Autodiscover oEmbed URLs from HTML.

synapse/rest/media/v1/oembed.py

@@ -96,6 +96,32 @@ def get_oembed_url(self, url: str) -> Optional[str]:
         # No match.
         return None
 
+    def autodiscover_from_html(self, tree: "etree.Element") -> Optional[str]:
+        """
+        Search an HTML document for oEmbed autodiscovery information.
+
+        Args:
+            tree: The parsed HTML body.
+
+        Returns:
+            The URL to use for oEmbed information, or None if no URL was found.
+        """
+        # Search for link elements with the proper rel and type attributes.
+        for tag in tree.xpath(
+            "//link[@rel='alternate'][@type='application/json+oembed']"
+        ):
+            if "href" in tag.attrib:
+                return tag.attrib["href"]
+
+        # Some providers (e.g. Flickr) use alternative instead of alternate.
+        for tag in tree.xpath(
+            "//link[@rel='alternative'][@type='application/json+oembed']"
+        ):
+            if "href" in tag.attrib:
+                return tag.attrib["href"]
+
+        return None
+
     def parse_oembed_response(self, url: str, raw_body: bytes) -> OEmbedResult:
         """
         Parse the oEmbed response into an Open Graph response.

synapse/rest/media/v1/preview_url_resource.py

@@ -298,7 +298,21 @@ async def _do_preview(self, url: str, user: str, ts: int) -> bytes:
             encoding = get_html_media_encoding(body, media_info.media_type)
             tree = decode_body(body, encoding)
             if tree is not None:
-                og = _calc_og(tree, media_info.uri)
+                # 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 = {}

tests/rest/media/v1/test_url_preview.py

@@ -721,3 +721,104 @@ def test_oembed_format(self):
                 "og:description": "Content Preview",
             },
         )
+
+    def test_oembed_autodiscovery(self):
+        """
+        Autodiscovery works by finding the link in the HTML response and then requesting an oEmbed URL.
+
+        1. Request a preview of a URL which is not known to the oEmbed code.
+        2. It returns HTML including a link to an oEmbed preview.
+        3. The oEmbed preview is requested and returns a URL for an image.
+        4. The image is requested for thumbnailing.
+
+        """
+        # This is a little cheesy in that we use the www subdomain (which isn't the
+        # list of oEmbed patterns) to get "raw" HTML response.
+        self.lookups["www.twitter.com"] = [(IPv4Address, "10.1.2.3")]
+        self.lookups["publish.twitter.com"] = [(IPv4Address, "10.1.2.3")]
+        self.lookups["cdn.twitter.com"] = [(IPv4Address, "10.1.2.3")]
+
+        result = b"""
+        <link rel="alternate" type="application/json+oembed"
+            href="http://publish.twitter.com/oembed?url=http%3A%2F%2Fcdn.twitter.com%2Fmatrixdotorg%2Fstatus%2F12345&format=json"
+            title="matrixdotorg" />
+        """
+
+        channel = self.make_request(
+            "GET",
+            "preview_url?url=http://www.twitter.com/matrixdotorg/status/12345",
+            shorthand=False,
+            await_result=False,
+        )
+        self.pump()
+
+        client = self.reactor.tcpClients[0][2].buildProtocol(None)
+        server = AccumulatingProtocol()
+        server.makeConnection(FakeTransport(client, self.reactor))
+        client.makeConnection(FakeTransport(server, self.reactor))
+        client.dataReceived(
+            (
+                b"HTTP/1.0 200 OK\r\nContent-Length: %d\r\n"
+                b'Content-Type: text/html; charset="utf8"\r\n\r\n'
+            )
+            % (len(result),)
+            + result
+        )
+
+        self.pump()
+
+        # The oEmbed response.
+        result2 = {
+            "version": "1.0",
+            "type": "photo",
+            "url": "http://cdn.twitter.com/matrixdotorg",
+        }
+        oembed_content = json.dumps(result2).encode("utf-8")
+
+        # Ensure a second request is made to the oEmbed URL.
+        client = self.reactor.tcpClients[1][2].buildProtocol(None)
+        server = AccumulatingProtocol()
+        server.makeConnection(FakeTransport(client, self.reactor))
+        client.makeConnection(FakeTransport(server, self.reactor))
+        client.dataReceived(
+            (
+                b"HTTP/1.0 200 OK\r\nContent-Length: %d\r\n"
+                b'Content-Type: application/json; charset="utf8"\r\n\r\n'
+            )
+            % (len(oembed_content),)
+            + oembed_content
+        )
+
+        self.pump()
+
+        # Ensure the URL is what was requested.
+        self.assertIn(b"/oembed?", server.data)
+
+        # Ensure a third request is made to the photo URL.
+        client = self.reactor.tcpClients[2][2].buildProtocol(None)
+        server = AccumulatingProtocol()
+        server.makeConnection(FakeTransport(client, self.reactor))
+        client.makeConnection(FakeTransport(server, self.reactor))
+        client.dataReceived(
+            (
+                b"HTTP/1.0 200 OK\r\nContent-Length: %d\r\n"
+                b"Content-Type: image/png\r\n\r\n"
+            )
+            % (len(SMALL_PNG),)
+            + SMALL_PNG
+        )
+
+        self.pump()
+
+        # Ensure the URL is what was requested.
+        self.assertIn(b"/matrixdotorg", server.data)
+
+        self.assertEqual(channel.code, 200)
+        body = channel.json_body
+        self.assertEqual(
+            body["og:url"], "http://www.twitter.com/matrixdotorg/status/12345"
+        )
+        self.assertTrue(body["og:image"].startswith("mxc://"))
+        self.assertEqual(body["og:image:height"], 1)
+        self.assertEqual(body["og:image:width"], 1)
+        self.assertEqual(body["og:image:type"], "image/png")