Allow auth backends to provide login and password

This is used to implement an auth backend that takes the credentials from an HTTP header (e.g. accounts are managed by an reverse proxy)

radicale/auth.py

@@ -67,6 +67,10 @@ def load(configuration, logger):
     logger.debug("Authentication type is %s", auth_type)
     if auth_type == "None":
         class_ = NoneAuth
+    elif auth_type == "remote_user":
+        class_ = RemoteUserAuth
+    elif auth_type == "http_x_remote_user":
+        class_ = HttpXRemoteUserAuth
     elif auth_type == "htpasswd":
         class_ = Auth
     else:
@@ -79,6 +83,14 @@ class BaseAuth:
         self.configuration = configuration
         self.logger = logger
 
+    def get_external_login(self, environ):
+        """Optionally provide the login and password externally.
+
+        Returns a tuple (login, password) or ().
+
+        """
+        return ()
+
     def is_authenticated(self, user, password):
         """Validate credentials.
 
@@ -201,3 +213,13 @@ class Auth(BaseAuth):
                     if login_ok & password_ok:
                         return True
         return False
+
+
+class RemoteUserAuth(NoneAuth):
+    def get_external_login(self, environ):
+        return environ.get("REMOTE_USER", ""), ""
+
+
+class HttpXRemoteUserAuth(NoneAuth):
+    def get_external_login(self, environ):
+        return environ.get("HTTP_X_REMOTE_USER", ""), ""