So far it didn't have to handle the case where one hash is empty and the other
isn't, but in the next commit we need that, so let's handle that case correctly.
There's enough logic in the function now that it's worth covering it with tests.
The rebase.updateRefs feature of git is very useful to rebase a stack of
branches and keep everything nicely stacked; however, it is usually in the way
when you make a copy of a branch and want to rebase it "away" from the original
branch in some way or other. For example, the original branch might sit on main,
and you want to rebase the copy onto devel to see if things still compile there.
Or you want to do some heavy history rewriting experiments on the copy, but keep
the original branch in case the experiments fail. Or you want to split a branch
in two because it contains two unrelated sets of changes; so you make a copy,
and drop half of the commits from the copy, then check out the original branch
and drop the other half of the commits from it.
In all these cases, git's updateRefs feature insists on moving the original
branch along with the copy in the first rebase that you make on the copy. I
think this is a bug in git, it should create update-ref todos only for branches
that point into the middle of your branch (because only then do they form a
stack), not when they point at the head (because then it's a copy). I had a long
discussion about this on the git mailing list [1], but people either don't agree
or don't care enough.
So we fix this on our side: whenever we start a rebase for whatever reason, be
it interactive, non-interactive, or behind-the-scenes, we drop any update-ref
todos that are at the very top of the todo list, which fixes all the
above-mentioned scenarios nicely.
I will admit that there's one scenario where git's behavior is the desired one,
and the fix in this PR makes it worse: when you create a new branch off of an
existing one, with the intention of creating a stack of branches, but before you
make the first commit on the new branch you realize some problem with the first
branch (e.g. a commit that needs to be reworded or dropped). It this case you do
want both branches to be affected by the change. In my experience this scenario
is much rarer than the other ones that I described above, and it's also much
easier to recover from: just check out the other branch again and hard-reset it
to the rebased one.
[1]
https://public-inbox.org/git/354f9fed-567f-42c8-9da9-148a5e223022@haller-berlin.de/
Sometimes it takes a while to get PRs accepted upstream, and this blocks our
progress. Since I'm pretty much the only one making changes there anyway, it
makes sense to point to my fork directly.
It is a bad idea to read a git-rebase-todo file, remove some update-ref todos,
and write it back out behind git's back. This will cause git to actually remove
the branches referenced by those update-ref todos when the rebase is continued.
The reason is that git remembers the refs affected by update-ref todos at the
beginning of the rebase, and remembers information about them in the file
.git/rebase-merge/update-refs. Then, whenever the user performs a "git rebase
--edit-todo" command, it updates that file based on whether update-ref todos
were added or removed by that edit. If we rewrite the git-rebase-todo file
behind git's back, this updating doesn't happen.
Fix this by not updating the git-rebase-todo file directly in this case, but
performing a "git rebase --edit-todo" command where we set ourselves as the
editor and change the file in there. This makes git update the bookkeeping
information properly.
Ideally we would use this method for all cases where we change the
git-rebase-todo file (e.g. moving todos up/down, or changing the type of a
todo); this would be cleaner because we wouldn't mess with git's private
implementation details. I tried this, but unfortunately it isn't fast enough.
Right now, moving a todo up or down takes between 1 and 2ms on my machine;
changing it to do a "git rebase --edit-todo" slows it down to over 100ms, which
is unacceptable.
This fixes two problems with the "amend commit with staged changes" command:
1. Amending to a fixup commit didn't work (this would create a commmit with the
title "fixup! fixup! original title" and keep that at the top of the branch)
2. Unrelated fixup commits would be squashed too.
The added integration test verifies that both of these problems are fixed.
Instead of passing a bunch of different options in
PrepareInteractiveRebaseCommandOpts, where it was unclear how they interact if
several are set, have only a single field "instruction" which can be set to one
of various different instructions.
The functionality of replacing the entire todo file with our own is no longer
available; it is only possible to prepend todos to the existing file.
Also, instead of using different env vars for the various rebase operations that
we want to tell the daemon to do, use a single one that contains a json-encoded
struct with all available instructions. This makes the protocol much clearer,
and makes it easier to extend in the future.
