add API endpoint for BGP announce set modification

[rcgoodfellow]

Fixes #6022

[rcgoodfellow]

Basic testing performed in a4x2 and things work as expected.

[david-crespo]

What's the difference between create and update if they take the same body?

[rcgoodfellow]

What's the difference between create and update if they take the same body?

• If the announce set does not exist, nothing.
• If the announce set exists, but is empty, nothing.
• If the announce set exists, and the update is purely additive, nothing.
• If the announce set exists, and the update is not purely additive, then create will not remove existing entries, but update will.

More concisely, the update is a complete specification, and the announce set will match the request payload exactly on successful return, whereas a create can only add entries. I think for create, we may want to consider returning an error code if the announce set already exists.

[david-crespo]

Ok. I think that should be added as a description in the doc comment on each endpoint. Something like "Replaces existing announce set with the one specified." and "Adds new entries but does not modify existing ones." That mostly solves the issue in the following musings, so take them however you want.

The names update and create are a little counterintuitive to me for these operations — update less so than create. Update here works similarly to updating a project or whatever: you expect the state after the request to match what you passed in. "Create" is worse because when I see "create set" I expect it to create a set, not union the existing set with what I pass in.

Erroring if the set already exists would fix that, but that also means create becomes a special case of update for when there is no existing set, which makes me wonder whether we really need both. It seems like we could do ok with just update. That's the model we follow for firewall rules, though I admit it's kind of a pain and requires shenanigans in the client to simulate adding one rule at a time.

[rcgoodfellow]

Something like "Replaces existing announce set with the one specified."

Done

It seems like we could do ok with just update

So say we all.