Re: [PATCH 2/2] setup: don't fail if commondir reference is deleted.

[Eric Sunshine <sunshine@sunshineco.com>]

On Thu, Feb 21, 2019 at 12:07 PM Phillip Wood <phillip.wood@talktalk.net> wrote:
> On 21/02/2019 13:50, Michal Suchánek wrote:
> >> On Tue, Feb 19, 2019 at 12:05 AM Michal Suchanek <msuchanek@suse.de> wrote:
> > The problem is we don't forbid worktree names ending with ".lock".
> > Which means that if we start to forbid them now existing worktrees
> > might become inaccessible.
>
> I think it is also racy as the renaming breaks the use of mkdir erroring
> out if the directory already exists. One solution is to have a lock
> entry in $GIT_COMMON_DIR/worktree-locks and make sure the code that
> iterates over the entries in $GIT_COMMON_DIR/worktrees skips any that
> have a corresponding ignores in $GIT_COMMON_DIR/worktree-locks. If the
> worktree-locks/<dir> is created before worktree/<dir> then it should be
> race free (you will have to remove the lock if the real entry cannot be
> created and then increment the counter and try again). Entries could
> also be locked on removal to prevent a race there.

I wonder, though, how much this helps or hinders the use-case which
prompted this patch series in the first place; to wit, creating
hundreds or thousands of worktrees. Doing so serially was too slow, so
the many "git worktree add" invocations were instead run in parallel
(which led to "discovery" of race conditions). Using a global worktree
lock would serialize worktree creation, thus slowing it down once
again.