From mboxrd@z Thu Jan  1 00:00:00 1970
From: Junio C Hamano <gitster@pobox.com>
Subject: Re: Reachability lists in git
Date: Tue, 18 Nov 2014 13:22:26 -0800
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	<Pine.LNX.4.44L0.1411181610070.2918-100000@iolanthe.rowland.org>
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Cc: Jonathan Nieder <jrnieder@gmail.com>, <git@vger.kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
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In-Reply-To: <Pine.LNX.4.44L0.1411181610070.2918-100000@iolanthe.rowland.org>
	(Alan Stern's message of "Tue, 18 Nov 2014 16:16:57 -0500 (EST)")
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Alan Stern <stern@rowland.harvard.edu> writes:

>> > 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).
> ...
> Thus, if B introduced a bug, that bug would not be present in Y.  But Y 
> might be better for testing than X, because Y might fix some other 
> problems that are present in X.

The problem with that line of reasoning is that in real life there
will be unbound number of Y's that forked from a point before
somebody wrote B.  Which one among these Y's would you pick and why?

If Y has fixed another problem that is present in X and make it
easier to test, Z, a direct descendant of Y (i.e. Z^1 = Y), may have
fixed yet another problem that is unrelated to the problem B
introduced and it may make the result even easier to test.  Where do
you stop?

Still confused...