Re: [Summit topic] Server-side merge/rebase: needs and wants?

[Taylor Blau <me@ttaylorr.com>]

I was discussing this with Elijah today in IRC. I thought that I sent
the following message to the list, but somehow dropped it from the CC
list, and only sent it to Elijah and Johannes.

Here it is in its entirety, this time copying the list.

n Thu, Oct 21, 2021 at 01:56:06PM +0200, Johannes Schindelin wrote:
>  5.  The challenge is not necessarily the technical challenges, but the UX for
>      server tools that live “above” the git executable.
>
>      1. What kind of output is needed? Machine-readable error messages?
>
>      2. What Git objects must be created: a tree? A commit?
>
>      3. How to handle, report, and store conflicts? Index is not typically
>         available on the server.

I looked a little bit more into what GitHub would need in order to make
the switch. For background, we currently perform merges and rebases
using libgit2 as the backend, for the obvious reason which is that in a
pre-ORT world we could not write an intermediate result without having
an index around.

(As a fun aside, we used to expand our bare copy of a repository into a
temporary working directory, perform the merge there, and then delete
the directory. We definitely don't do that anymore ;)).

It looks like our current libgit2 usage more-or-less returns an
(object_id, list<file>) tuple, where:

  - a non-NULL object_id is the result of a successful (i.e.,
    conflict-free) merge; specifically the oid of the resulting root
    tree

  - a NULL object_id and a non-empty list of files indicates that the
    merge could not be completed without manual conflict resolution, and
    the list of files indicates where the conflicts were

When we try to process a conflicted merge, we display the list of files
where conflicts were present in the web UI. We do have a UI to resolve
conflicts, but we populate the contents of that UI by telling libgit2 to
perform the same merge on *just that file*, and writing out the file
with its conflict markers as the result (and sending that result out to
a web editor).

So I think an ORT-powered server-side merge would have to be able to:

  - write out the contents of a merge (with a tree, not a commit), and
    indicate whether or not that merge was successful with an exit code

  - write out the list of files that had conflicts upon failure

Given my limited knowledge of the ORT implementation, it seems like
writing out the conflicts themselves would be pretty easy. But GitHub
probably wouldn't use it, or at least not immediately, since we rely
heavily on being able to recreate the conflicts file-by-file as they are
needed.

Anyway, I happened to be looking into all of this during the summit, but
never wrote any of it down. So I figured that this might be helpful in
case folks are interested in pursuing this further. If so, let me know
if there are any other questions about what GitHub might want on the
backend, and I'll try to answer as best I can.

Thanks,
Taylor