Segfault when rebasing with --autosquash

[Paul Ganssle <paul@ganssle.io>]

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Hi all,

I have recently been using the git absorb
<https://github.com/tummychow/git-absorb> tool to automatically split up
various minor adjustments to my branches into the right fixup commits,
which I then autosquash at the very end. This has worked very well for
the most part, but twice now it has managed to get into a state where
`git rebase -i --autosquash` leads to a segfault in git. This may also
be a bug in `git absorb`, but I think the segfault must at least be a
bug in git.

Unfortunately, I have been unable to /deliberately/ create a repository
that exhibits this behavior using `git absorb`, but last time it
happened I created a fork of my repo and trimmed out as many commits as
I could while still exhibiting the behavior, you can find it here, along
with instructions on how to trigger the bug:
https://github.com/pganssle-bug-mwes/git_autosquash_bug_mwe

I'll also note that I have confirmed that a fresh clone, git gc and git
prune do not help here. It's easy enough to work around — just do an
interactive rebase without --autosquash and manually squash in any
commits that say `fixup <commit_id>` instead of `fixup <commit_message>`
and you will get back into a state where --autosquash works.

Please do let me know if anyone comes up with a better diagnosis for
this (there may be an associated bug in git absorb, and knowing more
about this bug might elucidate where that could come in from git absorb).

Thanks!
Paul



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