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Diffstat (limited to 'synapse/media/preview_html.py')
| -rw-r--r-- | synapse/media/preview_html.py | 501 |
1 files changed, 501 insertions, 0 deletions
diff --git a/synapse/media/preview_html.py b/synapse/media/preview_html.py new file mode 100644 index 0000000000..516d0434f0 --- /dev/null +++ b/synapse/media/preview_html.py @@ -0,0 +1,501 @@ +# 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 |