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

1008 lines
36 KiB
Python

# 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 codecs
import datetime
import errno
import fnmatch
import itertools
import logging
import os
import re
import shutil
import sys
import traceback
from typing import TYPE_CHECKING, Dict, Generator, Iterable, Optional, Set, Tuple, Union
from urllib import parse as urlparse
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.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 lxml import etree
from synapse.rest.media.v1.media_repository import MediaRepository
from synapse.server import HomeServer
logger = logging.getLogger(__name__)
_charset_match = re.compile(
br'<\s*meta[^>]*charset\s*=\s*"?([a-z0-9_-]+)"?', flags=re.I
)
_xml_encoding_match = re.compile(
br'\s*<\s*\?\s*xml[^>]*encoding="([a-z0-9_-]+)"', flags=re.I
)
_content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
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 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):
"""
Generating URL previews is a complicated task which many potential pitfalls.
See docs/development/url_previews.md for discussion of the design and
algorithm followed in this module.
"""
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_datastore()
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 = urlparse.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
if pattern.startswith("^"):
if not re.match(pattern, getattr(url_tuple, attrib)):
match = False
continue
else:
if not fnmatch.fnmatch(getattr(url_tuple, attrib), 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._download_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
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 = {}
if oembed_url:
oembed_info = await self._download_url(oembed_url, user)
og, expiration_ms = await self._handle_oembed_response(
url, oembed_info, expiration_ms
)
# If there was no oEmbed URL (or oEmbed parsing failed), attempt
# to generate the Open Graph information from the HTML.
if not oembed_url or not og:
og = _calc_og(tree, media_info.uri)
await self._precache_image_url(user, media_info, og)
else:
og = {}
elif oembed_url:
# Handle the oEmbed information.
og, 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 = {}
# 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, user: UserID) -> MediaInfo:
# 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):
try:
logger.debug("Trying to get preview for url '%s'", url)
length, headers, uri, code = await self.client.get_file(
url,
output_stream=f,
max_size=self.max_spider_size,
headers={"Accept-Language": self.url_preview_accept_language},
)
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,
)
await finish()
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
try:
time_now_ms = self.clock.time_msec()
await self.store.store_local_media(
media_id=file_id,
media_type=media_type,
time_now_ms=time_now_ms,
upload_name=download_name,
media_length=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=media_type,
media_length=length,
download_name=download_name,
created_ts_ms=time_now_ms,
filesystem_id=file_id,
filename=fname,
uri=uri,
response_code=code,
expires=expires,
etag=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 or not og["og:image"]:
return
# 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.
image_info = await self._download_url(
_rebase_url(og["og:image"], media_info.uri), user
)
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", og["og:image"])
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
else:
del og["og:image"]
async def _handle_oembed_response(
self, url: str, media_info: MediaInfo, expiration_ms: int
) -> Tuple[JsonDict, 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 (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 {}, 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, 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.info("Still running DB updates; skipping 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: %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: %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.info("Deleted %d entries from url cache", len(removed_media))
else:
logger.debug("No entries removed from url 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: %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: %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.info("Deleted %d media from url cache", len(removed_media))
else:
logger.debug("No media removed from url cache")
def _normalise_encoding(encoding: str) -> Optional[str]:
"""Use the Python codec's name as the normalised entry."""
try:
return codecs.lookup(encoding).name
except LookupError:
return None
def get_html_media_encodings(body: bytes, content_type: Optional[str]) -> Iterable[str]:
"""
Get potential encoding of the body based on the (presumably) HTML body or the content-type header.
The precedence used for finding a character encoding is:
1. <meta> tag with a charset declared.
2. The XML document's character encoding attribute.
3. The Content-Type header.
4. Fallback to utf-8.
5. Fallback to windows-1252.
This roughly follows the algorithm used by BeautifulSoup's bs4.dammit.EncodingDetector.
Args:
body: The HTML document, as bytes.
content_type: The Content-Type header.
Returns:
The character encoding of the body, as a string.
"""
# There's no point in returning an encoding more than once.
attempted_encodings: Set[str] = set()
# Limit searches to the first 1kb, since it ought to be at the top.
body_start = body[:1024]
# Check if it has an encoding set in a meta tag.
match = _charset_match.search(body_start)
if match:
encoding = _normalise_encoding(match.group(1).decode("ascii"))
if encoding:
attempted_encodings.add(encoding)
yield encoding
# TODO Support <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
# Check if it has an XML document with an encoding.
match = _xml_encoding_match.match(body_start)
if match:
encoding = _normalise_encoding(match.group(1).decode("ascii"))
if encoding and encoding not in attempted_encodings:
attempted_encodings.add(encoding)
yield encoding
# Check the HTTP Content-Type header for a character set.
if content_type:
content_match = _content_type_match.match(content_type)
if content_match:
encoding = _normalise_encoding(content_match.group(1))
if encoding and encoding not in attempted_encodings:
attempted_encodings.add(encoding)
yield encoding
# Finally, fallback to UTF-8, then windows-1252.
for fallback in ("utf-8", "cp1252"):
if fallback not in attempted_encodings:
yield fallback
def decode_body(
body: bytes, uri: str, content_type: Optional[str] = None
) -> Optional["etree.Element"]:
"""
This uses lxml to parse the HTML document.
Args:
body: The HTML document, as bytes.
uri: The URI used to download the body.
content_type: The Content-Type header.
Returns:
The parsed HTML body, or None if an error occurred during processed.
"""
# If there's no body, nothing useful is going to be found.
if not body:
return None
# The idea here is that multiple encodings are tried until one works.
# Unfortunately the result is never used and then LXML will decode the string
# again with the found encoding.
for encoding in get_html_media_encodings(body, content_type):
try:
body.decode(encoding)
except Exception:
pass
else:
break
else:
logger.warning("Unable to decode HTML body for %s", uri)
return None
from lxml import etree
# Create an HTML parser.
parser = etree.HTMLParser(recover=True, encoding=encoding)
# Attempt to parse the body. Returns None if the body was successfully
# parsed, but no tree was found.
return etree.fromstring(body, parser)
def _calc_og(tree: "etree.Element", media_uri: str) -> Dict[str, Optional[str]]:
"""
Calculate metadata for an HTML document.
This uses lxml to search the HTML document for Open Graph data.
Args:
tree: The parsed HTML document.
media_url: The URI used to download the body.
Returns:
The Open Graph response as a dictionary.
"""
# if we see any image URLs in the OG response, then spider them
# (although the client could choose to do this by asking for previews of those
# URLs to avoid DoSing the server)
# "og:type" : "video",
# "og:url" : "https://www.youtube.com/watch?v=LXDBoHyjmtw",
# "og:site_name" : "YouTube",
# "og:video:type" : "application/x-shockwave-flash",
# "og:description" : "Fun stuff happening here",
# "og:title" : "RemoteJam - Matrix team hack for Disrupt Europe Hackathon",
# "og:image" : "https://i.ytimg.com/vi/LXDBoHyjmtw/maxresdefault.jpg",
# "og:video:url" : "http://www.youtube.com/v/LXDBoHyjmtw?version=3&autohide=1",
# "og:video:width" : "1280"
# "og:video:height" : "720",
# "og:video:secure_url": "https://www.youtube.com/v/LXDBoHyjmtw?version=3",
og: Dict[str, Optional[str]] = {}
for tag in tree.xpath("//*/meta[starts-with(@property, 'og:')]"):
if "content" in tag.attrib:
# if we've got more than 50 tags, someone is taking the piss
if len(og) >= 50:
logger.warning("Skipping OG for page with too many 'og:' tags")
return {}
og[tag.attrib["property"]] = tag.attrib["content"]
# TODO: grab article: meta tags too, e.g.:
# "article:publisher" : "https://www.facebook.com/thethudonline" />
# "article:author" content="https://www.facebook.com/thethudonline" />
# "article:tag" content="baby" />
# "article:section" content="Breaking News" />
# "article:published_time" content="2016-03-31T19:58:24+00:00" />
# "article:modified_time" content="2016-04-01T18:31:53+00:00" />
if "og:title" not in og:
# do some basic spidering of the HTML
title = tree.xpath("(//title)[1] | (//h1)[1] | (//h2)[1] | (//h3)[1]")
if title and title[0].text is not None:
og["og:title"] = title[0].text.strip()
else:
og["og:title"] = None
if "og:image" not in og:
# TODO: extract a favicon failing all else
meta_image = tree.xpath(
"//*/meta[translate(@itemprop, 'IMAGE', 'image')='image']/@content"
)
if meta_image:
og["og:image"] = _rebase_url(meta_image[0], media_uri)
else:
# TODO: consider inlined CSS styles as well as width & height attribs
images = tree.xpath("//img[@src][number(@width)>10][number(@height)>10]")
images = sorted(
images,
key=lambda i: (
-1 * float(i.attrib["width"]) * float(i.attrib["height"])
),
)
if not images:
images = tree.xpath("//img[@src]")
if images:
og["og:image"] = images[0].attrib["src"]
if "og:description" not in og:
meta_description = tree.xpath(
"//*/meta"
"[translate(@name, 'DESCRIPTION', 'description')='description']"
"/@content"
)
if meta_description:
og["og:description"] = meta_description[0]
else:
og["og:description"] = _calc_description(tree)
elif og["og:description"]:
# This must be a non-empty string at this point.
assert isinstance(og["og:description"], str)
og["og:description"] = summarize_paragraphs([og["og:description"]])
# TODO: delete the url downloads to stop diskfilling,
# as we only ever cared about its OG
return og
def _calc_description(tree: "etree.Element") -> Optional[str]:
"""
Calculate a text description based on an HTML document.
Grabs any text nodes which are inside the <body/> tag, unless they are within
an HTML5 semantic markup tag (<header/>, <nav/>, <aside/>, <footer/>), or
if they are within a <script/> or <style/> tag.
This is a very very very coarse approximation to a plain text render of the page.
Args:
tree: The parsed HTML document.
Returns:
The plain text description, or None if one cannot be generated.
"""
# We don't just use XPATH here as that is slow on some machines.
from lxml import etree
TAGS_TO_REMOVE = (
"header",
"nav",
"aside",
"footer",
"script",
"noscript",
"style",
etree.Comment,
)
# Split all the text nodes into paragraphs (by splitting on new
# lines)
text_nodes = (
re.sub(r"\s+", "\n", el).strip()
for el in _iterate_over_text(tree.find("body"), *TAGS_TO_REMOVE)
)
return summarize_paragraphs(text_nodes)
def _iterate_over_text(
tree: "etree.Element", *tags_to_ignore: Iterable[Union[str, "etree.Comment"]]
) -> Generator[str, None, None]:
"""Iterate over the tree returning text nodes in a depth first fashion,
skipping text nodes inside certain tags.
"""
# This is basically a stack that we extend using itertools.chain.
# This will either consist of an element to iterate over *or* a string
# to be returned.
elements = iter([tree])
while True:
el = next(elements, None)
if el is None:
return
if isinstance(el, str):
yield el
elif el.tag not in tags_to_ignore:
# el.text is the text before the first child, so we can immediately
# return it if the text exists.
if el.text:
yield el.text
# We add to the stack all the elements children, interspersed with
# each child's tail text (if it exists). The tail text of a node
# is text that comes *after* the node, so we always include it even
# if we ignore the child node.
elements = itertools.chain(
itertools.chain.from_iterable( # Basically a flatmap
[child, child.tail] if child.tail else [child]
for child in el.iterchildren()
),
elements,
)
def _rebase_url(url: str, base: str) -> str:
base_parts = list(urlparse.urlparse(base))
url_parts = list(urlparse.urlparse(url))
if not url_parts[0]: # fix up schema
url_parts[0] = base_parts[0] or "http"
if not url_parts[1]: # fix up hostname
url_parts[1] = base_parts[1]
if not url_parts[2].startswith("/"):
url_parts[2] = re.sub(r"/[^/]+$", "/", base_parts[2]) + url_parts[2]
return urlparse.urlunparse(url_parts)
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 summarize_paragraphs(
text_nodes: Iterable[str], min_size: int = 200, max_size: int = 500
) -> Optional[str]:
"""
Try to get a summary respecting first paragraph and then word boundaries.
Args:
text_nodes: The paragraphs to summarize.
min_size: The minimum number of words to include.
max_size: The maximum number of words to include.
Returns:
A summary of the text nodes, or None if that was not possible.
"""
# TODO: Respect sentences?
description = ""
# Keep adding paragraphs until we get to the MIN_SIZE.
for text_node in text_nodes:
if len(description) < min_size:
text_node = re.sub(r"[\t \r\n]+", " ", text_node)
description += text_node + "\n\n"
else:
break
description = description.strip()
description = re.sub(r"[\t ]+", " ", description)
description = re.sub(r"[\t \r\n]*[\r\n]+", "\n\n", description)
# If the concatenation of paragraphs to get above MIN_SIZE
# took us over MAX_SIZE, then we need to truncate mid paragraph
if len(description) > max_size:
new_desc = ""
# This splits the paragraph into words, but keeping the
# (preceding) whitespace intact so we can easily concat
# words back together.
for match in re.finditer(r"\s*\S+", description):
word = match.group()
# Keep adding words while the total length is less than
# MAX_SIZE.
if len(word) + len(new_desc) < max_size:
new_desc += word
else:
# At this point the next word *will* take us over
# MAX_SIZE, but we also want to ensure that its not
# a huge word. If it is add it anyway and we'll
# truncate later.
if len(new_desc) < min_size:
new_desc += word
break
# Double check that we're not over the limit
if len(new_desc) > max_size:
new_desc = new_desc[:max_size]
# We always add an ellipsis because at the very least
# we chopped mid paragraph.
description = new_desc.strip() + ""
return description if description else None