initial WIP of a tentative preview_url endpoint - incomplete, untested, experimental, etc. just putting it here for safekeeping for now

docs/url_previews.rst

@@ -0,0 +1,74 @@
+URL Previews
+============
+
+Design notes on a URL previewing service for Matrix:
+
+Options are:
+
+ 1. Have an AS which listens for URLs, downloads them, and inserts an event that describes their metadata.
+   * Pros:
+     * Decouples the implementation entirely from Synapse.
+     * Uses existing Matrix events & content repo to store the metadata.
+   * Cons:
+     * Which AS should provide this service for a room, and why should you trust it?
+     * Doesn't work well with E2E; you'd have to cut the AS into every room
+     * the AS would end up subscribing to every room anyway.
+
+ 2. Have a generic preview API (nothing to do with Matrix) that provides a previewing service:
+   * Pros:
+     * Simple and flexible; can be used by any clients at any point
+   * Cons:
+     * If each HS provides one of these independently, all the HSes in a room may needlessly DoS the target URI
+     * We need somewhere to store the URL metadata rather than just using Matrix itself
+     * We can't piggyback on matrix to distribute the metadata between HSes.
+
+ 3. Make the synapse of the sending user responsible for spidering the URL and inserting an event asynchronously which describes the metadata.
+   * Pros:
+     * Works transparently for all clients
+     * Piggy-backs nicely on using Matrix for distributing the metadata.
+     * No confusion as to which AS
+   * Cons:
+     * Doesn't work with E2E
+     * We might want to decouple the implementation of the spider from the HS, given spider behaviour can be quite complicated and evolve much more rapidly than the HS.  It's more like a bot than a core part of the server.
+
+ 4. Make the sending client use the preview API and insert the event itself when successful.
+   * Pros:
+      * Works well with E2E
+      * No custom server functionality
+      * Lets the client customise the preview that they send (like on FB)
+   * Cons:
+      * Entirely specific to the sending client, whereas it'd be nice if /any/ URL was correctly previewed if clients support it.
+
+ 5. Have the option of specifying a shared (centralised) previewing service used by a room, to avoid all the different HSes in the room DoSing the target.
+
+Best solution is probably a combination of both 2 and 4.
+ * Sending clients do their best to create and send a preview at the point of sending the message, perhaps delaying the message until the preview is computed?  (This also lets the user validate the preview before sending)
+ * Receiving clients have the option of going and creating their own preview if one doesn't arrive soon enough (or if the original sender didn't create one)
+
+This is a bit magical though in that the preview could come from two entirely different sources - the sending HS or your local one.  However, this can always be exposed to users: "Generate your own URL previews if none are available?"
+
+This is tantamount also to senders calculating their own thumbnails for sending in advance of the main content - we are trusting the sender not to lie about the content in the thumbnail.  Whereas currently thumbnails are calculated by the receiving homeserver to avoid this attack.
+
+However, this kind of phishing attack does exist whether we let senders pick their thumbnails or not, in that a malicious sender can send normal text messages around the attachment claiming it to be legitimate.  We could rely on (future) reputation/abuse management to punish users who phish (be it with bogus metadata or bogus descriptions).   Bogus metadata is particularly bad though, especially if it's avoidable.
+
+As a first cut, let's do #2 and have the receiver hit the API to calculate its own previews (as it does currently for image thumbnails).  We can then extend/optimise this to option 4 as a special extra if needed.
+
+API
+---
+
+GET /_matrix/media/r0/previewUrl?url=http://wherever.com
+200 OK
+{
+    "og:type"        : "article"
+    "og:url"         : "https://twitter.com/matrixdotorg/status/684074366691356672"
+    "og:title"       : "Matrix on Twitter"
+    "og:image"       : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png"
+    "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance & bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”"
+    "og:site_name"   : "Twitter"
+}
+
+* Downloads the URL
+  * If HTML, just stores it in RAM and parses it for OG meta tags
+    * Download any media OG meta tags to the media repo, and refer to them in the OG via mxc:// URIs.
+  * If a media filetype we know we can thumbnail: store it on disk, and hand it to the thumbnailer. Generate OG meta tags from the thumbnailer contents.
+  * Otherwise, don't bother downloading further.

synapse/config/repository.py

@@ -53,6 +53,7 @@ class ContentRepositoryConfig(Config):
     def read_config(self, config):
         self.max_upload_size = self.parse_size(config["max_upload_size"])
         self.max_image_pixels = self.parse_size(config["max_image_pixels"])
+        self.max_spider_size = self.parse_size(config["max_spider_size"])
         self.media_store_path = self.ensure_directory(config["media_store_path"])
         self.uploads_path = self.ensure_directory(config["uploads_path"])
         self.dynamic_thumbnails = config["dynamic_thumbnails"]
@@ -73,6 +74,9 @@ class ContentRepositoryConfig(Config):
         # The largest allowed upload size in bytes
         max_upload_size: "10M"
 
+        # The largest allowed URL preview spidering size in bytes
+        max_spider_size: "10M"
+
         # Maximum number of pixels that will be thumbnailed
         max_image_pixels: "32M"
 
@@ -80,7 +84,7 @@ class ContentRepositoryConfig(Config):
         # the resolution requested by the client. If true then whenever
         # a new resolution is requested by the client the server will
         # generate a new thumbnail. If false the server will pick a thumbnail
-        # from a precalcualted list.
+        # from a precalculated list.
         dynamic_thumbnails: false
 
         # List of thumbnail to precalculate when an image is uploaded.

synapse/http/client.py

@@ -238,6 +238,87 @@ class SimpleHttpClient(object):
         else:
             raise CodeMessageException(response.code, body)
 
+    # XXX: FIXME: This is horribly copy-pasted from matrixfederationclient.
+    # The two should be factored out.
+
+    @defer.inlineCallbacks
+    def get_file(self, url, output_stream, args={}, max_size=None):
+        """GETs a file from a given URL
+        Args:
+            url (str): The URL to GET
+            output_stream (file): File to write the response body to.
+        Returns:
+            A (int,dict) tuple of the file length and a dict of the response
+            headers.
+        """
+
+        def body_callback(method, url_bytes, headers_dict):
+            self.sign_request(destination, method, url_bytes, headers_dict)
+            return None
+
+        response = yield self.request(
+            "GET",
+            url.encode("ascii"),
+            headers=Headers({
+                b"User-Agent": [self.user_agent],
+            })
+        )
+
+        headers = dict(response.headers.getAllRawHeaders())
+
+        if headers['Content-Length'] > max_size:
+            logger.warn("Requested URL is too large > %r bytes" % (self.max_size,))
+            # XXX: do we want to explicitly drop the connection here somehow? if so, how?
+            raise # what should we be raising here?
+
+        # TODO: if our Content-Type is HTML or something, just read the first
+        # N bytes into RAM rather than saving it all to disk only to read it
+        # straight back in again
+
+        try:
+            length = yield preserve_context_over_fn(
+                _readBodyToFile,
+                response, output_stream, max_size
+            )
+        except:
+            logger.exception("Failed to download body")
+            raise
+
+        defer.returnValue((length, headers))
+
+
+# XXX: FIXME: This is horribly copy-pasted from matrixfederationclient.
+# The two should be factored out.
+
+class _ReadBodyToFileProtocol(protocol.Protocol):
+    def __init__(self, stream, deferred, max_size):
+        self.stream = stream
+        self.deferred = deferred
+        self.length = 0
+        self.max_size = max_size
+
+    def dataReceived(self, data):
+        self.stream.write(data)
+        self.length += len(data)
+        if self.max_size is not None and self.length >= self.max_size:
+            logger.warn("Requested URL is too large > %r bytes" % (self.max_size,))
+            self.deferred = defer.Deferred()
+            self.transport.loseConnection()
+
+    def connectionLost(self, reason):
+        if reason.check(ResponseDone):
+            self.deferred.callback(self.length)
+        else:
+            self.deferred.errback(reason)
+
+
+# XXX: FIXME: This is horribly copy-pasted from matrixfederationclient.
+# The two should be factored out.
+
+def _readBodyToFile(response, stream, max_size):
+    d = defer.Deferred()
+    response.deliverBody(_ReadBodyToFileProtocol(stream, d, max_size))
+    return d
 
 class CaptchaServerHttpClient(SimpleHttpClient):
     """

synapse/rest/media/v1/media_repository.py

@@ -17,6 +17,7 @@ from .upload_resource import UploadResource
 from .download_resource import DownloadResource
 from .thumbnail_resource import ThumbnailResource
 from .identicon_resource import IdenticonResource
+from .preview_url_resource import PreviewUrlResource
 from .filepath import MediaFilePaths
 
 from twisted.web.resource import Resource
@@ -78,3 +79,5 @@ class MediaRepositoryResource(Resource):
         self.putChild("download", DownloadResource(hs, filepaths))
         self.putChild("thumbnail", ThumbnailResource(hs, filepaths))
         self.putChild("identicon", IdenticonResource())
+        self.putChild("preview_url", PreviewUrlResource(hs, filepaths))
+

synapse/rest/media/v1/preview_url_resource.py

@@ -0,0 +1,164 @@
+# Copyright 2016 OpenMarket Ltd
+#
+# 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.
+
+from twisted.web.resource import Resource
+from lxml import html
+from synapse.http.client import SimpleHttpClient
+from synapse.http.server import respond_with_json_bytes
+from simplejson import json
+
+import logging
+logger = logging.getLogger(__name__)
+
+class PreviewUrlResource(Resource):
+    isLeaf = True
+
+    def __init__(self, hs, filepaths):
+        Resource.__init__(self)
+        self.client = SimpleHttpClient(hs)
+        self.filepaths = filepaths
+        self.max_spider_size = hs.config.max_spider_size
+        self.server_name = hs.hostname
+        self.clock = hs.get_clock()
+
+    def render_GET(self, request):
+        self._async_render_GET(request)
+        return NOT_DONE_YET
+
+    @request_handler
+    @defer.inlineCallbacks
+    def _async_render_GET(self, request):
+        url = request.args.get("url")
+        
+        try:
+            # TODO: keep track of whether there's an ongoing request for this preview
+            # and block and return their details if there is one.
+
+            media_info = self._download_url(url)
+        except:
+            os.remove(fname)
+            raise
+
+        if self._is_media(media_type):
+            dims = yield self._generate_local_thumbnails(
+                    media_info.filesystem_id, media_info
+                  )
+
+            og = {
+                "og:description" : media_info.download_name,
+                "og:image" : "mxc://%s/%s" % (self.server_name, media_info.filesystem_id),
+                "og:image:type" : media_info.media_type,
+                "og:image:width" : dims.width,
+                "og:image:height" : dims.height,
+            }
+
+            # define our OG response for this media
+        elif self._is_html(media_type):
+            tree = html.parse(media_info.filename)
+
+            # suck it up into lxml and define our OG response.
+            # if we see any 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"        : "article"
+            # "og:url"         : "https://twitter.com/matrixdotorg/status/684074366691356672"
+            # "og:title"       : "Matrix on Twitter"
+            # "og:image"       : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png"
+            # "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance & bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”"
+            # "og:site_name"   : "Twitter"
+
+            og = {}
+            for tag in tree.xpath("//*/meta[starts-with(@property, 'og:')]"):
+                og[tag.attrib['property']] = tag.attrib['content']
+
+            # TODO: store our OG details in a cache (and expire them when stale)
+            # TODO: delete the content to stop diskfilling, as we only ever cared about its OG
+
+        respond_with_json_bytes(request, 200, json.dumps(og), send_cors=True)
+
+    def _download_url(url):
+        requester = yield self.auth.get_user_by_req(request)
+
+        # XXX: horrible duplication with base_resource's _download_remote_file()
+        file_id = random_string(24)
+
+        fname = self.filepaths.local_media_filepath(file_id)
+        self._makedirs(fname)
+
+        try:
+            with open(fname, "wb") as f:
+                length, headers = yield self.client.get_file(
+                    url, output_stream=f, max_size=self.max_spider_size,
+                )
+            media_type = headers["Content-Type"][0]
+            time_now_ms = self.clock.time_msec()
+
+            content_disposition = headers.get("Content-Disposition", None)
+            if content_disposition:
+                _, params = cgi.parse_header(content_disposition[0],)
+                download_name = None
+
+                # First check if there is a valid UTF-8 filename
+                download_name_utf8 = params.get("filename*", None)
+                if download_name_utf8:
+                    if download_name_utf8.lower().startswith("utf-8''"):
+                        download_name = download_name_utf8[7:]
+
+                # If there isn't check for an ascii name.
+                if not download_name:
+                    download_name_ascii = params.get("filename", None)
+                    if download_name_ascii and is_ascii(download_name_ascii):
+                        download_name = download_name_ascii
+
+                if download_name:
+                    download_name = urlparse.unquote(download_name)
+                    try:
+                        download_name = download_name.decode("utf-8")
+                    except UnicodeDecodeError:
+                        download_name = None
+            else:
+                download_name = None
+
+            yield self.store.store_local_media(
+                media_id=fname,
+                media_type=media_type,
+                time_now_ms=self.clock.time_msec(),
+                upload_name=download_name,
+                media_length=length,
+                user_id=requester.user,
+            )
+
+        except:
+            os.remove(fname)
+            raise
+
+        return {
+            "media_type": media_type,
+            "media_length": length,
+            "download_name": download_name,
+            "created_ts": time_now_ms,
+            "filesystem_id": file_id,
+            "filename": fname,
+        }
+
+
+    def _is_media(content_type):
+        if content_type.lower().startswith("image/"):
+            return True
+
+    def _is_html(content_type):
+        content_type = content_type.lower()
+        if content_type == "text/html" or
+           content_type.startswith("application/xhtml"):
+            return True