Re: [PATCH v3 2/2] setup: don't fail if commondir reference is deleted.

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, 21 Feb 2019 04:27:21 -0500
Eric Sunshine <sunshine@sunshineco.com> wrote:

> On Wed, Feb 20, 2019 at 1:35 PM Eric Sunshine <sunshine@sunshineco.com> wrote:
> > On Wed, Feb 20, 2019 at 12:16 PM Michal Suchánek <msuchanek@suse.de> wrote:  
> > > On Wed, 20 Feb 2019 11:55:46 -0500
> > > Eric Sunshine <sunshine@sunshineco.com> wrote:  
> > > > On Wed, Feb 20, 2019 at 11:17 AM Michal Suchanek <msuchanek@suse.de> wrote:  
> > > > > Another even rarer issue is that the file might be zero size because another
> > > > > process initializing a worktree opened the file but has not written is content
> > > > > yet.  
> > > >
> > > > Based upon the explanation thus far, I'm having trouble understanding
> > > > under what circumstances these race conditions can arise. Are you
> > > > trying to invoke Git commands in a particular worktree even as the
> > > > worktree itself is being created?  
> > >
> > > It's explained in the following paragraph. If you have multiple
> > > worktrees some *other* worktreee may be uninitialized.  
> >
> > I understand that, but setup.c:get_common_dir_noenv() is concerned
> > only with _this_ worktree -- the one in which the Git command is being
> > run -- so it's not clear if or how some other partially-initialized
> > worktree could have any impact. (And, I'm having trouble fathoming how
> > it could, which is why I'm asking these questions).  
> 
> I still can't see how setup.c:get_common_dir_noenv() could be
> responsible for the behavior you're describing of _any_ Git command
> erroring out due to _any_ worktree being incompletely-initialized.
> However, I can imagine "git worktree add" itself being racy and
> failing due to a missing or empty "commondir" file for some other
> worktree since that command _does_ consult other worktree entries when
> validating the "add" operation via
> builtin/worktree.c:validate_worktree_add() which calls
> get_worktrees(). If get_worktrees() is subject to that raciness
> problem, then "git worktree add" will inherit that undesirable
> raciness behavior (as will other "git worktree" commands which call
> get_worktrees(), such as "git worktree list").
> 
> > Is it possible that when you saw that error message, it actually arose
> > from some code other than setup.c:get_common_dir_noenv()?  
> 
> So, I'm suspecting get_worktrincludes both itees() or some function it calls (and so
> on) as the racy culprit.

Yes, that's my explanation for the situation as well.

Thanks

Michal