Add an admin API endpoint to find a user based on its external ID in an auth provider.

changelog.d/13810.feature

@@ -0,0 +1 @@
+Add an admin API endpoint to find a user based on its external ID in an auth provider.

docs/admin_api/user_admin_api.md

@@ -1155,3 +1155,27 @@ GET /_synapse/admin/v1/username_available?username=$localpart
 
 The request and response format is the same as the
 [/_matrix/client/r0/register/available](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available) API.
+
+### Find a user based on their ID in an auth provider
+
+The API is:
+
+```
+GET /_synapse/admin/v1/auth_providers/$provider/users/$external_id
+```
+
+A response body like the following is returned:
+
+```json
+{
+    "user_id": "@hello:example.org"
+}
+```
+
+**Parameters**
+
+The following parameters should be set in the URL:
+
+- `provider` - The ID of the authentication provider, as advertised by the [`GET /_matrix/client/v3/login`](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3login) API in the `m.login.sso` authentication method.
+- `external_id` - The user ID from the authentication provider. Usually corresponds to the `sub` claim for OIDC providers, or to the `uid` attestation for SAML2 providers.
+

synapse/rest/admin/__init__.py

@@ -80,6 +80,7 @@
     SearchUsersRestServlet,
     ShadowBanRestServlet,
     UserAdminServlet,
+    UserByExternalId,
     UserMembershipRestServlet,
     UserRegisterServlet,
     UserRestServletV2,
@@ -275,6 +276,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    UserByExternalId(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if hs.config.worker.worker_app is None:

synapse/rest/admin/users.py

@@ -1156,3 +1156,30 @@ async def on_GET(
                 "rooms": by_room_data,
             },
         }
+
+
+class UserByExternalId(RestServlet):
+    """Find a user based on an external ID from an auth provider"""
+
+    PATTERNS = admin_patterns(
+        "/auth_providers/(?P<provider>[^/]*)/users/(?P<external_id>[^/]*)"
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_GET(
+        self,
+        request: SynapseRequest,
+        provider: str,
+        external_id: str,
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        user_id = await self._store.get_user_by_external_id(provider, external_id)
+
+        if user_id is None:
+            raise NotFoundError("User not found")
+
+        return HTTPStatus.OK, {"user_id": user_id}