Question about using augment-structure to augment the root node of a Yang data structure?

[phulien]

Hi,

I have a question regarding YANG data structure extensions (RFC 8791) and the use of augment-structure. Specifically, I am inquiring whether it is possible to augment to the root of a YANG data structure. For instance, with the provided YANG model, yanglint successfully validates the model if the last sx:augment-structure /foo:mystruct statement is commented out. However, if that statement is included, yanglint reports an error:

• Yang model

module foo {
  namespace "foo";
  prefix foo;

  import ietf-yang-structure-ext { prefix sx; }

  sx:structure mystruct {
    container mycontainer {
      leaf leaf1 {
        type string;
      }
    }
  }

  sx:augment-structure /foo:mystruct/foo:mycontainer {
    leaf leaf2 {
      type string;
    }
  }

  // Augment to the root of the structure, so `leaf3` will be a sibling of `mycontainer`.
  sx:augment-structure /foo:mystruct {
    leaf leaf3 {
      type string;
    }
  }
}  

• Yanglint error

libyang err : Augment extension target node "/foo:mystruct" from module "foo" was not found. (/foo:{extension='sx:augment-structure'}/{augment='/foo:mystruct'})  

Moreover, if we cannot use augment-structure to add augments to the root of a structure, could you please suggest how to make the following leafref path valid in the Yang model below?

• a.yang

module a {
  namespace "urn:a";
  prefix a;

  import b {
    prefix b;
  }
  import ietf-yang-structure-ext {
    prefix sx;
  }

  grouping leafrefs {
    leaf leaf1 {
      type leafref {
        path "/a:top/a:leaf2";
      }
    }
  }

  grouping leaves {
    leaf leaf2 {
      type string;
    }
  }

  container top {
    config false;
    uses leaves;
  }


  // Augments 'a:leaf1' into '/b:mystruct/b:mycontainer'. This augmented
  // 'a:leaf1' will be a sibling of 'b:myleaf'.
  //
  // XPath within a structure can only reference nodes inside the same
  // structure. Therefore, '/a:top/a:leaf2' must also be augmented into
  // 'b:mystruct' to satisfy the leafref path.
  sx:augment-structure "/b:mystruct/b:mycontainer" {
    uses leafrefs;
  }

  // This 'augment-structure' attempts to satisfy the leafref path by augmenting
  // '/a:top/a:leaf2' into the root of '/b:mystruct'. However, yanglint reports
  // an error because we are augmenting to the root of a structure.
  sx:augment-structure "/b:mystruct" {
    container top {
      uses leaves;
    }
  }

  // This 'augment-structure' is valid in yanglint but does not satisfy the
  // leafref path because it augments '/b:top/a:leaf2', not '/a:top/a:leaf2'.
  sx:augment-structure "/b:mystruct/b:top" {
    uses leaves;
  }
}

• b.yang

module b {
  namespace "urn:b";
  prefix b;

  import ietf-yang-structure-ext {
    prefix sx;
  }

  sx:structure mystruct {
    container mycontainer {
      leaf myleaf {
        type string;
      }
    }
    container top { }
  }
}

Yanglint reports the following error for the second augment-structure statement:

libyang err : Augment extension target node "/b:mystruct" from module "a" was not found. (/a:{extension='sx:augment-structure'}/{augment='/b:mystruct'})

And the following error for the third augment-structure statement:

libyang err : Not found node "top" in path. (/b:mycontainer/a:leaf1)

[michalvasko]

I am not able to tell you whether this is legal. The YANG module describes the augment-structure path as:

The mandatory 'path' parameter value identifies the YANG
conceptual data node that is being augmented and is
represented as an absolute-schema-nodeid string, where the
first node in the absolute-schema-nodeid string identifies the
YANG data structure to augment, and the rest of the nodes in
the string identifies the node within the YANG structure to
augment.

"identifies the YANG conceptual data node" would mean that the augment target must be a data node (similarly to a standard augment) but I am not entirely sure. But that is the issue that augments are expected only on nodes, which is why it is not applied. Supporting this should not be too much trouble but like I said, I am not sure it should be supported. If you think it should, ask on the netmod mailing list for clarification.

As for your leafref problem, I do not think you can solve it if such an augment-structure is really not supported.

[phulien]

Thank you, @michalvasko. I also sent the same question to netmod. On the other hand, we found that this use of augment-structure is claimed to be supported in the following links:

• https://docs.yumaworks.com/en/latest/yangauto/define-abstract-yang.html
• https://dr2lopez.github.io/yang-provenance/draft-lopez-opsawg-yang-provenance.html

I will update when I get an answer from netmod.

[michalvasko]

Okay, if other projects support it I suppose we can, too. Should be working in the latest devel. But I am still curious about what netmod will say.

[phulien]

I got confirmation from Andy Bierman from netmod that augmenting to the root of a YANG data structure should be supported.
See: https://mailarchive.ietf.org/arch/msg/netmod/DD0ZbjsuPlQEH5E6I29zApUNRus/

[michalvasko]

Yes, but you have conveniently ignored the text I quoted so I will reference it in the mailing list. Like I said, I have no strong opinion either way but this is not how you ask for a proper clarification.

[phulien]

Oh, sorry. I sent the question here and to netmod at the same time, but there was a problem with the email I sent to netmod, it was not delivered. I contacted the netmod mailing list to resolve the issue, which took several days, and they asked me to resend the original email. I did not intend to ignore the text you quoted. Sorry if this caused any misunderstanding. Please feel free to continue the mailing list discussion if you want to clarify it further.

[michalvasko]

Okay, now can this issue be considered solved.