Re: Reachability lists in git

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 18, 2014, 21:22 UTC
Message-ID: <xmqq1tp0uru5.fsf@gitster.dls.corp.google.com>
In-Reply-To: <Pine.LNX.4.44L0.1411181610070.2918-100000@iolanthe.rowland.org>

Alan Stern <stern@rowland.harvard.edu> writes:

Show 13 quoted lines

>> > 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...