From: Jeff King <peff@peff.net>
Date: Thu, 19 Dec 2019 04:50:07 GMT
Subject: Re: How to determine a branch whether has a specified commit
Message-ID: <20191219045007.GA89333@coredump.intra.peff.net>
In-Reply-To: <CAGyf7-GK4SJ8oqJ-n1gCdNOq0Dwsxy4V7Eep7qXTsd51pVz7rA@mail.gmail.com>

On Wed, Dec 18, 2019 at 07:09:36PM -0800, Bryan Turner wrote:

> > I think
> >     git branch --contains <commit> | grep <branch name>
> 
> "Combine" the two commands: git branch --contains <commit> <branch
> name>. That will either output the branch's name, if it contains the
> commit, or nothing if it doesn't.

Or "git for-each-ref --contains <commit> refs/heads/<branch>" if you're
scripting.

> A different approach would be something like: git rev-list --count
> <commit> ^<branch name>. If that returns a count of 0, it means the
> commit is reachable from the branch. Otherwise, it'll output some
> positive count indicating how many commits (including the specific
> commit you asked about) are not reachable from the branch. (I suspect
> you don't really need the count, so you could also use "-1" instead of
> "--count".)

That works, but is not quite as careful as some other techniques with
respect to looking past commits with clock skew. A more careful way is
to check that "git merge-base <commit> <branch>" is equal to <commit>.

That clock-skew thing will eventually go away once we start using
generation numbers consistently (but I don't think we do yet in
limit_list(), which handles the "^" part of the traversal).

-Peff