git merges of tags

Stephen Rothwell· May 18, 2017, 23:23 UTC · lore

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

Linus Torvalds· May 19, 2017, 18:30 UTC · re: Stephen Rothwell · lore

Re: git merges of tags

On Thu, May 18, 2017 at 4:23 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:

Show 5 quoted lines

>
> 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