From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 20 Jan 2012 19:18:50 GMT
Subject: Re: [Q] Determing if a commit is reachable from the HEAD ?
Message-ID: <7v7h0mfahx.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20120120180620.GA8504@codeaurora.org>

David Brown <davidb@codeaurora.org> writes:

> On Fri, Jan 20, 2012 at 03:13:58PM +0100, Andreas Schwab wrote:
>> Brian Foster <brian.foster@maxim-ic.com> writes:
>> 
>> >  In a script, how can I determine commit Y is reachable
>> >  from the current HEAD ?
>> 
>> test $(git merge-base HEAD Y) = $(git rev-parse Y)
>
> Almost.  It works as long as there is only one merge base.  You really
> need to check if $(git rev-parse Y) is one of $(git merge-base --all
> HEAD Y)

Can you give us an example of a topology to which "merge-base --all HEAD Y"
gives more than one output and Y is still reachable from HEAD?

It is my understanding that merge-base computation will give only Y and
nothing else when Y is reachable from HEAD. I also think this assumption
is used by some of the internal code in Git, and that is why I care.