[Kde-pim] Re: Creating remote branch called HEAD corrupts remote clones

[Junio C Hamano]

Stephen Kelly <steveire at gmail.com> writes:

> There were also messages like this:
>
> $ git pull
> remote: Counting objects: 5, done.
> remote: Total 3 (delta 0), reused 0 (delta 0)
> Unpacking objects: 100% (3/3), done.
> From /home/kde-devel/dev/src/playground/git/tmp/remote
>  + 1434cd2...dd30974 HEAD       -> origin/HEAD  (forced update)

Stephen, thanks, I think you have found a bug, not in this step but one
step before this 'git pull', where Bob pushes to the remote for the first
time after making a commit.

This issue is inherent in the way how the 'separate remotes' layout
introduced in 1.6.0 arranges the remote tracking mappings.

The refs/remotes/origin/HEAD in Bob's repository is supposed to be a
symbolic ref that points at the primary branch of the 'origin' remote
(typically its master), e.g. "ref: refs/remotes/origin/master".  But in
general, local 'refs/remotes/origin/X' for any value of X is to copy
'refs/heads/X' from the 'origin'.

Oops.  If the origin repository has 'refs/heads/HEAD', these rules
obviously conflict with each other.

In this particular case, Bob pushes the change to his refs/heads/master to
the remote to update its refs/heads/master.  The push at the same time
tries to pretend that it fetched from the remote to update Bob's tracking
branches, so refs/remotes/origin/master in Bob's repository is also
updated to point at this commit.

However, because Bob's refs/remotes/origin/HEAD is a symbolic ref that
points at his refs/remotes/origin/master, its value is also updated by
this push.

Bob's next fetch from remote will then notice that remotes/origin/HEAD he
has is different from refs/heads/HEAD the remote has, and tries to update
it, which would obviously a non-fast-forward.

I personally think it is reasonable to forbid HEAD or anything all caps
that ends with "_HEAD" as branch names.  Opinions?
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