Re: [edk2-devel] [PATCH] OvmfPkg/Bhyve: clean up TPM_ENABLE remnants

["Laszlo Ersek" <lersek@redhat.com>]

On 06/23/21 20:44, Kinney, Michael D wrote:
>
> Hi Laszlo,
>
> Thank you for the test case.
>
> I created 2 PRs against edk2-codereview using your patches.
> I made minor update to commit messages to pass patch check.
>
>     https://github.com/tianocore/edk2-codereview/pull/18
>     https://github.com/tianocore/edk2-codereview/pull/19
>
> Found another issue with PatchCheck for the Mergify merge commit and
> fixed that.
>
> Mergify did process #18 and merged it in after passing all CI. Mergify
> rebased #19 successfully and merged it after passing all CI. I do not
> think this was your expected result.

Indeed, my "desired" result at least would have been for mergify to
reject the rebase.

> I looked more closely at the patches you provided.  They were not
> overlapping in the lines of Readme.rst.  This is why no merge conflict
> was detected.

More precisely, a contextual conflict *was* determined between the
patches, but that conflict was auto-resolved.

This is risky when done in an automated fashion. It is an extremely
convenient feature of git, when used interactively; that is, when the
auto-merge (automatic conflict resolution) is semantically verified by a
human. Git takes away the chore of conflict resolution, presents a
"likely good" end result, and a human only needs to *look* at the end
result, not *implement* it.

But that "human look" is exactly what's missing from mergify.

Basically what I'd like for mergify is to turn off automatic conflict
resolution.

More or less, speaking in terms of the stand-alone "patch" utility
<https://man7.org/linux/man-pages/man1/patch.1.html>, my preference is
to set the "fuzz factor" to zero.


One way a human reviews such context differences is with git-range-diff.
Continuing my previous example commands:

$ git range-diff --color master..b2 b1..b2-rebase

1:  02dc81e58bd6 ! 1:  2cf39d4b1790 world
    @@ -6,8 +6,8 @@
     --- a/ReadMe.rst
     +++ b/ReadMe.rst
     @@
    -
      A modern, feature-rich, cross-platform firmware development
    +   HELLO
      environment for the UEFI and PI specifications from www.uefi.org.
     +  WORLD

This output shows that the "world" addition is the same (it is identical
between pre-rebase and post-rebase in the commit), but the context has
changed. During the rebase, the leading empty line of the context
disappeared, and a HELLO line in the middle of the leading context
appeared.

This result may or may not be semantically correct; it needs a human
decision. What if the original purpose of the "world" patch author was
to say WORLD but only without HELLO? When they looked at the code, there
was no HELLO yet.

git-range-diff is very powerful, but reading its output takes some
getting used to. (Colorization with the "--color" option is basically
required for understanding; I can't reproduce it in this email, alas.)

I don't want to obsess about this forever, I just want us all to be
aware that this risk exists.

Thanks,
Laszlo

>
> I then created 2 new PRs that added text to the same line # in Readme.rst.
>
>     https://github.com/tianocore/edk2-codereview/pull/21
>     https://github.com/tianocore/edk2-codereview/pull/22
>
> PR #21 passed all CI tests and was merged.  Mergify then attempted to
> rebase #22 and got a merge conflict and is still in the open state waiting
> for the developer to manually handle the merge conflict.

This case is not worrisome; when there is a clear conflict that cannot be auto-resolved, I'm not concerned.

My concern is the sneaky contextual conflict that *appears* auto-resolvable, but is semantically broken. Those things exist.

Thanks
Laszlo