Re: BUG: commit-reach.c:66: bad generation skip

[Derrick Stolee <stolee@gmail.com>]

On 2/7/2021 5:31 PM, Filippo Valsorda wrote:
> 
> On Sun, Feb 7, 2021 at 11:28 PM Filippo Valsorda <valsorda@google.com> wrote:

>> [System Info]
>> git version:
>> git version 2.30.0

_This_ is interesting. I haven't heard of this problem happening
in a released version of Git.

I'm CC'ing Jonathan Nieder who recently saw this happening, but that
was on a newer version than 2.30.0 with a topic that is not part
of 2.30.0. But maybe the version shipped internally is versioned
without extra information on top of the latest tag? (I see your
@google.com email, which makes me think you have an internal version.)

>> What happened instead? (Actual behavior)
>>
>> BUG: commit-reach.c:66: bad generation skip     ad18 >        7 at
>> be28e5abc5ddca0d6b2d8c91b7bb9c05717154e7

This was recently reported, but it was thought to be due to an
in-process topic branch. If you recently had that version installed,
then your data would have needed a rewrite anyway.

This hints towards more problems with generation numbers across
split commit-graph layers. (The child has generation number 7, so
it is likely part of a thin layer.)

This might be sufficient evidence to convert this BUG() to a
warning with something like:

  warning: The commit-graph file has incorrect reachability index
  warning: information. Results may be incorrect until commit-graph
  warning: is rewritten with 'git commit-graph write'.

This could also be a die() instead of warning(), as that command
itself might return incorrect results.

> I was able to workaround this bug with a "git gc" invocation.

Yes, this will delete (and possibly replace) your commit-graph file
where the bad data lies.

To better understand your situation, could you please run the
following commands?

	git config --get-regexp graph
	git config features.experimental

I'm specifically looking for values of fetch.writeCommitGraph and
gc.writeCommitGraph.

Thanks,
-Stolee