Simplify the flow for SSO UIA (#8881)

* SsoHandler: remove inheritance from BaseHandler

* Simplify the flow for SSO UIA

We don't need to do all the magic for mapping users when we are doing UIA, so
let's factor that out.

synapse/handlers/sso.py

@@ -17,8 +17,9 @@ from typing import TYPE_CHECKING, Awaitable, Callable, List, Optional
 
 import attr
 
+from twisted.web.http import Request
+
 from synapse.api.errors import RedirectException
-from synapse.handlers._base import BaseHandler
 from synapse.http.server import respond_with_html
 from synapse.types import UserID, contains_invalid_mxid_characters
 
@@ -42,14 +43,16 @@ class UserAttributes:
     emails = attr.ib(type=List[str], default=attr.Factory(list))
 
 
-class SsoHandler(BaseHandler):
+class SsoHandler:
     # The number of attempts to ask the mapping provider for when generating an MXID.
     _MAP_USERNAME_RETRIES = 1000
 
     def __init__(self, hs: "HomeServer"):
-        super().__init__(hs)
+        self._store = hs.get_datastore()
+        self._server_name = hs.hostname
         self._registration_handler = hs.get_registration_handler()
         self._error_template = hs.config.sso_error_template
+        self._auth_handler = hs.get_auth_handler()
 
     def render_error(
         self, request, error: str, error_description: Optional[str] = None
@@ -95,7 +98,7 @@ class SsoHandler(BaseHandler):
         )
 
         # Check if we already have a mapping for this user.
-        previously_registered_user_id = await self.store.get_user_by_external_id(
+        previously_registered_user_id = await self._store.get_user_by_external_id(
             auth_provider_id, remote_user_id,
         )
 
@@ -181,7 +184,7 @@ class SsoHandler(BaseHandler):
             previously_registered_user_id = await grandfather_existing_users()
             if previously_registered_user_id:
                 # Future logins should also match this user ID.
-                await self.store.record_user_external_id(
+                await self._store.record_user_external_id(
                     auth_provider_id, remote_user_id, previously_registered_user_id
                 )
                 return previously_registered_user_id
@@ -214,8 +217,8 @@ class SsoHandler(BaseHandler):
                 )
 
             # Check if this mxid already exists
-            user_id = UserID(attributes.localpart, self.server_name).to_string()
-            if not await self.store.get_users_by_id_case_insensitive(user_id):
+            user_id = UserID(attributes.localpart, self._server_name).to_string()
+            if not await self._store.get_users_by_id_case_insensitive(user_id):
                 # This mxid is free
                 break
         else:
@@ -238,7 +241,47 @@ class SsoHandler(BaseHandler):
             user_agent_ips=[(user_agent, ip_address)],
         )
 
-        await self.store.record_user_external_id(
+        await self._store.record_user_external_id(
             auth_provider_id, remote_user_id, registered_user_id
         )
         return registered_user_id
+
+    async def complete_sso_ui_auth_request(
+        self,
+        auth_provider_id: str,
+        remote_user_id: str,
+        ui_auth_session_id: str,
+        request: Request,
+    ) -> None:
+        """
+        Given an SSO ID, retrieve the user ID for it and complete UIA.
+
+        Note that this requires that the user is mapped in the "user_external_ids"
+        table. This will be the case if they have ever logged in via SAML or OIDC in
+        recentish synapse versions, but may not be for older users.
+
+        Args:
+            auth_provider_id: A unique identifier for this SSO provider, e.g.
+                "oidc" or "saml".
+            remote_user_id: The unique identifier from the SSO provider.
+            ui_auth_session_id: The ID of the user-interactive auth session.
+            request: The request to complete.
+        """
+
+        user_id = await self.get_sso_user_by_remote_user_id(
+            auth_provider_id, remote_user_id,
+        )
+
+        if not user_id:
+            logger.warning(
+                "Remote user %s/%s has not previously logged in here: UIA will fail",
+                auth_provider_id,
+                remote_user_id,
+            )
+            # Let the UIA flow handle this the same as if they presented creds for a
+            # different user.
+            user_id = ""
+
+        await self._auth_handler.complete_sso_ui_auth(
+            user_id, ui_auth_session_id, request
+        )