Re: [Q] Determing if a commit is reachable from the HEAD ?

From: David Brown <davidb@codeaurora.org>
Date: Jan 20, 2012, 23:33 UTC
Message-ID: <20120120233335.GA20302@codeaurora.org>
In-Reply-To: <7v7h0mfahx.fsf@alter.siamese.dyndns.org>

On Fri, Jan 20, 2012 at 11:18:50AM -0800, Junio C Hamano wrote:

Show 20 quoted lines

> 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.

Hmm, I thought I'd convinced myself that this was possible. Now, I can't come up with a way of doing it that doesn't involve improper commits with earlier timestamps than their parents.

Sorry about that, David

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