Does git branch -m have side effects for other dev

问题:

We've already learned how to switch which branch points to what using git branch -m. If I do this, is it going to make life difficult for the other people pulling from my repository?

Say I do a bunch of stuff on a branch topic1 and then do a

git branch -m master old_master
git branch -m topic1 master
git push origin master

and then somebody else pulls master from the my remote repository, what will they have to do to make everything point to the right place? Will I have to tell everybody to repeat my steps?

Is this akin to the problem of rebasing commits after pushing them and leaving other developers with dangling objects?

回答1:

I'm not exactly sure what your repo looks like but here's the worst-case scenario.

Suppose your origin repository looks like this

origin:
o---o---A---B---C  master

And your local repository looks like this,

JimPuls:
o---o---A---B---C  master, origin/master
         \
          D---E---F topic1

Then, after your branch renames your local repository looks like this:

JimPuls:
o---o---A---B---C  old_master, origin/master
         \
          D---E---F master

Now, when you push master to origin that'll be a non-fast-forward update. After the push, the origin repository will look like this:

origin:
o---o---A...B...C  (B & C are orphaned commits)
         \
          D---E---F master

This can be cruel to your friends who may have done commits on top of C. For example, if Sally was working with you her repository may look like this:

Sally:
o---o---A---B---C  origin/master
                 \
                  G---H---I master

Now, if you do your non-fast-forward push and Sally does a fetch her repository will look like this:

Sally:
          D---E---F  origin/master
         /
o---o---A---B---C  
                 \
                  G---H---I  master

Now Sally has to figure out how to get her work (G, H, I) back into the repository. If she simply does a merge with origin/master then the changes in B and C will be back in the repository (oops!). Instead, she'll have to cherry-pick or rebase her G-H-I changes onto origin/master.

It's cool that Git lets you do that but it's kind of asking for trouble. You're really hoping that Sally notices the situation. This is why you should warn all the other contributors when you do this so they can deal with the change appropriately.

NOTE: the above is a worst-case scenario. If your topic1 branch departed from master at C then that change is a fast-forward and there are no problems.

回答2:

Basically your operations are the same as:

# git checkout master
# git reset --hard topic1
# git push origin master

And they will have exactly that effect: Everybody else will get the topic1 branch (it’s named master for them, though) and its ancestry up to the point where master and topic1 first diverged. The old master branch is then lying around in their repositories and will be garbage collected at some point in the future because nothing points to it anymore.

If topic1 is a branch that originated from the current HEAD of master you will be fine here. Otherwise you will get into the “rewriting history” situation which can make a mess of your e.g. your tags. You need to think carefully about what you’re really trying to achieve. Maybe a simple git merge will serve you better?