Padding machines can cause invalid cell on client intro circs
While stress testing vanguards, I got a dropped cell on a CIRCUIT_PADDING circuit that was previously HS_CLIENT_INTRO:HSCI_INTRO_SENT.
This could be an instance of #31002. It could be that we are switching to padding, and then some issue happened where the padding machine did not shut down properly. Or it could be another instance of #29927, but with padding involved this time.
Unfortunately I do not have client logs of this to find out, as it was a Tor Browser client. I am working to reproduce elsewhere, but it is rare.
FWIW, this is harmless. I will be adding an exemption to vanguards to demote the log to INFO there, but still close the circuit (since it is padding anyway and not used anymore).