Re: [netmod] Question on when-construction involving identities not detected as an error during module compilation

[Martin Bjorklund <[e-mail]>]

Hi,

"Bogaert, Bart (Nokia - BE/Antwerp)" <[e-mail]> wrote:
> Hi,
> 
> We have a question on YANG module compilation.  Assume the following
> model:
> 
> module test-feat-compile {
>   yang-version 1.1;
>   namespace "http://www.example.com/test-feat-compile";
>   prefix "tfc";
> 
>   identity failure-reason {
>     description
>       "The reason a failure occurred.";
>   }
> 
>   identity no-failure {
>     description
>       "No failure has occurred.";
>   }
> 
>   identity general-error {
>     base failure-reason;
>     description
>       "A general error occurred.";
>   }
> 
>   container failure {
>     description
>       "Objects associated with a failure.";
> 
>     leaf failure-reason {
>       type identityref {
>         base failure-reason;
>       }
>       description
>         "The reason the failure occurred.";
>     }
> 
>     leaf failure-string {
>       when "../failure-reason != 'no-failure'" {
>         description
>           "Only valid when there is a failure.";
>       }
>       type string;
>       description
>         "A text string indicating the reason for the failure when
>          either no defined reason exists or additional information
>          is available beyond the definition of the reason.";
>     }
>   }
> }
> 
> Looking at the when clause there is something obviously wrong (at
> least this is how I see it):
> 
>   *   The leaf 'failure-reason' is of type identityref with 'failure-reason'
>       as base
>   *   Identity 'no-failure' does not have 'failure-reason' as base (see it
>       as having been forgotten)
> Question is: shouldn't this be reported at compile time?  Pyang
> doesn't and the other compiler we use doesn't report this either.
> 
> Even when 'no-failure' is modified into some arbitrary string it is
> still accepted while this arbitrary string is not defined anywhere as
> identity.
> 
> And when it comes to the correct syntax, RFC 7950 states (section
> 9.1.3) that identities in must and when statements should be prefixed
> (so tfc:no-failure in this case).  Also this "violation" passes.
> 
> So: what can we expect to be checked at compile time in this case?
> Now you can only detect these anomalies when using a system supporting
> the modules and detect that things do not work as intended.  It would
> be better to have these erroneous constructions being detected at
> compile time.  If there is something in the RFC that allows the
> compiler to behave as it does now it would be good to know.

Strictly speaking, the "when" expressions are not erroneous; they are
proper XPath expressions that happen to always return "false".

A good compiler could detect this and produce a warning, but it would
be incorrect to flag this as an error.


/martin