Combine the SSO Redirect Servlets (#9015)

* Implement CasHandler.handle_redirect_request

... to make it match OidcHandler and SamlHandler

* Clean up interface for OidcHandler.handle_redirect_request

Make it accept `client_redirect_url=None`.

* Clean up interface for `SamlHandler.handle_redirect_request`

... bring it into line with CAS and OIDC by making it take a Request parameter,
move the magic for `client_redirect_url` for UIA into the handler, and fix the
return type to be a `str` rather than a `bytes`.

* Define a common protocol for SSO auth provider impls

* Give SsoIdentityProvider an ID and register them

* Combine the SSO Redirect servlets

Now that the SsoHandler knows about the identity providers, we can combine the
various *RedirectServlets into a single implementation which delegates to the
right IdP.

* changelog

synapse/rest/client/v2_alpha/auth.py

@@ -14,15 +14,20 @@
 # limitations under the License.
 
 import logging
+from typing import TYPE_CHECKING
 
 from synapse.api.constants import LoginType
 from synapse.api.errors import SynapseError
 from synapse.api.urls import CLIENT_API_PREFIX
+from synapse.handlers.sso import SsoIdentityProvider
 from synapse.http.server import respond_with_html
 from synapse.http.servlet import RestServlet, parse_string
 
 from ._base import client_patterns
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
@@ -35,7 +40,7 @@ class AuthRestServlet(RestServlet):
 
     PATTERNS = client_patterns(r"/auth/(?P<stagetype>[\w\.]*)/fallback/web")
 
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         super().__init__()
         self.hs = hs
         self.auth = hs.get_auth()
@@ -85,31 +90,20 @@ class AuthRestServlet(RestServlet):
         elif stagetype == LoginType.SSO:
             # Display a confirmation page which prompts the user to
             # re-authenticate with their SSO provider.
+
             if self._cas_enabled:
-                # Generate a request to CAS that redirects back to an endpoint
-                # to verify the successful authentication.
-                sso_redirect_url = self._cas_handler.get_redirect_url(
-                    {"session": session},
-                )
-
+                sso_auth_provider = self._cas_handler  # type: SsoIdentityProvider
             elif self._saml_enabled:
-                # Some SAML identity providers (e.g. Google) require a
-                # RelayState parameter on requests. It is not necessary here, so
-                # pass in a dummy redirect URL (which will never get used).
-                client_redirect_url = b"unused"
-                sso_redirect_url = self._saml_handler.handle_redirect_request(
-                    client_redirect_url, session
-                )
-
+                sso_auth_provider = self._saml_handler
             elif self._oidc_enabled:
-                client_redirect_url = b""
-                sso_redirect_url = await self._oidc_handler.handle_redirect_request(
-                    request, client_redirect_url, session
-                )
-
+                sso_auth_provider = self._oidc_handler
             else:
                 raise SynapseError(400, "Homeserver not configured for SSO.")
 
+            sso_redirect_url = await sso_auth_provider.handle_redirect_request(
+                request, None, session
+            )
+
             html = await self.auth_handler.start_sso_ui_auth(sso_redirect_url, session)
 
         else: