Separate HTTP preview code and URL previewer. (#15269)

Separates REST layer code from the actual URL previewing.

2 files changed, 837 insertions, 792 deletions

diff --git a/synapse/media/url_previewer.py b/synapse/media/url_previewer.py
new file mode 100644
index 0000000000..c8a4a809f1
--- /dev/null
+++ b/synapse/media/url_previewer.py
@@ -0,0 +1,833 @@
+# Copyright 2016 OpenMarket Ltd
+# Copyright 2020-2023 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.logging.context import make_deferred_yieldable, run_in_background
+from synapse.media._base import FileInfo, get_filename_from_headers
+from synapse.media.media_storage import MediaStorage
+from synapse.media.oembed import OEmbedProvider
+from synapse.media.preview_html import decode_body, parse_html_to_open_graph
+from synapse.metrics.background_process_metrics import run_as_background_process
+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
+
+if TYPE_CHECKING:
+    from synapse.media.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 UrlPreviewer:
+    """
+    Generates an Open Graph (https://ogp.me/) responses (with some Matrix
+    specific additions) for a given URL.
+
+    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.
+
+    If any additional requests (e.g. from oEmbed autodiscovery, step 5.3 or
+    image thumbnailing, step 5.4 or 6.4) fails then the URL preview as a whole
+    does not fail. As much information as possible is returned.
+
+    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.
+    """
+
+    def __init__(
+        self,
+        hs: "HomeServer",
+        media_repo: "MediaRepository",
+        media_storage: MediaStorage,
+    ):
+        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 preview(self, url: str, user: UserID, ts: int) -> bytes:
+        # 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, user, ts)
+            observable = ObservableDeferred(download, consumeErrors=True)
+            self._cache[url] = observable
+        else:
+            logger.info("Returning cached response")
+
+        return await make_deferred_yieldable(observable.observe())
+
+    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:
+                    try:
+                        oembed_info = await self._handle_url(
+                            oembed_url, user, allow_data_urls=True
+                        )
+                    except Exception as e:
+                        # Fetching the oEmbed info failed, don't block the entire URL preview.
+                        logger.warning(
+                            "oEmbed fetch failed during URL preview: %s errored with %s",
+                            oembed_url,
+                            e,
+                        )
+                    else:
+                        (
+                            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")
+
+        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)
diff --git a/synapse/rest/media/preview_url_resource.py b/synapse/rest/media/preview_url_resource.py
index 7ada728757..58513c4be4 100644
--- a/synapse/rest/media/preview_url_resource.py
+++ b/synapse/rest/media/preview_url_resource.py
@@ -12,26 +12,9 @@
 # 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 typing import TYPE_CHECKING
 
-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,
@@ -39,71 +22,13 @@ from synapse.http.server import (
 )
 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.media._base import FileInfo, get_filename_from_headers
 from synapse.media.media_storage import MediaStorage
-from synapse.media.oembed import OEmbedProvider
-from synapse.media.preview_html import decode_body, parse_html_to_open_graph
-from synapse.metrics.background_process_metrics import run_as_background_process
-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 synapse.media.url_previewer import UrlPreviewer
 
 if TYPE_CHECKING:
     from synapse.media.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):
     """
@@ -121,54 +46,6 @@ class PreviewUrlResource(DirectServeJsonResource):
       * 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.
-
-    If any additional requests (e.g. from oEmbed autodiscovery, step 5.3 or
-    image thumbnailing, step 5.4 or 6.4) fails then the URL preview as a whole
-    does not fail. As much information as possible is returned.
-
-    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
@@ -183,48 +60,10 @@ class PreviewUrlResource(DirectServeJsonResource):
 
         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
-            )
+        self._url_previewer = UrlPreviewer(hs, media_repo, media_storage)
 
     async def _async_render_OPTIONS(self, request: SynapseRequest) -> None:
         request.setHeader(b"Allow", b"OPTIONS, GET")
@@ -238,632 +77,5 @@ class PreviewUrlResource(DirectServeJsonResource):
         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())
+        og = await self._url_previewer.preview(url, requester.user, ts)
         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:
-                    try:
-                        oembed_info = await self._handle_url(
-                            oembed_url, user, allow_data_urls=True
-                        )
-                    except Exception as e:
-                        # Fetching the oEmbed info failed, don't block the entire URL preview.
-                        logger.warning(
-                            "oEmbed fetch failed during URL preview: %s errored with %s",
-                            oembed_url,
-                            e,
-                        )
-                    else:
-                        (
-                            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")
-
-        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)