Generate guest access token on 3pid invites

This means that following the same link across multiple sessions or
devices can re-use the same guest account.

Note that this is somewhat of an abuse vector; we can't throw up
captchas on this flow, so this is a way of registering ephemeral
accounts for spam, whose sign-up we don't rate limit.

synapse/handlers/register.py

@@ -349,3 +349,18 @@ class RegistrationHandler(BaseHandler):
 
     def auth_handler(self):
         return self.hs.get_handlers().auth_handler
+
+    @defer.inlineCallbacks
+    def guest_access_token_for(self, medium, address, inviter_user_id):
+        access_token = yield self.store.get_3pid_guest_access_token(medium, address)
+        if access_token:
+            defer.returnValue(access_token)
+
+        _, access_token = yield self.register(
+            generate_token=True,
+            make_guest=True
+        )
+        access_token = yield self.store.save_or_get_3pid_guest_access_token(
+            medium, address, access_token, inviter_user_id
+        )
+        defer.returnValue(access_token)

synapse/handlers/room.py

@@ -848,6 +848,13 @@ class RoomMemberHandler(BaseHandler):
                 user.
         """
 
+        registration_handler = self.hs.get_handlers().registration_handler
+        guest_access_token = yield registration_handler.guest_access_token_for(
+            medium=medium,
+            address=address,
+            inviter_user_id=inviter_user_id,
+        )
+
         is_url = "%s%s/_matrix/identity/api/v1/store-invite" % (
             id_server_scheme, id_server,
         )
@@ -864,6 +871,7 @@ class RoomMemberHandler(BaseHandler):
                 "sender": inviter_user_id,
                 "sender_display_name": inviter_display_name,
                 "sender_avatar_url": inviter_avatar_url,
+                "guest_access_token": guest_access_token,
             }
         )
         # TODO: Check for success

synapse/storage/registration.py

@@ -387,3 +387,47 @@ class RegistrationStore(SQLBaseStore):
             "find_next_generated_user_id",
             _find_next_generated_user_id
         )))
+
+    @defer.inlineCallbacks
+    def get_3pid_guest_access_token(self, medium, address):
+        ret = yield self._simple_select_one(
+            "threepid_guest_access_tokens",
+            {
+                "medium": medium,
+                "address": address
+            },
+            ["guest_access_token"], True, 'get_3pid_guest_access_token'
+        )
+        if ret:
+            defer.returnValue(ret["guest_access_token"])
+        defer.returnValue(None)
+
+    @defer.inlineCallbacks
+    def save_or_get_3pid_guest_access_token(
+            self, medium, address, access_token, inviter_user_id
+    ):
+        """
+        Gets the 3pid's guest access token if exists, else saves access_token.
+
+        :param medium (str): Medium of the 3pid. Must be "email".
+        :param address (str): 3pid address.
+        :param access_token (str): The access token to persist if none is
+            already persisted.
+        :param inviter_user_id (str): User ID of the inviter.
+        :return (deferred str): Whichever access token is persisted at the end
+            of this function call.
+        """
+        def insert(txn):
+            txn.execute(
+                "INSERT INTO threepid_guest_access_tokens "
+                "(medium, address, guest_access_token, first_inviter) "
+                "VALUES (?, ?, ?, ?)",
+                (medium, address, access_token, inviter_user_id)
+            )
+
+        try:
+            yield self.runInteraction("save_3pid_guest_access_token", insert)
+            defer.returnValue(access_token)
+        except self.database_engine.module.IntegrityError:
+            ret = yield self.get_3pid_guest_access_token(medium, address)
+            defer.returnValue(ret)

synapse/storage/schema/delta/30/threepid_guest_access_tokens.sql

@@ -0,0 +1,24 @@
+/* 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.
+ */
+
+-- Stores guest account access tokens generated for unbound 3pids.
+CREATE TABLE threepid_guest_access_tokens(
+    medium TEXT, -- The medium of the 3pid. Must be "email".
+    address TEXT, -- The 3pid address.
+    guest_access_token TEXT, -- The access token for a guest user for this 3pid.
+    first_inviter TEXT -- User ID of the first user to invite this 3pid to a room.
+);
+
+CREATE UNIQUE INDEX threepid_guest_access_tokens_index ON threepid_guest_access_tokens(medium, address);