Re: [PATCH 1/5] rebase -r: demonstrate bug with conflicting merges

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Tue, 13 Nov 2018, Junio C Hamano wrote:

> "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
> 
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > When calling `merge` on a branch that has already been merged, that
> > `merge` is skipped quietly, but currently a MERGE_HEAD file is being
> > left behind and will then be grabbed by the next `pick` (that did
> > not want to create a *merge* commit).
> >
> > Demonstrate this.
> >
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >  t/t3430-rebase-merges.sh | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> 
> For a trivially small change/fix like this, it is OK and even
> preferrable to make 1+2 a single step, as applying t/ part only to
> try to see the breakage (or "am"ing everything and then "diff |
> apply -R" the part outside t/ for the same purpose) is easy enough.

I disagree. It helps both development and porting to different branches to
be able to cherry-pick the regression test individually. Please do not ask
me to violate this hard-learned principle.

> Because the patch 2 with your method ends up showing only the test
> set-up part in the context by changing _failure to _success, without
> showing what end-user visible breakage the step fixed, which usually
> comes near the end of the added test piece.  A single patch that
> gives tests that ought to succeed would not force the readers to
> switch between patches 1 and 2 while reading the fix.

That is why I put in a verbose commit message, so that you do not have to
guess. And even the test title talks about this.

Seriously, I am very much opposed to changing the patches in the direction
you suggested. In my mind, they would make the story substantially worse.

Thank you for your review,
Dscho

> 
> Of course, the above would not apply for a more involved case where
> the actual fix to the code needs to span multiple patches.
> 
> Thanks.
> 
> > diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
> > index aa7bfc88ec..1f08a33687 100755
> > --- a/t/t3430-rebase-merges.sh
> > +++ b/t/t3430-rebase-merges.sh
> > @@ -396,4 +396,20 @@ test_expect_success 'with --autosquash and --exec' '
> >  	grep "G: +G" actual
> >  '
> >  
> > +test_expect_failure '--continue after resolving conflicts after a merge' '
> > +	git checkout -b already-has-g E &&
> > +	git cherry-pick E..G &&
> > +	test_commit H2 &&
> > +
> > +	git checkout -b conflicts-in-merge H &&
> > +	test_commit H2 H2.t conflicts H2-conflict &&
> > +	test_must_fail git rebase -r already-has-g &&
> > +	grep conflicts H2.t &&
> 
> Is this making sure that the above test_must_fail succeeded because
> of a conflict and not due to any other failure?  I would have used
> "ls-files -u H2.t" to see if the index is unmerged, which probably
> is a more direct way to test what this is trying to test, but if we
> are in the conflicted state, the one side of << == >> has this
> string (the other has "H2" in it, presumably?), so in practice this
> should be good enough.
> 
> > +	echo resolved >H2.t &&
> > +	git add -u &&
> 
> and we resolve to continue.
> 
> > +	git rebase --continue &&
> > +	test_must_fail git rev-parse --verify HEAD^2 &&
> 
> Even if we made an octopus by mistake, the above will catch it,
> which is good.
> 
> > +	test_path_is_missing .git/MERGE_HEAD
> > +'
> > +
> >  test_done
> 
> And from the proposed log message, I am reading that the last two
> things (i.e. resulting tip is a child with a single parent and there
> is no leftover MERGE_HEAD file) fail without the fix.  
> 
> This is enough material to convince me or anybody that the bug is
> worth fixing.  Thanks for being careful noticing a glitch during
> your real (and otherwise unrelated to the bug) work and following
> through.
>