Re: [PATCH 4/5] t6022, t6046: test expected behavior instead of testing a proxy for it

[Elijah Newren <newren@gmail.com>]

On Thu, Mar 12, 2020 at 9:48 AM Elijah Newren <newren@gmail.com> wrote:
>
> On Thu, Mar 12, 2020 at 6:20 AM SZEDER Gábor <szeder.dev@gmail.com> wrote:
> >
> > On Thu, Feb 27, 2020 at 12:14:23AM +0000, Elijah Newren via GitGitGadget wrote:
> > > From: Elijah Newren <newren@gmail.com>
> > >
> > > In t6022, we were testing for file being overwritten (or not) based on
> > > an output message instead of checking for the file being overwritten.
> > > Since we can check for the file being overwritten via mtime updates,
> > > check that instead.
> > >
> > > In t6046, we were largely checking for both the expected behavior and a
> > > proxy for it, which is unnecessary.  The calls to test-tool also were a
> > > bit cryptic.  Make them a little clearer.
> > >
> > > Signed-off-by: Elijah Newren <newren@gmail.com>
> > > ---
> > >  t/t6022-merge-rename.sh                | 15 ++++-
> > >  t/t6046-merge-skip-unneeded-updates.sh | 89 +++++++++++++++++---------
> > >  2 files changed, 70 insertions(+), 34 deletions(-)
> > >
> > > diff --git a/t/t6022-merge-rename.sh b/t/t6022-merge-rename.sh
> > > index 6f196aaf276..d97cf48495b 100755
> > > --- a/t/t6022-merge-rename.sh
> > > +++ b/t/t6022-merge-rename.sh
> > > @@ -242,12 +242,23 @@ test_expect_success 'merge of identical changes in a renamed file' '
> > >       rm -f A M N &&
> > >       git reset --hard &&
> > >       git checkout change+rename &&
> > > +
> > > +     test-tool chmtime =31337 B &&
> > > +     test-tool chmtime --get B >old-mtime &&
> >
> > Nit: I think it's possible to change the mtime and print it in a
> > single invocation with:
> >
> >   test-tool chmtime --get =1234 file
>
> Oh, cool.
>
> > >       GIT_MERGE_VERBOSITY=3 git merge change >out &&
> >
> > Nit: The output of 'git merge' is still redirected to a file, but ...
> >
> > > -     test_i18ngrep "^Skipped B" out &&
> >
> > ... the only command looking at the output is now removed.
>
> Indeed.
>
> > > +     test-tool chmtime --get B >new-mtime &&
> > > +     test_cmp old-mtime new-mtime &&
> > > +
> > >       git reset --hard HEAD^ &&
> > >       git checkout change &&
> > > +
> > > +     test-tool chmtime =-1 M &&
> > > +     test-tool chmtime --get M >old-mtime &&
> > >       GIT_MERGE_VERBOSITY=3 git merge change+rename >out &&
> > > -     test_i18ngrep ! "^Skipped B" out
> >
> > Likewise.
> >
> > > +     test-tool chmtime --get B >new-mtime &&
> > > +     test $(cat old-mtime) -lt $(cat new-mtime)
> >
> > I saw this test fail today in one of my custom CI builds:
> >
> >   +git checkout change
> >   Switched to branch 'change'
> >   +test-tool chmtime =-1 M
> >   +test-tool chmtime --get M
> >   +GIT_MERGE_VERBOSITY=3 git merge change+rename
> >   +test-tool chmtime --get B
> >   +cat old-mtime
> >   +cat new-mtime
> >   +test 1583967731 -lt 1583967731
> >   error: last command exited with $?=1
> >   not ok 12 - merge of identical changes in a renamed file
> >
> > The contents of 'out', i.e. the output of the 'git merge' command
> > before the failure is:
> >
> >   Auto-merging B
> >   Merge made by the 'recursive' strategy.
> >    A => B | 0
> >    1 file changed, 0 insertions(+), 0 deletions(-)
> >    rename A => B (100%)
> >
> > This is a rare failure, this is the first time I saw it, and to make
> > things worse, this one time it happened on big-endian and with all the
> > GIT_TEST_* knobs enabled.
> >
> >   https://travis-ci.org/github/szeder/git-cooking-topics-for-travis-ci/jobs/661294571#L4020
>
> This is very troubling.  The workflow is basically:
>   - Manually set the mtime to a file to something old (I happened to
> pick 1 second before now, but picking something from 1970 would have
> been fine too).
>   - Run a merge which is known to need to overwrite the file.  Your
> output ("Auto-merging B") suggests that we should have been in such a
> case.
>   - Verify that the file was actually updated as expected.  Since the
> OS is supposed to update the mtime when it writes the file, it should
> have set it to something recent, i.e. something *different* than what
> it had before.
>
> So, now I'm left wondering how the mtime possibly could have been not
> updated.  Maybe the file wasn't actually written?  (But if so, why
> didn't other people see the failure?  Or your stress runs not see it?)
>  Or maybe it was written but all file contents and metadata were
> delayed in writing to disk such that a subsequent command still sees
> the old file??  Or maybe it was written but the mtime update was
> delayed and the test was able to check it in that intermediate
> state???  Or perhaps the mtime check before the merge raced with the
> setting of the mtime backwards and got the mtime before it was
> rewound????
>
> I don't have a plausible scenario under which any of these should be
> possible; I'm at a loss.

Hmm.  Maybe leap seconds, or clock updates via ntp at an unfortunate
time?  Perhaps just setting the "old" time to something more than one
second in the past would avoid this?

Anyone have a clue if this is a useful guess or not?