Make error messages more useful: add a URI

[Nico Williams]

On Fri, Oct 03, 2014 at 12:07:34PM -0400, Benjamin Kaduk wrote:
> On Thu, 2 Oct 2014, Nico Williams wrote:
> > [libdefaults]
> >     err_fmt = %E. Please see https://whatever.example/%N
> > 
> > [...]
> 
> I'm slightly concerned that supporting more %foo tokens for expansion
> would lead to an API explosion at call sites that set the error message.
> Adding N new optional positional parameters to krb5_set_error_message to
> hold all the possible values seems bad.  Adding a single struct parameter
> to hold them all might be bad.  A single parameter to the head of a linked
> list of containers holding type-tagged data is probably too complex;
> adding separate routines to add a single expandable value to the context
> might make call sites too noisy, depending on how common it is to set
> extra attributes.
> 
> Did you have anything in particular in mind?

I hadn't thought about it much, but here's the list of tokens I
envision:

 %E -> normal error message
 %N -> error code (numeric)
 %S -> error code (symbolic; string)
 %A -> additional context information (string)

The last item would require changes at various krb5_set_error_message()
call sites, and might not materialize.  Things I'd expect to be included
in it, if it happens at all, might be: relevant environment variables
and values, and the ccache name (if defaulted).

Nico
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