Add an admin API endpoint to find a user based on its external ID in an auth provider. (#13810)

synapse/rest/admin/users.py

@@ -1156,3 +1156,30 @@ class AccountDataRestServlet(RestServlet):
                 "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}