Add new begin_auth callback in SSHClient

This commit adds a new begin_auth callback in the SSHClient class. When
AsyncSSH is acting as a client, this is called when authentication
begins, providing the username the client is attempting to authenticate
as, after evaluating any supplied SSH config and connection options.

Author: ronf

asyncssh/client.py

@@ -220,6 +220,15 @@ def auth_banner_received(self, msg: str, lang: str) -> None:
 
         """
 
+    def begin_auth(self, username: str) -> None:
+        """Begin client authentication
+
+           This method is called when client authentication is about to
+           begin, Applications may store the username passed here to
+           be used in future authentication callbacks.
+
+        """
+
     def auth_completed(self) -> None:
         """Authentication was completed successfully
 

asyncssh/connection.py

@@ -2292,6 +2292,9 @@ def _process_service_accept(self, _pkttype: int, _pktid: int,
 
             self._auth_in_progress = True
 
+            if self._owner: # pragma: no branch
+                self._owner.begin_auth(self._username)
+
             # This method is only in SSHClientConnection
             # pylint: disable=no-member
             cast('SSHClientConnection', self).try_next_auth()