linux-kernel - Re: git merges of tags

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Message-ID: <CA+55aFwaxsNZ0ACQkASdDTbSrdcHMJpFPtswK485tEWRBVVYrA@mail.gmail.com>
Date:   Fri, 19 May 2017 11:30:55 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Git Mailing list <git@...r.kernel.org>
Subject: Re: git merges of tags

On Thu, May 18, 2017 at 4:23 PM, Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>
> Just a reminder that if you are merging Linus' tree (or any tree
> really) via a tag, git was changed some time ago so that merging a tag
> will not do a fast forward (there is a good reason for this - I just
> can't recall it ATM).

The reason is that when you merge a signed tag, git squirrels away t
he signature into the merge commit, so that you can see and verify the
signage later (use "git log --show-signatures" to see the signatures
on the commits).

If you fast-forward, there isn't any new commit to add the signing data to.

> To do the fast forward, try "git merge <tag>^{}" ...

A slightly simpler syntax might be just "tag^0", but yes, the "^{}"
thing peels off any tags.

>   (unfortunately
> doing "git merge --ff <tag>" also does not do a fast forward - it also
> doesn't fail, it unexpectedly just creates a merge commit :-().

"--ff" is the default behavior, and means "allow fast forward", but
note that it is about "allowing", not "forcing".

You can use "--ff-only" to say that you will _only_ accept a
fast-forward, and git will error out  if it needs to create a merge.

                  Linus