Operating System: All
Platform: All

RFC7950:

9.9.3. The "require-instance" Statement
The "require-instance" statement, which is a substatement to the
"type" statement, MAY be present if the type is "instance-identifier"
or "leafref". It takes as an argument the string "true" or "false".
If this statement is not present, it defaults to "true".
If "require-instance" is "true", it means that the instance being
referred to MUST exist for the data to be valid. This constraint is
enforced according to the rules in Section 8 of RFC7950.
If "require-instance" is "false", it means that the instance being
referred to MAY exist in valid data.

API change - https://git.opendaylight.org/gerrit/#/c/47983/

Implementation - https://git.opendaylight.org/gerrit/#/c/49697/

when mounting XR 6.2.1 in Boron the openconfig-bgp model is mounted correctly.

when mounting XR 6.2.1 in Carbon this fails.

It turns out to be because the model has a leafref with a require-instance under it. This is ambiguous in RFC6020 (YANG 1.0) in that it's not mentioned in the text but is permitted in the ABNF, but is unambiguous in RFC7950 (present in both text and ABNF). The fix was as simple as side-loading a modified model with yang-version set to "1.1".

but WHY OH WHY did you change the code in Carbon to explicitly prevent this from working unless yang-version is set to 1.1? It breaks things that previously worked and it goes against Postel's law.