git fetch overwriting local tags

git fetch overwriting local tags

[Uwe Kleine-König]

Hello,

John and I wondered about git fetch overwriting local tags. I was sure
enough to claim that git fetch won't overwrite local tags with remote
tags having the same name. But after John pointed me to

	http://www.pythian.com/news/9067/on-the-perils-of-importing-remote-tags-in-git/

I tested that (using Debian's 1.7.7.3) and really, git does overwrite
local tags.

Here is my test script:

	mkdir a
	cd a
	echo some content > some_file
	git init 
	git add some_file
	git commit -m 'some commit log'
	git tag some_tag

	cd ..

	mkdir b
	cd b
	echo some different content > another_file
	git init 
	git add another_file
	git commit -m 'another commit log'
	git tag some_tag

	git fetch --tags ../a

After that I have:

	git log -1 --oneline some_tag
	c4ad89a some commit log

so b's tag was overwritten.

Is this intended?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

Re: git fetch overwriting local tags

[Jeff King]

On Wed, Nov 23, 2011 at 10:08:21AM +0100, Uwe Kleine-König wrote:

> John and I wondered about git fetch overwriting local tags. I was sure
> enough to claim that git fetch won't overwrite local tags with remote
> tags having the same name. But after John pointed me to
> 
> 	http://www.pythian.com/news/9067/on-the-perils-of-importing-remote-tags-in-git/
> 
> I tested that (using Debian's 1.7.7.3) and really, git does overwrite
> local tags.
> 
> Here is my test script:
> [...]
> 	git fetch --tags ../a
> [...]
> Is this intended?

Sort of.

By default, "git fetch" will "auto-follow" tags; if you fetch a commit
which is pointed to by a tag, then git will fetch that tag, too. So
generally, you shouldn't need to specify "--tags" at all, because you
will already be getting the relevant tags.

The "--tags" option, however, is a short-hand for saying "fetch all of
the tags", and is equivalent to providing the refspec:

  git fetch ../a refs/tags/*:refs/tags/*

Which of course will update your local tags with similarly-named ones
from the remote.  So in that sense, there is no bug, and it is working
as intended; the problem is that the author's intent was not the same as
your intent. :)

I'm not sure why you're using "--tags" in the first place. That might
help us figure out if there's another way to do what you want that is
safer.

That being said, it would be nice if "--tags" wasn't so surprising.
Three things that I think could help are:

  1. We usually require a "+" on the refspec (or "--force") to update
     non-fast-forward branches. But there is no such safety on tags
     (which generally shouldn't be updated at all). Should we at least
     be enforcing the same fast-forward rules on tag fetches (or even
     something more strict, like forbidding tag update at all unless
     forced)?

  2. We don't keep a reflog on tags. Generally there's no point. But
     it wouldn't be very expensive (since they don't usually change),
     and could provide a safety mechanism here.

  3. Keeping tags from remotes in separate namespaces, but collating
     them at lookup time. This has been discussed, and I think is
     generally a fine idea, but nobody has moved forward with code.

-Peff

Re: git fetch overwriting local tags

[Uwe Kleine-König]

Hi Jeff,

On Wed, Nov 23, 2011 at 05:16:58PM -0500, Jeff King wrote:
> On Wed, Nov 23, 2011 at 10:08:21AM +0100, Uwe Kleine-König wrote:
> 
> > John and I wondered about git fetch overwriting local tags. I was sure
> > enough to claim that git fetch won't overwrite local tags with remote
> > tags having the same name. But after John pointed me to
> > 
> > 	http://www.pythian.com/news/9067/on-the-perils-of-importing-remote-tags-in-git/
> > 
> > I tested that (using Debian's 1.7.7.3) and really, git does overwrite
> > local tags.
> > 
> > Here is my test script:
> > [...]
> > 	git fetch --tags ../a
> > [...]
> > Is this intended?
> 
> Sort of.
> 
> By default, "git fetch" will "auto-follow" tags; if you fetch a commit
> which is pointed to by a tag, then git will fetch that tag, too. So
> generally, you shouldn't need to specify "--tags" at all, because you
> will already be getting the relevant tags.
Hmm, if I do:

        mkdir a
        cd a
        echo some content > some_file
        git init
        git add some_file
        git commit -m 'some commit log'
        git tag some_tag

        cd ..

        mkdir b
        cd b
        echo some different content > another_file
        git init
        git add another_file
        git commit -m 'another commit log'

	git fetch ../a

I don't get the tag. That's why I added --tags. I guess that's because
some_tag is a lightweight tag. Hmm, but even if I change the command to
create the tag to

	git tag -a -m 'tag desc' some_tag

I don't get it without --tags?!

> The "--tags" option, however, is a short-hand for saying "fetch all of
> the tags", and is equivalent to providing the refspec:
> 
>   git fetch ../a refs/tags/*:refs/tags/*
> 
> Which of course will update your local tags with similarly-named ones
> from the remote.  So in that sense, there is no bug, and it is working
> as intended; the problem is that the author's intent was not the same as
> your intent. :)
> 
> I'm not sure why you're using "--tags" in the first place. That might
> help us figure out if there's another way to do what you want that is
> safer.
> 
> That being said, it would be nice if "--tags" wasn't so surprising.
> Three things that I think could help are:
> 
>   1. We usually require a "+" on the refspec (or "--force") to update
>      non-fast-forward branches. But there is no such safety on tags
>      (which generally shouldn't be updated at all). Should we at least
>      be enforcing the same fast-forward rules on tag fetches (or even
>      something more strict, like forbidding tag update at all unless
>      forced)?
That sounds fine for me.

>   2. We don't keep a reflog on tags. Generally there's no point. But
>      it wouldn't be very expensive (since they don't usually change),
>      and could provide a safety mechanism here.
I prefer 1, but that would be better than the current situation at
least.
 
>   3. Keeping tags from remotes in separate namespaces, but collating
>      them at lookup time. This has been discussed, and I think is
>      generally a fine idea, but nobody has moved forward with code.
That's something that John said in our discussion, too. That's the
suggestion I like the most.

Best regards and thanks for your time,
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |