Invalid cred usage not rejected

[michael-o]

What went wrong?

I have passed invalid creg usage to gssapi.Credentials(usage) and it has been accepted.

How do we reproduce?

$ python3
> Python 3.7.7 (default, Mar 19 2020, 21:26:00)
> [Clang 9.0.1 (git@github.com:llvm/llvm-project.git c1a0a213378a458fbea1a5c77b31 on freebsd12
> Type "help", "copyright", "credits" or "license" for more information.
 import gssapi
 cred = gssapi.Credentials(usage='nonsense')
 cred.usage
> 'both'

Component versions (python-gssapi, Kerberos, OS / distro, etc.)

Installed via FreeBSD Ports.

Expected behavior

My expectation is that it should yield to an exception as does MIT Kerberos:
https://github.com/krb5/krb5/blob/6d4eb6eb473c93f0db05409195448364382760a9/src/lib/gssapi/mechglue/g_acquire_cred.c#L73-L78

[frozencemetery]

In general I don't think we want to be in the business of input validation - we'd rather punt that to krb5. However, I think it makes sense because we perform translation and a typo here wouldn't be caught. Patch forthcoming.