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/v1/login.py

@@ -311,48 +311,31 @@ class LoginRestServlet(RestServlet):
         return result
 
 
-class BaseSSORedirectServlet(RestServlet):
-    """Common base class for /login/sso/redirect impls"""
-
+class SsoRedirectServlet(RestServlet):
     PATTERNS = client_patterns("/login/(cas|sso)/redirect", v1=True)
 
+    def __init__(self, hs: "HomeServer"):
+        # make sure that the relevant handlers are instantiated, so that they
+        # register themselves with the main SSOHandler.
+        if hs.config.cas_enabled:
+            hs.get_cas_handler()
+        elif hs.config.saml2_enabled:
+            hs.get_saml_handler()
+        elif hs.config.oidc_enabled:
+            hs.get_oidc_handler()
+        self._sso_handler = hs.get_sso_handler()
+
     async def on_GET(self, request: SynapseRequest):
-        args = request.args
-        if b"redirectUrl" not in args:
-            return 400, "Redirect URL not specified for SSO auth"
-        client_redirect_url = args[b"redirectUrl"][0]
-        sso_url = await self.get_sso_url(request, client_redirect_url)
+        client_redirect_url = parse_string(
+            request, "redirectUrl", required=True, encoding=None
+        )
+        sso_url = await self._sso_handler.handle_redirect_request(
+            request, client_redirect_url
+        )
+        logger.info("Redirecting to %s", sso_url)
         request.redirect(sso_url)
         finish_request(request)
 
-    async def get_sso_url(
-        self, request: SynapseRequest, client_redirect_url: bytes
-    ) -> bytes:
-        """Get the URL to redirect to, to perform SSO auth
-
-        Args:
-            request: The client request to redirect.
-            client_redirect_url: the URL that we should redirect the
-                client to when everything is done
-
-        Returns:
-            URL to redirect to
-        """
-        # to be implemented by subclasses
-        raise NotImplementedError()
-
-
-class CasRedirectServlet(BaseSSORedirectServlet):
-    def __init__(self, hs):
-        self._cas_handler = hs.get_cas_handler()
-
-    async def get_sso_url(
-        self, request: SynapseRequest, client_redirect_url: bytes
-    ) -> bytes:
-        return self._cas_handler.get_redirect_url(
-            {"redirectUrl": client_redirect_url}
-        ).encode("ascii")
-
 
 class CasTicketServlet(RestServlet):
     PATTERNS = client_patterns("/login/cas/ticket", v1=True)
@@ -379,40 +362,8 @@ class CasTicketServlet(RestServlet):
         )
 
 
-class SAMLRedirectServlet(BaseSSORedirectServlet):
-    PATTERNS = client_patterns("/login/sso/redirect", v1=True)
-
-    def __init__(self, hs):
-        self._saml_handler = hs.get_saml_handler()
-
-    async def get_sso_url(
-        self, request: SynapseRequest, client_redirect_url: bytes
-    ) -> bytes:
-        return self._saml_handler.handle_redirect_request(client_redirect_url)
-
-
-class OIDCRedirectServlet(BaseSSORedirectServlet):
-    """Implementation for /login/sso/redirect for the OIDC login flow."""
-
-    PATTERNS = client_patterns("/login/sso/redirect", v1=True)
-
-    def __init__(self, hs):
-        self._oidc_handler = hs.get_oidc_handler()
-
-    async def get_sso_url(
-        self, request: SynapseRequest, client_redirect_url: bytes
-    ) -> bytes:
-        return await self._oidc_handler.handle_redirect_request(
-            request, client_redirect_url
-        )
-
-
 def register_servlets(hs, http_server):
     LoginRestServlet(hs).register(http_server)
+    SsoRedirectServlet(hs).register(http_server)
     if hs.config.cas_enabled:
-        CasRedirectServlet(hs).register(http_server)
         CasTicketServlet(hs).register(http_server)
-    elif hs.config.saml2_enabled:
-        SAMLRedirectServlet(hs).register(http_server)
-    elif hs.config.oidc_enabled:
-        OIDCRedirectServlet(hs).register(http_server)