The current ls-remote script as written does not actually work; I think it only returns branches in the remote that:

The actual extension is a 100-line Python script...
The real Pandora's box this opens...

The real Pandora's box this opens is that now that arc is exposed to whatever issues arise with the raw Python/Mercurial API, there's a much weaker argument against not writing more extensions.

I put the extension in phabricator/ earlier by accident, but that was just me goofing my local paths. The extension is purely client side: hg can do ls-remote just fine (it's just "do hg pull, but stop half way through"), there's just no UI command for it.

I am considering reviving the command server since I don't remember it having any critical failures, but offhand:

I'm going to try writing a Mercurial extension instead.

I believe this is roughly the form we end up with, which is completely ridiculous and extremely difficult to test or have any confidence in:

As a tool developer, this kind of thing is infuriating:

This is also made more complicated because Mercurial branches and bookmarks are all in a single shared namespace to some degree. I'm not sure what happens if you try to fetch a bookmark that conflicts with a local branch name (presumably: it renames it as "divergent"?) or try to fetch a branch which conflicts with a local bookmark name (presumably: fails in a way that's hard to detect?).

When the user specifies arc land --onto marker, I'd like to identify whether marker is a bookmark or branch.

See T13545, T13547, T13548, for the next round.

This approximately exists now via T11968.

I'm not 100% sure I got this, but all of these seem to work now:

After D12336:

This is somewhat implemented by D21315, which is headed to master soon. See also T13546 and T12876.

Just extracting a few blocks of old code for reference:

I may actually make the minimum version whichever version has this fix:

Today, arc land in Mercurial does not support the "merge" land strategy.