Bug: `git commit --fixup` with duplicate commit messages

Bug: `git commit --fixup` with duplicate commit messages

[Oliver Joseph Ash]

Hi,

I believe I have found a bug in `git commit --fixup`.

Steps to reproduce:
1. Create a git history with two commits (A and B) with the same
commit message (e.g. foo)
2. Create a new commit using `git commit --fixup {SHA}`, referring to
the last commit SHA
3. Run an interactive rebase

Expected: the fixup commit should appear below the last commit in the todo list.

Actual: the fixup commit appears below the first commit in the todo list.

It seems that the fixup commit is moved below the most recent commit
with a matching commit message, however in this case there are
duplicate commit messages. I would expect the fixup commit to be moved
below the commit SHA I specified when I ran `git commit --fixup`.

Thanks,
Olly

Re: Bug: `git commit --fixup` with duplicate commit messages

[Johannes Schindelin]

Hi Oliver,

On Fri, 14 Dec 2018, Oliver Joseph Ash wrote:

> I believe I have found a bug in `git commit --fixup`.
> 
> Steps to reproduce:
> 1. Create a git history with two commits (A and B) with the same
> commit message (e.g. foo)
> 2. Create a new commit using `git commit --fixup {SHA}`, referring to
> the last commit SHA
> 3. Run an interactive rebase
> 
> Expected: the fixup commit should appear below the last commit in the todo list.
> 
> Actual: the fixup commit appears below the first commit in the todo list.
> 
> It seems that the fixup commit is moved below the most recent commit
> with a matching commit message, however in this case there are
> duplicate commit messages. I would expect the fixup commit to be moved
> below the commit SHA I specified when I ran `git commit --fixup`.

Yes, this is a problem of the design of this feature. It was done
partially, I believe, to survive multiple rebases *without* --autosquash
before a final rebase *with* --autosquash.

The behavior really is by design.

Ciao,
Johannes

Re: Bug: `git commit --fixup` with duplicate commit messages

[Rafael Ascensão]

On Fri, Dec 14, 2018 at 12:30:28PM +0000, Oliver Joseph Ash wrote:
> I believe I have found a bug in `git commit --fixup`.

That's not a bug, it's actually the documented behaviour of rebase
--autosquash.

As you figured out, the squash/fixup is based on whether the message
has the squash!/fixup! prefix and the subject matches. But it also
allows specifying hashes.

So for fixups, you can be explicit and use: git commit -m 'fixup! SHA'.

Similarly for squashes. (But a little less friendly as you'll need to
deal with passing an argument to -m that contains newlines).

But adding 'squash! SHA' when the editor opens should also work.

I believe the main reason this works based on subject message matching
is to be more friendly with scenarios where you're sharing series of
commits that include fixups and squashes. (Either via format patch, or
by rebasing the series on a newer base without --autosquash).

On such cases the commit you're trying to fixup will have a different
OID, so any hardcoded OID will be useless while commit message matching
works most of the time.

One potential improvement to this is to teach --fixup and --squash to
also add trailers like: "{fixup,squash}: SHA" or "target: SHA" and teach
--autosquash to respect it when possible.

Thoughts?

Re: Bug: `git commit --fixup` with duplicate commit messages

[John Passaro]

On Fri, 14 Dec 2018, Oliver Joseph Ash wrote:

> I believe I have found a bug in `git commit --fixup`.
>
> Steps to reproduce:
> 1. Create a git history with two commits (A and B) with the same
> commit message (e.g. foo)

> 2. Create a new commit using `git commit --fixup {SHA}`, referring to
> the last commit SHA
> 3. Run an interactive rebase
>
> Expected: the fixup commit should appear below the last commit in the todo list.

Perhaps another useful behavior, when the ambiguity exists (i.e. absent
the trailer Rafael suggested), would be to keep the existing behavior
but notify the user, and point to a new section in the docs on the
subject:


pick aaaa foo
fixup dddd fixup! foo
pick bbbb foo
# ambiguous autosquash: dddd may have targeted bbbb instead of aaaa.
pick cccc unrelated

# Rebase 0000..dddd onto 0000 (4 commands)
# This rebase includes one or more ambiguous autosquashes.
# See git help rebase for more information (under AMBIGUOUS AUTOSQUASH)
#
# Commands:
# (etc)

On Fri, Dec 14, 2018 at 9:00 AM Rafael Ascensão <rafa.almas@gmail.com> wrote:
>
> On Fri, Dec 14, 2018 at 12:30:28PM +0000, Oliver Joseph Ash wrote:
> > I believe I have found a bug in `git commit --fixup`.
>
> That's not a bug, it's actually the documented behaviour of rebase
> --autosquash.
>
> As you figured out, the squash/fixup is based on whether the message
> has the squash!/fixup! prefix and the subject matches. But it also
> allows specifying hashes.
>
> So for fixups, you can be explicit and use: git commit -m 'fixup! SHA'.
>
> Similarly for squashes. (But a little less friendly as you'll need to
> deal with passing an argument to -m that contains newlines).
>
> But adding 'squash! SHA' when the editor opens should also work.
>
> I believe the main reason this works based on subject message matching
> is to be more friendly with scenarios where you're sharing series of
> commits that include fixups and squashes. (Either via format patch, or
> by rebasing the series on a newer base without --autosquash).
>
> On such cases the commit you're trying to fixup will have a different
> OID, so any hardcoded OID will be useless while commit message matching
> works most of the time.
>
> One potential improvement to this is to teach --fixup and --squash to
> also add trailers like: "{fixup,squash}: SHA" or "target: SHA" and teach
> --autosquash to respect it when possible.
>
> Thoughts?

I like very much the proposal to use trailers to indicate the intended target.
"target: SHA" would probably be better. It could be used for a first pass and
if no match is found (in case of rebase or patch-by-mail) or the trailer is not
present, fall back to the default approach of matching by subject line and
noting other potential matches as described above).