger.debug("Trying to parse data url '%s'", url)
            with urlopen(url) as url_info:
                # TODO Can this be more efficient.
                output_stream.write(url_info.read())
        except Exception as e:
            logger.warning("Error parsing data: URL %s: %r", url, e)

            raise SynapseError(
                500,
                "Failed to parse data URL: %s"
                % (traceback.format_exception_only(sys.exc_info()[0], e),),
                Codes.UNKNOWN,
            )

        return DownloadResult(
            # Read back the length that has been written.
            length=output_stream.tell(),
            uri=url,
            # If it was parsed, consider this a 200 OK.
            response_code=200,
            # urlopen shoves the media-type from the data URL into the content type
            # header object.
            media_type=url_info.headers.get_content_type(),
            # Some features are not supported by data: URLs.
            download_name=None,
            expires=ONE_HOUR,
            etag=None,
        )

    async def _handle_url(
        self, url: str, user: UserID, allow_data_urls: bool = False
    ) -> MediaInfo:
        """
        Fetches content from a URL and parses the result to generate a MediaInfo.

        It uses the media storage provider to persist the fetched content and
        stores the mapping into the database.

        Args:
             url: The URL to fetch.
             user: The user who ahs requested this URL.
             allow_data_urls: True if data URLs should be allowed.

        Returns:
            A MediaInfo object describing the fetched content.
        """

        # TODO: we should probably honour robots.txt... except in practice
        # we're most likely being explicitly triggered by a human rather than a
        # bot, so are we really a robot?

        file_id = datetime.date.today().isoformat() + "_" + random_string(16)

        file_info = FileInfo(server_name=None, file_id=file_id, url_cache=True)

        with self.media_storage.store_into_file(file_info) as (f, fname, finish):
            if url.startswith("data:"):
                if not allow_data_urls:
                    raise SynapseError(
                        500, "Previewing of data: URLs is forbidden", Codes.UNKNOWN
                    )

                download_result = await self._parse_data_url(url, f)
            else:
                download_result = await self._download_url(url, f)

            await finish()

        try:
            time_now_ms = self.clock.time_msec()

            await self.store.store_local_media(
                media_id=file_id,
                media_type=download_result.media_type,
                time_now_ms=time_now_ms,
                upload_name=download_result.download_name,
                media_length=download_result.length,
                user_id=user,
                url_cache=url,
            )

        except Exception as e:
            logger.error("Error handling downloaded %s: %r", url, e)
            # TODO: we really ought to delete the downloaded file in this
            # case, since we won't have recorded it in the db, and will
            # therefore not expire it.
            raise

        return MediaInfo(
            media_type=download_result.media_type,
            media_length=download_result.length,
            download_name=download_result.download_name,
            created_ts_ms=time_now_ms,
            filesystem_id=file_id,
            filename=fname,
            uri=download_result.uri,
            response_code=download_result.response_code,
            expires=download_result.expires,
            etag=download_result.etag,
        )

    async def _precache_image_url(
        self, user: UserID, media_info: MediaInfo, og: JsonDict
    ) -> None:
        """
        Pre-cache the image (if one exists) for posterity

        Args:
            user: The user requesting the preview.
            media_info: The media being previewed.
            og: The Open Graph dictionary. This is modified with image information.
        """
        # If there's no image or it is blank, there's nothing to do.
        if "og:image" not in og:
            return

        # Remove the raw image URL, this will be replaced with an MXC URL, if successful.
        image_url = og.pop("og:image")
        if not image_url:
            return

        # The image URL from the HTML might be relative to the previewed page,
        # convert it to an URL which can be requested directly.
        url_parts = urlparse(image_url)
        if url_parts.scheme != "data":
            image_url = urljoin(media_info.uri, image_url)

        # FIXME: it might be cleaner to use the same flow as the main /preview_url
        # request itself and benefit from the same caching etc.  But for now we
        # just rely on the caching on the master request to speed things up.
        try:
            image_info = await self._handle_url(image_url, user, allow_data_urls=True)
        except Exception as e:
            # Pre-caching the image failed, don't block the entire URL preview.
            logger.warning(
                "Pre-caching image failed during URL preview: %s errored with %s",
                image_url,
                e,
            )
            return

        if _is_media(image_info.media_type):
            # TODO: make sure we don't choke on white-on-transparent images
            file_id = image_info.filesystem_id
            dims = await self.media_repo._generate_thumbnails(
                None, file_id, file_id, image_info.media_type, url_cache=True
            )
            if dims:
                og["og:image:width"] = dims["width"]
                og["og:image:height"] = dims["height"]
            else:
                logger.warning("Couldn't get dims for %s", image_url)

            og["og:image"] = f"mxc://{self.server_name}/{image_info.filesystem_id}"
            og["og:image:type"] = image_info.media_type
            og["matrix:image:size"] = image_info.media_length

    async def _handle_oembed_response(
        self, url: str, media_info: MediaInfo, expiration_ms: int
    ) -> Tuple[JsonDict, Optional[str], int]:
        """
        Parse the downloaded oEmbed info.

        Args:
            url: The URL which is being previewed (not the one which was
                requested).
            media_info: The media being previewed.
            expiration_ms: The length of time, in milliseconds, the media is valid for.

        Returns:
            A tuple of:
                The Open Graph dictionary, if the oEmbed info can be parsed.
                The author name if it could be retrieved from oEmbed.
                The (possibly updated) length of time, in milliseconds, the media is valid for.
        """
        # If JSON was not returned, there's nothing to do.
        if not _is_json(media_info.media_type):
            return {}, None, expiration_ms

        with open(media_info.filename, "rb") as file:
            body = file.read()

        oembed_response = self._oembed.parse_oembed_response(url, body)
        open_graph_result = oembed_response.open_graph_result

        # Use the cache age from the oEmbed result, if one was given.
        if open_graph_result and oembed_response.cache_age is not None:
            expiration_ms = oembed_response.cache_age

        return open_graph_result, oembed_response.author_name, expiration_ms

    def _start_expire_url_cache_data(self) -> Deferred:
        return run_as_background_process(
            "expire_url_cache_data", self._expire_url_cache_data
        )

    async def _expire_url_cache_data(self) -> None:
        """Clean up expired url cache content, media and thumbnails."""

        assert self._worker_run_media_background_jobs

        now = self.clock.time_msec()

        logger.debug("Running url preview cache expiry")

        if not (await self.store.db_pool.updates.has_completed_background_updates()):
            logger.debug("Still running DB updates; skipping url preview cache expiry")
            return

        def try_remove_parent_dirs(dirs: Iterable[str]) -> None:
            """Attempt to remove the given chain of parent directories

            Args:
                dirs: The list of directory paths to delete, with children appearing
                    before their parents.
            """
            for dir in dirs:
                try:
                    os.rmdir(dir)
                except FileNotFoundError:
                    # Already deleted, continue with deleting the rest
                    pass
                except OSError as e:
                    # Failed, skip deleting the rest of the parent dirs
                    if e.errno != errno.ENOTEMPTY:
                        logger.warning(
                            "Failed to remove media directory while clearing url preview cache: %r: %s",
                            dir,
                            e,
                        )
                    break

        # First we delete expired url cache entries
        media_ids = await self.store.get_expired_url_cache(now)

        removed_media = []
        for media_id in media_ids:
            fname = self.filepaths.url_cache_filepath(media_id)
            try:
                os.remove(fname)
            except FileNotFoundError:
                pass  # If the path doesn't exist, meh
            except OSError as e:
                logger.warning(
                    "Failed to remove media while clearing url preview cache: %r: %s",
                    media_id,
                    e,
                )
                continue

            removed_media.append(media_id)

            dirs = self.filepaths.url_cache_filepath_dirs_to_delete(media_id)
            try_remove_parent_dirs(dirs)

        await self.store.delete_url_cache(removed_media)

        if removed_media:
            logger.debug(
                "Deleted %d entries from url preview cache", len(removed_media)
            )
        else:
            logger.debug("No entries removed from url preview cache")

        # Now we delete old images associated with the url cache.
        # These may be cached for a bit on the client (i.e., they
        # may have a room open with a preview url thing open).
        # So we wait a couple of days before deleting, just in case.
        expire_before = now - IMAGE_CACHE_EXPIRY_MS
        media_ids = await self.store.get_url_cache_media_before(expire_before)

        removed_media = []
        for media_id in media_ids:
            fname = self.filepaths.url_cache_filepath(media_id)
            try:
                os.remove(fname)
            except FileNotFoundError:
                pass  # If the path doesn't exist, meh
            except OSError as e:
                logger.warning(
                    "Failed to remove media from url preview cache: %r: %s", media_id, e
                )
                continue

            dirs = self.filepaths.url_cache_filepath_dirs_to_delete(media_id)
            try_remove_parent_dirs(dirs)

            thumbnail_dir = self.filepaths.url_cache_thumbnail_directory(media_id)
            try:
                shutil.rmtree(thumbnail_dir)
            except FileNotFoundError:
                pass  # If the path doesn't exist, meh
            except OSError as e:
                logger.warning(
                    "Failed to remove media from url preview cache: %r: %s", media_id, e
                )
                continue

            removed_media.append(media_id)

            dirs = self.filepaths.url_cache_thumbnail_dirs_to_delete(media_id)
            # Note that one of the directories to be deleted has already been
            # removed by the `rmtree` above.
            try_remove_parent_dirs(dirs)

        await self.store.delete_url_cache_media(removed_media)

        if removed_media:
            logger.debug("Deleted %d media from url preview cache", len(removed_media))
        else:
            logger.debug("No media removed from url preview cache")


def _is_media(content_type: str) -> bool:
    return content_type.lower().startswith("image/")


def _is_html(content_type: str) -> bool:
    content_type = content_type.lower()
    return content_type.startswith("text/html") or content_type.startswith(
        "application/xhtml"
    )


def _is_json(content_type: str) -> bool:
    return content_type.lower().startswith("application/json")


def _is_previewable(content_type: str) -> bool:
    """Returns True for content types for which we will perform URL preview and False
    otherwise."""

    return _is_html(content_type) or _is_media(content_type) or _is_json(content_type)