From: Alan Stern <stern@rowland.harvard.edu>
Date: Tue, 18 Nov 2014 20:45:43 GMT
Subject: Re: Reachability lists in git
Message-ID: <Pine.LNX.4.44L0.1411181541590.2918-100000@iolanthe.rowland.org>
In-Reply-To: <20141118203204.GM6527@google.com>

On Tue, 18 Nov 2014, Jonathan Nieder wrote:

> Alan Stern wrote:
> 
> > Tracking down regressions.  Bisection isn't perfect.  Suppose a
> > bisection run ends up saying that B is the first bad commit.  It's easy
> > enough to build B and test it, to verify that it really is bad.
> >
> > But to be sure that B introduced the fault, it would help to find the
> > latest commit that doesn't include B's changes -- that is, the latest
> > commit that B isn't reachable from (or the maximal elements in the set
> > of all such commits).
> 
> Isn't that B^ (or B^ and B^2, if B is a merge)?

No.  Here's a simple example:

            Y
           /
          /
         X--B

In this diagram, X = B^.  But B isn't reachable from either X or Y, 
whereas it is reachable from one of X's children (namely Y).  Therefore 
Y is the unique maximal commit which B is not reachable from.

Alan Stern