Matthieu Moy <Matthieu.Moy@xxxxxxxxxxxxxxx> writes:
>> But what if bad-A and bad-B have more than one merge bases? We
>> won't know which side the badness came from.
>>
>> o---o---o---bad-A
>> / \ /
>> -----Good---o---o---o /
>> \ / \
>> o---o---o---bad-B
>>
>> Being able to bisect the region of DAG bound by "^Good bad-A bad-B"
>> may have value in such a case. I dunno.
>
> I could help finding several guilty commits, but anyway you can't
> guarantee you'll find them all as soon as you use a binary search: if
> the history looks like
>
> --- Good --- Bad --- Good --- Good --- Bad --- Good --- Bad
>
> then without examining all commits, you can't tell how many good->bad
> switches occured.
>
> But keeping several bad commits wouldn't help keeping the set of
> potentially guilty commits small: bad commits appear on the positive
> side in "^Good bad-A bad-B", so having more bad commits mean having a
> larger DAG to explore (which is a bit counter-intuitive: without
> thinking about it I'd have said "more info => less commits to explore").
>
> So, if finding all guilty commits is not possible, I'm not sure how
> valuable it is to try to find several of them.
The criss-cross merge example, is not trying to find multiple
sources of badness. It still assumes [*1*] that there is only one
event that introduced the badness observed at bad-A and bad-B, both
of which inherited the badness from the same such event. Unlike a
case with a single/unique merge-base, we cannot say "we can start
from the merge-base, as their common badness must be coming from the
same place". The badness may exist in the first 'o' on the same
line as bad-A in the above picture, which is an ancestor of one
merge-base on that line and does not break the other merge base on
the same line as bad-B, for example.
> OTOH, keeping several good commits is needed to find a commit for which
> all parents are good and the commit is bad.
Yes, that is correct.
[Footnote]
*1* The assumption is what makes "bisect" workable. If the
assumption does not hold, then "bisect" would not give a useful
answer "where did I screw up?". It gives a fairly useless "I
found one bad commit whose parent is good---there is no
guarantee if that has anything to do with the badness you are
seeing at the tip".