Re: [PATCH v3 2/2] receive-pack: Protect current branch for bare repository worktree

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Mon, 8 Nov 2021, Junio C Hamano wrote:

> Junio C Hamano <gitster@pobox.com> writes:
>
> >> @@ -1456,11 +1456,11 @@ static const char *update_worktree(unsigned char *sha1, const struct worktree *w
> >>  		work_tree = worktree->path;
> >>  	else if (git_work_tree_cfg)
> >>  		work_tree = git_work_tree_cfg;
> >
> > Not a fault of this patch at all, but I am not sure if this existing
> > bit of code is correct.  Everything else in this function works by
> > assuming that the worktree that comes from the caller was checked
> > with find_shared_symref("HEAD", name) to ensure that, if not NULL,
> > it has the branch checked out and updating to the new commit given
> > as the other parameter makes sense.
> >
> > But this "fall back to configured worktree" is taken when the gave
> > us NULL worktree or worktree without the .path member (i.e. no
> > checkout), and it must have come from a NULL return from the call to
> > find_shared_symref().  IOW, the function said "no worktree
> > associated with the repository checks out that branch being
> > updated."  I doubt it is a bug to update the working tree of the
> > repository with the commit pushed to some branch that is *not* HEAD,
> > only because core.worktree was set to point at an explicit location.
>
> Not "I doubt", but I suspect it is a bug.  Sorry.
>
> But in practice, especially with the new code structure, we'd never
> flip do_update_worktree on unless find_shared_symref() says that the
> ref we are updating in the function is what is checked out, which
> means worktree is always non-NULL when we call update_worktree().
>
> So, unless there is some situation where worktree->path is NULL for
> a worktree with a checkout, the "else if" above is a dead code, I
> think.
>
> Similarly, I suspect that is_bare_repository() call the patch moved
> into the if/else if/ chain is even reachable with the updated
> caller.  find_shared_symref() is always called, and unless it gives
> a non-NULL worktree, do_update_worktree never becomes true.
>
> Anyway, enough bug finding in the existing code.  I think the
> update-instead was Dscho's invention and when the codepath was
> updated to be worktree ready, Dscho helped Hariom to do so, so
> I'll CC Dscho to see if he has input.

It's such a blast from the past! I first worked on this in 1404bcbb6b3
(receive-pack: add another option for receive.denyCurrentBranch,
2014-11-26), and Hariom & I worked on this last year, before the pandemic
hit over here (and therefore it feels like a decade ago).

The `worktree` variable was introduced in 4ef346482d6
(receive.denyCurrentBranch: respect all worktrees, 2020-02-23), and since
the patch under discussion does away with the `is_bare_repository()` call,
I think that we now can safely change these lines:

        if (do_update_worktree) {
                ret = update_worktree(new_oid->hash, find_shared_symref("HEAD", name));
                if (ret)
                        return ret;
        }

to pass `worktree` directly to the `update_worktree()` function, rather
than calling `find_shared_symref()` again.

And since that is the case, I think your analysis is correct, we always
call `update_worktree()` with a `worktree` parameter that is non-`NULL`.

As to the riddle about why we check `git_work_tree_cfg` at all? Back when
I introduced support for `denyCurrentBranch = updateInstead`, there were
no worktrees, the only way to give a bare repository a worktree was via
that config.

And from how I read the code in `worktree`, both "main" and "linked"
worktrees do have a `path` attribute that is non-`NULL`. We therefore
really have to look at the `is_bare` attribute to know whether
`worktree->path` _actually_ refers to a worktree. But as you also pointed
out, `find_shared_symref()` skips any worktree with non-zero `is_bare`.

We also can be pretty certain that only the `if (worktree &&
worktree->path)` arm is hit, we should probably turn the code into:

	if (!worktree || (!worktree->path && !worktree->is_bare))
                BUG("update_worktree() called without a path");

	if (worktree->is_bare)
		return "denyCurrentBranch = updateInstead needs a worktree";

	work_tree = worktree->path;

Ciao,
Dscho