Re: Worktree creation race

[Duy Nguyen <pclouds@gmail.com>]

Thanks for including me. Apparently I did miss some emails :)

On Fri, Feb 1, 2019 at 1:27 PM Eric Sunshine <sunshine@sunshineco.com> wrote:
>
> On Mon, Jan 28, 2019 at 7:58 AM Marketa Calabkova <mcalabkova@suse.cz> wrote:
> > On 15/01/2019 15:03, Marketa Calabkova wrote:
> > > I am writing to report a bug. The original report is from my colleague, I am also providing his suggestions.
> > >
> > > There is insufficient locking for worktree addition. Adding worktree may fail.
> > >
> > > The problem is that git reads the directory entries in $GIT_DIR/worktrees,
> > > finds a worktree name that does not exist, tries to create it, and if an
> > > error is returned adding the worktree fails. When multiple git processes
> > > do this in parallel only one adds a worktree and the others fail. Git should
> > > reread the directory and find a new name that does not exist when creating
> > > the worktree directory fails because another git process already created it.
> > >
> > > I suppose adding PID in the tree name would mitigate the issue to the point it will be very unlikely to encounter.
> > >
> > > I need more than the tree in the temporary directory so using the temporary directory directly as a tree is out of question.
> > >
> > > cd gitrepo
> > > git commit --allow-empty -m Empty
> > > for n in $(seq 10000) ; do ( tmp=$(mktemp -d /dev/shm/gittest/test.XXXXXXXXXXX) ; mkdir $tmp/test ; git worktree add --detach $tmp/test ; ) & done
> > >
> > > (you should see many messages like:
> > > fatal: could not create directory of '.git/worktrees/test284': File exists)
> > >
> > Does anyone has a suggestion what to do with this bug? It looks like a
> > one-line fix probably in builtin/worktree.c, but I have no idea how to
> > do it. Sorry.
>
> I doubt this is a one-line fix, and I don't think it has anything to
> do with reading entries in $GIT_DIR/worktrees.

I never thought people would create worktrees "like crazy" to end up
worrying about races like this. The mkdir loop would be one way to go.
But I'm going to add a new option to let the user control this
directory name. This is necessary since this name is now exposed via
"worktrees/<name>" reference space and should also be reported in "git
worktree list". Avoiding the race is a nice bonus.

> add_worktree() already attempts to give a unique identifier to each
> worktree by adding a numeric suffix and incrementing that suffix if
> the name already exists (such as the 284 in your example error
> message) but there is definitely a race-condition between the time it
> stat()s the name and the time it mkdir()s it.
>
> One possible fix would be to unconditionally use the PID, as you
> suggest, though, this is not necessarily foolproof against races
> either (though it makes collisions very unlikely).
>
> Another possibility would be to skip the stat() and instead do the
> mkdir() in a loop, incrementing the sequence number each time through
> the loop. That should eliminate the race entirely (I think).
-- 
Duy