Fixed #823 (#824)

* Added a failing test for the multi-oauth-router issue

* Fixed the #823 regression.

Using a regex for the backend name validation instead of an enum.

* Fixed formatting errors

* Moved the `AuthenticationBackendName` enum to `Authenticator`

This prevents an issue with OpenAPI schema generation caused by two
endpoints accepting a parameter with a duplicate name.

fastapi_users/authentication/__init__.py

@@ -1,3 +1,4 @@
+import enum
 import re
 from inspect import Parameter, Signature
 from typing import Callable, Optional, Sequence
@@ -42,6 +43,7 @@ class Authenticator:
     """
 
     backends: Sequence[BaseAuthentication]
+    backends_enum: enum.Enum
 
     def __init__(
         self,
@@ -49,6 +51,10 @@ class Authenticator:
         get_user_manager: UserManagerDependency[models.UC, models.UD],
     ):
         self.backends = backends
+        self.backends_enum = enum.Enum(  # type: ignore
+            "AuthenticationBackendName",
+            {backend.name: backend.name for backend in backends},  # type: ignore
+        )
         self.get_user_manager = get_user_manager
 
     def current_user(

fastapi_users/router/oauth.py

@@ -1,4 +1,3 @@
-import enum
 from typing import Dict, List
 
 import jwt
@@ -44,11 +43,6 @@ def get_oauth_router(
             route_name=callback_route_name,
         )
 
-    AuthenticationBackendName: enum.EnumMeta = enum.Enum(
-        "AuthenticationBackendName",
-        {backend.name: backend.name for backend in authenticator.backends},
-    )
-
     @router.get(
         "/authorize",
         name="oauth:authorize",
@@ -56,7 +50,7 @@ def get_oauth_router(
     )
     async def authorize(
         request: Request,
-        authentication_backend: AuthenticationBackendName,
+        authentication_backend: authenticator.backends_enum,  # type: ignore
         scopes: List[str] = Query(None),
     ):
         if redirect_url is not None: