Split out a separate endpoint to complete SSO registration (#9262)

There are going to be a couple of paths to get to the final step of SSO reg, and I want the URL in the browser to consistent. So, let's move the final step onto a separate path, which we redirect to.

7 files changed, 145 insertions, 26 deletions

diff --git a/changelog.d/9262.feature b/changelog.d/9262.feature
new file mode 100644
index 0000000000..c21b197ca1
--- /dev/null
+++ b/changelog.d/9262.feature
@@ -0,0 +1 @@
+Improve the user experience of setting up an account via single-sign on.
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 57a2f5237c..86d6f73674 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -62,6 +62,7 @@ from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
 from synapse.rest.synapse.client.pick_idp import PickIdpResource
 from synapse.rest.synapse.client.pick_username import pick_username_resource
+from synapse.rest.synapse.client.sso_register import SsoRegisterResource
 from synapse.rest.well_known import WellKnownResource
 from synapse.server import HomeServer
 from synapse.storage import DataStore
@@ -192,6 +193,7 @@ class SynapseHomeServer(HomeServer):
                     "/_synapse/admin": AdminRestResource(self),
                     "/_synapse/client/pick_username": pick_username_resource(self),
                     "/_synapse/client/pick_idp": PickIdpResource(self),
+                    "/_synapse/client/sso_register": SsoRegisterResource(self),
                 }
             )
 
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 3308b037d2..50c5ae142a 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -21,12 +21,13 @@ import attr
 from typing_extensions import NoReturn, Protocol
 
 from twisted.web.http import Request
+from twisted.web.iweb import IRequest
 
 from synapse.api.constants import LoginType
 from synapse.api.errors import Codes, NotFoundError, RedirectException, SynapseError
 from synapse.handlers.ui_auth import UIAuthSessionDataConstants
 from synapse.http import get_request_user_agent
-from synapse.http.server import respond_with_html
+from synapse.http.server import respond_with_html, respond_with_redirect
 from synapse.http.site import SynapseRequest
 from synapse.types import JsonDict, UserID, contains_invalid_mxid_characters
 from synapse.util.async_helpers import Linearizer
@@ -141,6 +142,9 @@ class UsernameMappingSession:
     # expiry time for the session, in milliseconds
     expiry_time_ms = attr.ib(type=int)
 
+    # choices made by the user
+    chosen_localpart = attr.ib(type=Optional[str], default=None)
+
 
 # the HTTP cookie used to track the mapping session id
 USERNAME_MAPPING_SESSION_COOKIE_NAME = b"username_mapping_session"
@@ -647,6 +651,25 @@ class SsoHandler:
         )
         respond_with_html(request, 200, html)
 
+    def get_mapping_session(self, session_id: str) -> UsernameMappingSession:
+        """Look up the given username mapping session
+
+        If it is not found, raises a SynapseError with an http code of 400
+
+        Args:
+            session_id: session to look up
+        Returns:
+            active mapping session
+        Raises:
+            SynapseError if the session is not found/has expired
+        """
+        self._expire_old_sessions()
+        session = self._username_mapping_sessions.get(session_id)
+        if session:
+            return session
+        logger.info("Couldn't find session id %s", session_id)
+        raise SynapseError(400, "unknown session")
+
     async def check_username_availability(
         self, localpart: str, session_id: str,
     ) -> bool:
@@ -663,12 +686,7 @@ class SsoHandler:
 
         # make sure that there is a valid mapping session, to stop people dictionary-
         # scanning for accounts
-
-        self._expire_old_sessions()
-        session = self._username_mapping_sessions.get(session_id)
-        if not session:
-            logger.info("Couldn't find session id %s", session_id)
-            raise SynapseError(400, "unknown session")
+        self.get_mapping_session(session_id)
 
         logger.info(
             "[session %s] Checking for availability of username %s",
@@ -696,16 +714,33 @@ class SsoHandler:
             localpart: localpart requested by the user
             session_id: ID of the username mapping session, extracted from a cookie
         """
-        self._expire_old_sessions()
-        session = self._username_mapping_sessions.get(session_id)
-        if not session:
-            logger.info("Couldn't find session id %s", session_id)
-            raise SynapseError(400, "unknown session")
+        session = self.get_mapping_session(session_id)
+
+        # update the session with the user's choices
+        session.chosen_localpart = localpart
+
+        # we're done; now we can register the user
+        respond_with_redirect(request, b"/_synapse/client/sso_register")
+
+    async def register_sso_user(self, request: Request, session_id: str) -> None:
+        """Called once we have all the info we need to register a new user.
 
-        logger.info("[session %s] Registering localpart %s", session_id, localpart)
+        Does so and serves an HTTP response
+
+        Args:
+            request: HTTP request
+            session_id: ID of the username mapping session, extracted from a cookie
+        """
+        session = self.get_mapping_session(session_id)
+
+        logger.info(
+            "[session %s] Registering localpart %s",
+            session_id,
+            session.chosen_localpart,
+        )
 
         attributes = UserAttributes(
-            localpart=localpart,
+            localpart=session.chosen_localpart,
             display_name=session.display_name,
             emails=session.emails,
         )
@@ -720,7 +755,12 @@ class SsoHandler:
             request.getClientIP(),
         )
 
-        logger.info("[session %s] Registered userid %s", session_id, user_id)
+        logger.info(
+            "[session %s] Registered userid %s with attributes %s",
+            session_id,
+            user_id,
+            attributes,
+        )
 
         # delete the mapping session and the cookie
         del self._username_mapping_sessions[session_id]
@@ -751,3 +791,14 @@ class SsoHandler:
         for session_id in to_expire:
             logger.info("Expiring mapping session %s", session_id)
             del self._username_mapping_sessions[session_id]
+
+
+def get_username_mapping_session_cookie_from_request(request: IRequest) -> str:
+    """Extract the session ID from the cookie
+
+    Raises a SynapseError if the cookie isn't found
+    """
+    session_id = request.getCookie(USERNAME_MAPPING_SESSION_COOKIE_NAME)
+    if not session_id:
+        raise SynapseError(code=400, msg="missing session_id")
+    return session_id.decode("ascii", errors="replace")
diff --git a/synapse/http/server.py b/synapse/http/server.py
index d69d579b3a..8249732b27 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -761,6 +761,13 @@ def set_clickjacking_protection_headers(request: Request):
     request.setHeader(b"Content-Security-Policy", b"frame-ancestors 'none';")
 
 
+def respond_with_redirect(request: Request, url: bytes) -> None:
+    """Write a 302 response to the request, if it is still alive."""
+    logger.debug("Redirect to %s", url.decode("utf-8"))
+    request.redirect(url)
+    finish_request(request)
+
+
 def finish_request(request: Request):
     """ Finish writing the response to the request.
 
diff --git a/synapse/rest/synapse/client/pick_username.py b/synapse/rest/synapse/client/pick_username.py
index d3b6803e65..1bc737bad0 100644
--- a/synapse/rest/synapse/client/pick_username.py
+++ b/synapse/rest/synapse/client/pick_username.py
@@ -12,6 +12,7 @@
 # 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 typing import TYPE_CHECKING
 
 import pkg_resources
@@ -20,8 +21,7 @@ from twisted.web.http import Request
 from twisted.web.resource import Resource
 from twisted.web.static import File
 
-from synapse.api.errors import SynapseError
-from synapse.handlers.sso import USERNAME_MAPPING_SESSION_COOKIE_NAME
+from synapse.handlers.sso import get_username_mapping_session_cookie_from_request
 from synapse.http.server import DirectServeHtmlResource, DirectServeJsonResource
 from synapse.http.servlet import parse_string
 from synapse.http.site import SynapseRequest
@@ -61,12 +61,10 @@ class AvailabilityCheckResource(DirectServeJsonResource):
     async def _async_render_GET(self, request: Request):
         localpart = parse_string(request, "username", required=True)
 
-        session_id = request.getCookie(USERNAME_MAPPING_SESSION_COOKIE_NAME)
-        if not session_id:
-            raise SynapseError(code=400, msg="missing session_id")
+        session_id = get_username_mapping_session_cookie_from_request(request)
 
         is_available = await self._sso_handler.check_username_availability(
-            localpart, session_id.decode("ascii", errors="replace")
+            localpart, session_id
         )
         return 200, {"available": is_available}
 
@@ -79,10 +77,8 @@ class SubmitResource(DirectServeHtmlResource):
     async def _async_render_POST(self, request: SynapseRequest):
         localpart = parse_string(request, "username", required=True)
 
-        session_id = request.getCookie(USERNAME_MAPPING_SESSION_COOKIE_NAME)
-        if not session_id:
-            raise SynapseError(code=400, msg="missing session_id")
+        session_id = get_username_mapping_session_cookie_from_request(request)
 
         await self._sso_handler.handle_submit_username_request(
-            request, localpart, session_id.decode("ascii", errors="replace")
+            request, localpart, session_id
         )
diff --git a/synapse/rest/synapse/client/sso_register.py b/synapse/rest/synapse/client/sso_register.py
new file mode 100644
index 0000000000..dfefeb7796
--- /dev/null
+++ b/synapse/rest/synapse/client/sso_register.py
@@ -0,0 +1,50 @@
+# -*- coding: utf-8 -*-
+# 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 logging
+from typing import TYPE_CHECKING
+
+from twisted.web.http import Request
+
+from synapse.api.errors import SynapseError
+from synapse.handlers.sso import get_username_mapping_session_cookie_from_request
+from synapse.http.server import DirectServeHtmlResource
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class SsoRegisterResource(DirectServeHtmlResource):
+    """A resource which completes SSO registration
+
+    This resource gets mounted at /_synapse/client/sso_register, and is shown
+    after we collect username and/or consent for a new SSO user. It (finally) registers
+    the user, and confirms redirect to the client
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self._sso_handler = hs.get_sso_handler()
+
+    async def _async_render_GET(self, request: Request) -> None:
+        try:
+            session_id = get_username_mapping_session_cookie_from_request(request)
+        except SynapseError as e:
+            logger.warning("Error fetching session cookie: %s", e)
+            self._sso_handler.render_error(request, "bad_session", e.msg, code=e.code)
+            return
+        await self._sso_handler.register_sso_user(request, session_id)
diff --git a/tests/rest/client/v1/test_login.py b/tests/rest/client/v1/test_login.py
index e2bb945453..f01215ed1c 100644
--- a/tests/rest/client/v1/test_login.py
+++ b/tests/rest/client/v1/test_login.py
@@ -31,6 +31,7 @@ from synapse.rest.client.v2_alpha import devices, register
 from synapse.rest.client.v2_alpha.account import WhoamiRestServlet
 from synapse.rest.synapse.client.pick_idp import PickIdpResource
 from synapse.rest.synapse.client.pick_username import pick_username_resource
+from synapse.rest.synapse.client.sso_register import SsoRegisterResource
 from synapse.types import create_requester
 
 from tests import unittest
@@ -1215,6 +1216,7 @@ class UsernamePickerTestCase(HomeserverTestCase):
 
         d = super().create_resource_dict()
         d["/_synapse/client/pick_username"] = pick_username_resource(self.hs)
+        d["/_synapse/client/sso_register"] = SsoRegisterResource(self.hs)
         d["/_synapse/oidc"] = OIDCResource(self.hs)
         return d
 
@@ -1253,7 +1255,7 @@ class UsernamePickerTestCase(HomeserverTestCase):
         self.assertApproximates(session.expiry_time_ms, expected_expiry, tolerance=1000)
 
         # Now, submit a username to the username picker, which should serve a redirect
-        # back to the client
+        # to the completion page
         submit_path = picker_url + "/submit"
         content = urlencode({b"username": b"bobby"}).encode("utf8")
         chan = self.make_request(
@@ -1270,6 +1272,16 @@ class UsernamePickerTestCase(HomeserverTestCase):
         )
         self.assertEqual(chan.code, 302, chan.result)
         location_headers = chan.headers.getRawHeaders("Location")
+
+        # send a request to the completion page, which should 302 to the client redirectUrl
+        chan = self.make_request(
+            "GET",
+            path=location_headers[0],
+            custom_headers=[("Cookie", "username_mapping_session=" + session_id)],
+        )
+        self.assertEqual(chan.code, 302, chan.result)
+        location_headers = chan.headers.getRawHeaders("Location")
+
         # ensure that the returned location matches the requested redirect URL
         path, query = location_headers[0].split("?", 1)
         self.assertEqual(path, "https://x")