Re: [PATCH] bisect: error out when given any good rev that is not an ancestor of the bad rev

Junio C Hamano [off-list ref] writes:

Christian Couder [off-list ref] writes:

quoted

Before this patch "git bisect" doesn't really work when it is given
some good revs that are siblings of the bad rev.

For example if there is the following history:

A-B-C-D
   \E-F

and we launch "git bisect start D F" then only C and D will be
considered as possible first bad commit. This is wrong because A, B and
E may be bad too if the bug exists everywhere except in F that fixes it.

Please don't.

bisect is about finding a single regression by partitioning the graph into
older good section and newer bad section with a *single* "first bad
commit".

Your "this could also be possible" scenario is already outside the
realm.  You are assuming A, B and F is good, and D is bad.  But if E is
bad, then that breakage cannot possibly affect the transition between B
and D from good to bad (E cannot break D), so C must *also* be bad.

... which means you are dealing with *two* breakages.  That's outside what
bisect deals with.

And this does not need to have forked development.  If the graph were like
this:

  A-B-C-D-E-F

and if F is bad and B is good, with your logic, after checking that D is
already bad, we cannot discount E --- after somehow fixing D, we _might_
also be introducing another breakage with E.  You cannot even check for
that anyway, but the logic is the same.