Re: Detached checkout will clobber branch head when using symlink HEAD

[Junio C Hamano <gitster@pobox.com>]

I do not think that is a correct approach.

The offending callchain is:

  update_ref(..., "HEAD", REF_NODEREF, ...);
  -> lock_any_ref_for_update("HEAD", ..., REF_NODEREF);
   -> lock_ref_sha1_basic("HEAD", ..., REF_NODEREF, ...);
      . calls resolve_ref() to read HEAD to arrive at
        refs/heads/master
      . however, it notices REF_NODEREF and adjusts the ref to be updated
        back to "HEAD";
    -> hold_lock_file_for_update(..., "HEAD", 1);
     -> lock_file(..., "HEAD");
        . resolves symlink "HEAD" to "refs/heads/master", and
          locks it!  This creates "refs/heads/master.lock", that is
          then renamed to "refs/heads/master" when unlocked.

In other words, the breakage is in lock_file() and not in resolve_ref().
The latter gives the same output to the caller whether the HEAD is
symbolic link or textual symref -- at least it should.

The behaviour of lock_file() to resolve symlink at this point in the code
comes from d58e8d3 (When locking in a symlinked repository, try to lock
the original, 2007-07-25), and as explained in the log message of that
commit, we cannot unconditionally remove it.

Three patches to fix this issue, that come on top of the fix to t7201
(--track from detached HEAD should fail) I sent out last night, will
follow.