[Acme] Retrying challenges - spec bug?

[Rob Stradling <[e-mail]>]

https://tools.ietf.org/html/rfc8555#section-8.2 says:
   'The server MUST add an entry to the "error" field in the challenge
    after each failed validation query.'

And https://tools.ietf.org/html/rfc8555#section-8 says:
   'A challenge object with an error MUST have status equal to
    "invalid".'

The state transition diagram for challenge objects 
(https://tools.ietf.org/html/rfc8555#section-7.1.6) appears to 
indicate(*) that "invalid" is a final state for a challenge object, 
meaning that it is no longer possible for it to transition to "valid" 
and that retrying the challenge would therefore be pointless.

ISTM that the "error" field could be a very useful feedback mechanism 
inbetween retries, and that a challenge should only go to the "invalid" 
state once the ACME server has stopped retrying validation queries for 
that challenge.  Is this what the authors intended?

Do folks agree that 'A challenge object with an error MUST have status 
equal to "invalid"' is a bug in the spec?


(*) I wonder if I'm reading the state transition diagrams correctly...
In section 7.1.6, the state transition diagram for authorization objects 
shows that "invalid" is a final state...right?  But if that's the case, 
why does this sentence not list "invalid" as a final state?

   'The order also moves to the "invalid" state if it expires or one of
    its authorizations enters a final state other than "valid"
    ("expired", "revoked", or "deactivated").'

-- 
Rob Stradling
Senior Research & Development Scientist
Sectigo Limited