This will be merged, pending discussion on pros/cons of preprocessing vs. relaxing schema restriction.

A bit of background: the error here is is a result of the Relax-NG schema requiring a leaf node to have a 'properties' entry; that is considered a sanity check by the schema author, but emerges as an issue when we moved to a modular structure for the *.xml.i include files. Leaving that restriction as-is, the solution of merging leaf nodes in the lxml script should be straightforward (the general case more intricate), so I will look at amending the override-default script.

@artooro This sounds reasonable, and I don't imagine a problem, though I have yet to try it; if you would like to submit a pull request with fix, I will review.

A simple experiment using GraphQL to configure a DHCP server is here:
https://github.com/vyos/vyos-1x/compare/current...jestabro:graphql

Already backported: ff7b2b0e62510ef8de28c9c4bfa34badeabec775

@erkin One additional detail: there is an attempt at clean up on entering configuration mode (in UnionfsCstore::setupSession); nonetheless, there are clearly potential (and potentially serious) issues here. Cf. T3054 for the comment on setupSession; there has been no serious investigation of the issue yet.

Draft PRs:
https://github.com/vyos/vyos-utils/pull/1
https://github.com/vyos/vyos-1x/pull/869

Needs testing, but initial implementation here:

Opening a new task for the issue described in the last comment, above. The original problem re. error messages was resolved in vyos-utils rVYOSONEX68d1b283f766 .

There are two separate issues in this task: quoting of hw-id values (resolved in Sagitta/Equuleus); re-ordering of interfaces with respect to hw-id (Open).