Refactor media modules. (#15146)

* Removes the `v1` directory from `test.rest.media.v1`.
* Moves the non-REST code from `synapse.rest.media.v1` to `synapse.media`.
* Flatten the `v1` directory from `synapse.rest.media`,  but leave compatiblity
  with 3rd party media repositories and spam checkers.

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