"Cannot fetch git.git" (worktrees at fault? or origin/HEAD) ?

[Stefan Beller <sbeller@google.com>]

So I ran "git fetch --all" inside git.git that you are all familiar with.
All fetches failed with a similar error as:
Fetching kernelorg
fatal: bad object HEAD
error: https://kernel.googlesource.com/pub/scm/git/git did not send
all necessary objects

error: Could not fetch kernelorg

Working with an older version of Git (2.13.0) gives me

$ GIT_TRACE=/u/trace GIT_TRACE_PACKET=/u/out git fetch git://github.com/git/git
From git://github.com/git/git
 * branch                  HEAD       -> FETCH_HEAD

After fiddling around and some printf debugging,
clean fsck(!)
I could not make sense of it, so bisecting

$ git bisect bad
d0c39a49ccb5dfe7feba4325c3374d99ab123c59 is the first bad commit
commit d0c39a49ccb5dfe7feba4325c3374d99ab123c59
Author: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Date:   Wed Aug 23 19:36:59 2017 +0700

    revision.c: --all adds HEAD from all worktrees

    Unless single_worktree is set, --all now adds HEAD from all worktrees.

    Since reachable.c code does not use setup_revisions(), we need to call
    other_head_refs_submodule() explicitly there to have the same effect on
    "git prune", so that we won't accidentally delete objects needed by some
    other HEADs.

    A new FIXME is added because we would need something like

        int refs_other_head_refs(struct ref_store *, each_ref_fn, cb_data);

    in addition to other_head_refs() to handle it, which might require

        int get_submodule_worktrees(const char *submodule, int flags);

    It could be a separate topic to reduce the scope of this one.

    Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

And here we go, I do use worktrees nowadays!
$ git worktree list
/usr/local/google/home/sbeller/OSS/git                   117ddefdb4
(detached HEAD)
/u/git                                                   d0c39a49cc
(detached HEAD)
/usr/local/google/home/sbeller/OSS/git_origin_master     b14f27f917
(detached HEAD)
/usr/local/google/home/sbeller/OSS/submodule_remote_dot  3d9025bd69
(detached HEAD)

$ git show 3d9025bd69
fatal: ambiguous argument '3d9025bd69': unknown revision or path not
in the working tree.

ok, so I presume I just delete that working tree to "fix my copy of git"
sbeller@sbeller:/u/git$ rm -rf
/usr/local/google/home/sbeller/OSS/submodule_remote_dot
sbeller@sbeller:/u/git$ git worktree prune
sbeller@sbeller:/u/git$ git worktree list
/usr/local/google/home/sbeller/OSS/git                117ddefdb4 (detached HEAD)
/u/git                                                d0c39a49cc (detached HEAD)
/usr/local/google/home/sbeller/OSS/git_origin_master  b14f27f917 (detached HEAD)
$ git bisect reset
Previous HEAD position was d0c39a49cc... revision.c: --all adds HEAD
from all worktrees
HEAD is now at 660fb3dfa8... Sync with maint
$ make install
$ git fetch --all
# works fine!

Any idea which direction we need to heading for a real fix?