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git merges of tags

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  git merges of tags Stephen Rothwell <sfr@canb.auug.org.au> - 2017-05-19 01:30 +0200
    Re: git merges of tags Linus Torvalds <torvalds@linux-foundation.org> - 2017-05-19 20:40 +0200

#1644954 — git merges of tags

Hi all,

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).  This is a problem when your current head of
branch has been merged into (e.g.) Linus' tree and then you do "git
merge <tag>" on a tag that is later than the merge point.  You end up
with an unnecessary merge commit rather than just fast forwarding.

To do the fast forward, try "git merge <tag>^{}" ... (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 :-().
-- 
Cheers,
Stephen Rothwell

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#1645841

On Thu, May 18, 2017 at 4:23 PM, Stephen Rothwell <sfr@canb.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

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