Fix a regression that mapping providers should be able to redirect users. (#8878)

This was broken in #8801.

changelog.d/8878.bugfix

@@ -0,0 +1 @@
+Fix a regression in v1.24.0rc1 which failed to allow SAML mapping providers which were unable to redirect users to an additional page.

docs/sso_mapping_providers.md

@@ -169,6 +169,13 @@ A custom mapping provider must specify the following methods:
        * `emails` - A list of emails for the new user. If not provided, will
                     default to an empty list.
        
+       Alternatively it can raise a `synapse.api.errors.RedirectException` to
+       redirect the user to another page. This is useful to prompt the user for
+       additional information, e.g. if you want them to provide their own username.
+       It is the responsibility of the mapping provider to either redirect back
+       to `client_redirect_url` (including any additional information) or to
+       complete registration using methods from the `ModuleApi`.
+
 ### Default SAML Mapping Provider
 
 Synapse has a built-in SAML mapping provider if a custom provider isn't

synapse/handlers/oidc_handler.py

@@ -888,7 +888,7 @@ class OidcHandler(BaseHandler):
                 # continue to already be in use. Note that the error raised is
                 # arbitrary and will get turned into a MappingException.
                 if failures:
-                    raise RuntimeError(
+                    raise MappingException(
                         "Mapping provider does not support de-duplicating Matrix IDs"
                     )
 

synapse/handlers/sso.py

@@ -17,6 +17,7 @@ from typing import TYPE_CHECKING, Awaitable, Callable, List, Optional
 
 import attr
 
+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
@@ -28,7 +29,9 @@ logger = logging.getLogger(__name__)
 
 
 class MappingException(Exception):
-    """Used to catch errors when mapping the UserInfo object
+    """Used to catch errors when mapping an SSO response to user attributes.
+
+    Note that the msg that is raised is shown to end-users.
     """
 
 
@@ -145,6 +148,14 @@ class SsoHandler(BaseHandler):
             sso_to_matrix_id_mapper: A callable to generate the user attributes.
                 The only parameter is an integer which represents the amount of
                 times the returned mxid localpart mapping has failed.
+
+                It is expected that the mapper can raise two exceptions, which
+                will get passed through to the caller:
+
+                    MappingException if there was a problem mapping the response
+                        to the user.
+                    RedirectException to redirect to an additional page (e.g.
+                        to prompt the user for more information).
             grandfather_existing_users: A callable which can return an previously
                 existing matrix ID. The SSO ID is then linked to the returned
                 matrix ID.
@@ -154,8 +165,8 @@ class SsoHandler(BaseHandler):
 
         Raises:
             MappingException if there was a problem mapping the response to a user.
-            RedirectException: some mapping providers may raise this if they need
+            RedirectException: if the mapping provider needs to redirect the user
-                to redirect to an interstitial page.
+                to an additional page. (e.g. to prompt for more information)
 
         """
         # first of all, check if we already have a mapping for this user
@@ -179,10 +190,16 @@ class SsoHandler(BaseHandler):
         for i in range(self._MAP_USERNAME_RETRIES):
             try:
                 attributes = await sso_to_matrix_id_mapper(i)
+            except (RedirectException, MappingException):
+                # Mapping providers are allowed to issue a redirect (e.g. to ask
+                # the user for more information) and can issue a mapping exception
+                # if a name cannot be generated.
+                raise
             except Exception as e:
+                # Any other exception is unexpected.
                 raise MappingException(
-                    "Could not extract user attributes from SSO response: " + str(e)
+                    "Could not extract user attributes from SSO response."
-                )
+                ) from e
 
             logger.debug(
                 "Retrieved user attributes from user mapping provider: %r (attempt %d)",

tests/handlers/test_oidc.py

@@ -705,8 +705,7 @@ class OidcHandlerTestCase(HomeserverTestCase):
             MappingException,
         )
         self.assertEqual(
-            str(e.value),
+            str(e.value), "Mapping provider does not support de-duplicating Matrix IDs",
-            "Could not extract user attributes from SSO response: Mapping provider does not support de-duplicating Matrix IDs",
         )
 
     @override_config({"oidc_config": {"allow_existing_users": True}})

tests/handlers/test_saml.py

@@ -14,6 +14,7 @@
 
 import attr
 
+from synapse.api.errors import RedirectException
 from synapse.handlers.sso import MappingException
 
 from tests.unittest import HomeserverTestCase, override_config
@@ -49,6 +50,13 @@ class TestMappingProvider:
         return {"mxid_localpart": localpart, "displayname": None}
 
 
+class TestRedirectMappingProvider(TestMappingProvider):
+    def saml_response_to_user_attributes(
+        self, saml_response, failures, client_redirect_url
+    ):
+        raise RedirectException(b"https://custom-saml-redirect/")
+
+
 class SamlHandlerTestCase(HomeserverTestCase):
     def default_config(self):
         config = super().default_config()
@@ -166,3 +174,23 @@ class SamlHandlerTestCase(HomeserverTestCase):
         self.assertEqual(
             str(e.value), "Unable to generate a Matrix ID from the SSO response"
         )
+
+    @override_config(
+        {
+            "saml2_config": {
+                "user_mapping_provider": {
+                    "module": __name__ + ".TestRedirectMappingProvider"
+                },
+            }
+        }
+    )
+    def test_map_saml_response_redirect(self):
+        saml_response = FakeAuthnResponse({"uid": "test", "username": "test_user"})
+        redirect_url = ""
+        e = self.get_failure(
+            self.handler._map_saml_response_to_user(
+                saml_response, redirect_url, "user-agent", "10.10.10.10"
+            ),
+            RedirectException,
+        )
+        self.assertEqual(e.value.location, b"https://custom-saml-redirect/")