On Re-tagging

What should you do when you tag a wrong commit and you would want to re-tag?

If you never pushed anything out, just re-tag it. Use "-f" to replace the old one. And you’re done.

But if you have pushed things out (or others could just read your repository directly), then others will have already seen the old tag. In that case you can do one of two things:

However, Git does not (and it should not) change tags behind users back. So if somebody already got the old tag, doing a git pull on your tree shouldn’t just make them overwrite the old one.

If somebody got a release tag from you, you cannot just change the tag for them by updating your own one. This is a big security issue, in that people MUST be able to trust their tag-names. If you really want to do the insane thing, you need to just fess up to it, and tell people that you messed up. You can do that by making a very public announcement saying:

Ok, I messed up, and I pushed out an earlier version tagged as X. I
then fixed something, and retagged the *fixed* tree as X again.
If you got the wrong tag, and want the new one, please delete
the old one and fetch the new one by doing:
        git tag -d X
        git fetch origin tag X
to get my updated tag.
You can test which tag you have by doing
        git rev-parse X
which should return 0123456789abcdef.. if you have the new version.
Sorry for the inconvenience.

Does this seem a bit complicated? It should be. There is no way that it would be correct to just "fix" it automatically. People need to know that their tags might have been changed.

On Automatic following

If you are following somebody else’s tree, you are most likely using remote-tracking branches (eg. refs/remotes/origin/master). You usually want the tags from the other end.

On the other hand, if you are fetching because you would want a one-shot merge from somebody else, you typically do not want to get tags from there. This happens more often for people near the toplevel but not limited to them. Mere mortals when pulling from each other do not necessarily want to automatically get private anchor point tags from the other person.

Often, "please pull" messages on the mailing list just provide two pieces of information: a repo URL and a branch name; this is designed to be easily cut&pasted at the end of a git fetch command line:

Linus, please pull from
        git://git..../proj.git master
to get the following updates...

becomes:

$ git pull git://git..../proj.git master

In such a case, you do not want to automatically follow the other person’s tags.

One important aspect of Git is its distributed nature, which largely means there is no inherent "upstream" or "downstream" in the system. On the face of it, the above example might seem to indicate that the tag namespace is owned by the upper echelon of people and that tags only flow downwards, but that is not the case. It only shows that the usage pattern determines who are interested in whose tags.

A one-shot pull is a sign that a commit history is now crossing the boundary between one circle of people (e.g. "people who are primarily interested in the networking part of the kernel") who may have their own set of tags (e.g. "this is the third release candidate from the networking group to be proposed for general consumption with 2.6.21 release") to another circle of people (e.g. "people who integrate various subsystem improvements"). The latter are usually not interested in the detailed tags used internally in the former group (that is what "internal" means). That is why it is desirable not to follow tags automatically in this case.

It may well be that among networking people, they may want to exchange the tags internal to their group, but in that workflow they are most likely tracking each other’s progress by having remote-tracking branches. Again, the heuristic to automatically follow such tags is a good thing.

On Backdating Tags

If you have imported some changes from another VCS and would like to add tags for major releases of your work, it is useful to be able to specify the date to embed inside of the tag object; such data in the tag object affects, for example, the ordering of tags in the gitweb interface.

To set the date used in future tag objects, set the environment variable GIT_COMMITTER_DATE (see the later discussion of possible values; the most common form is "YYYY-MM-DD HH:MM").

For example:

$ GIT_COMMITTER_DATE="2006-10-02 10:31" git tag -s v1.0.1