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.

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
         )

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)