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MatrixSynapse/synapse/media/preview_html.py

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# Copyright 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 logging
import re
from typing import (
TYPE_CHECKING,
Callable,
Dict,
Generator,
Iterable,
List,
Optional,
Set,
Union,
)
if TYPE_CHECKING:
from lxml import etree
logger = logging.getLogger(__name__)
_charset_match = re.compile(
rb'<\s*meta[^>]*charset\s*=\s*"?([a-z0-9_-]+)"?', flags=re.I
)
_xml_encoding_match = re.compile(
rb'\s*<\s*\?\s*xml[^>]*encoding="([a-z0-9_-]+)"', flags=re.I
)
_content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
# Certain elements aren't meant for display.
ARIA_ROLES_TO_IGNORE = {"directory", "menu", "menubar", "toolbar"}
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 _get_meta_tags(
tree: "etree.Element",
property: str,
prefix: str,
property_mapper: Optional[Callable[[str], Optional[str]]] = None,
) -> Dict[str, Optional[str]]:
"""
Search for meta tags prefixed with a particular string.
Args:
tree: The parsed HTML document.
property: The name of the property which contains the tag name, e.g.
"property" for Open Graph.
prefix: The prefix on the property to search for, e.g. "og" for Open Graph.
property_mapper: An optional callable to map the property to the Open Graph
form. Can return None for a key to ignore that key.
Returns:
A map of tag name to value.
"""
results: Dict[str, Optional[str]] = {}
for tag in tree.xpath(
f"//*/meta[starts-with(@{property}, '{prefix}:')][@content][not(@content='')]"
):
# if we've got more than 50 tags, someone is taking the piss
if len(results) >= 50:
logger.warning(
"Skipping parsing of Open Graph for page with too many '%s:' tags",
prefix,
)
return {}
key = tag.attrib[property]
if property_mapper:
key = property_mapper(key)
# None is a special value used to ignore a value.
if key is None:
continue
results[key] = tag.attrib["content"]
return results
def _map_twitter_to_open_graph(key: str) -> Optional[str]:
"""
Map a Twitter card property to the analogous Open Graph property.
Args:
key: The Twitter card property (starts with "twitter:").
Returns:
The Open Graph property (starts with "og:") or None to have this property
be ignored.
"""
# Twitter card properties with no analogous Open Graph property.
if key == "twitter:card" or key == "twitter:creator":
return None
if key == "twitter:site":
return "og:site_name"
# Otherwise, swap twitter to og.
return "og" + key[7:]
def parse_html_to_open_graph(tree: "etree.Element") -> Dict[str, Optional[str]]:
"""
Parse the HTML document into an Open Graph response.
This uses lxml to search the HTML document for Open Graph data (or
synthesizes it from the document).
Args:
tree: The parsed HTML document.
Returns:
The Open Graph response as a dictionary.
"""
# Search for Open Graph (og:) meta tags, e.g.:
#
# "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 = _get_meta_tags(tree, "property", "og")
# TODO: Search for properties specific to the different Open Graph types,
# such as article: meta tags, 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" />
# Search for Twitter Card (twitter:) meta tags, e.g.:
#
# "twitter:site" : "@matrixdotorg"
# "twitter:creator" : "@matrixdotorg"
#
# Twitter cards tags also duplicate Open Graph tags.
#
# See https://developer.twitter.com/en/docs/twitter-for-websites/cards/guides/getting-started
twitter = _get_meta_tags(tree, "name", "twitter", _map_twitter_to_open_graph)
# Merge the Twitter values with the Open Graph values, but do not overwrite
# information from Open Graph tags.
for key, value in twitter.items():
if key not in og:
og[key] = value
if "og:title" not in og:
# Attempt to find a title from the title tag, or the biggest header on the page.
title = tree.xpath("((//title)[1] | (//h1)[1] | (//h2)[1] | (//h3)[1])/text()")
if title:
og["og:title"] = title[0].strip()
else:
og["og:title"] = None
if "og:image" not in og:
meta_image = tree.xpath(
"//*/meta[translate(@itemprop, 'IMAGE', 'image')='image'][not(@content='')]/@content[1]"
)
# If a meta image is found, use it.
if meta_image:
og["og:image"] = meta_image[0]
else:
# Try to find images which are larger than 10px by 10px.
#
# 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 no images were found, try to find *any* images.
if not images:
images = tree.xpath("//img[@src][1]")
if images:
og["og:image"] = images[0].attrib["src"]
# Finally, fallback to the favicon if nothing else.
else:
favicons = tree.xpath("//link[@href][contains(@rel, 'icon')]/@href[1]")
if favicons:
og["og:image"] = favicons[0]
if "og:description" not in og:
# Check the first meta description tag for content.
meta_description = tree.xpath(
"//*/meta[translate(@name, 'DESCRIPTION', 'description')='description'][not(@content='')]/@content[1]"
)
# If a meta description is found with content, use it.
if meta_description:
og["og:description"] = meta_description[0]
else:
og["og:description"] = parse_html_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 parse_html_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/>, <svg/> or <style/> tag, or if they are within
a tag whose content is usually only shown to old browsers
(<iframe/>, <video/>, <canvas/>, <picture/>).
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",
"svg",
"iframe",
"video",
"canvas",
"img",
"picture",
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: Optional["etree.Element"],
tags_to_ignore: Set[Union[str, "etree.Comment"]],
stack_limit: int = 1024,
) -> Generator[str, None, None]:
"""Iterate over the tree returning text nodes in a depth first fashion,
skipping text nodes inside certain tags.
Args:
tree: The parent element to iterate. Can be None if there isn't one.
tags_to_ignore: Set of tags to ignore
stack_limit: Maximum stack size limit for depth-first traversal.
Nodes will be dropped if this limit is hit, which may truncate the
textual result.
Intended to limit the maximum working memory when generating a preview.
"""
if tree is None:
return
# This is a stack whose items are elements to iterate over *or* strings
# to be returned.
elements: List[Union[str, "etree.Element"]] = [tree]
while elements:
el = elements.pop()
if isinstance(el, str):
yield el
elif el.tag not in tags_to_ignore:
# If the element isn't meant for display, ignore it.
if el.get("role") in ARIA_ROLES_TO_IGNORE:
continue
# 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 element's children, interspersed with
# each child's tail text (if it exists).
#
# We iterate in reverse order so that earlier pieces of text appear
# closer to the top of the stack.
for child in el.iterchildren(reversed=True):
if len(elements) > stack_limit:
# We've hit our limit for working memory
break
if child.tail:
# 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.append(child.tail)
elements.append(child)
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
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