Hello,

On Thu, 21 Feb 2019 17:27:04 +0000
Phillip Wood [off-list ref] wrote:

Hi Eric

On 21/02/2019 17:12, Eric Sunshine wrote:

quoted

On Thu, Feb 21, 2019 at 12:07 PM Phillip Wood [off-list ref] wrote:  

quoted

On 21/02/2019 13:50, Michal Suchánek wrote:  

quoted

quoted

On Tue, Feb 19, 2019 at 12:05 AM Michal Suchanek [off-list ref] 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.  

The idea is that there are per-worktree lock stored under worktree-locks 
(hence the plural name). Using a separate directory for the locks gets 
round the problems of name clashes between the lock for a worktree 
called foo and one called foo.lock and means we can rely on mkdir 
erroring out if the worktree name already exists as there is no renaming.

I suppose this separate directory would work. When are you supposed to
take the lock, though?

When adding worktree, sure.

When managing worktrees, sure. Otherwise you would see the incomplete
worktrees.

When doing anything in git? Probably. Because otherwise you could
accidentally use the incomplete worktree. Or somebody deleting worktree
would fail removing it because you would keep adding files to it.

Isn't git supposed to allow parallel access to the repository?

As things stand if you wanted to implement worktree locking you would
need to lock the worktree for *every* operation that touches it, and
for many operations you would have to lock/unlock *all* worktrees one by
one to find the worktree you are supposed to work on.

I don't feel like adding locking to all of git to fix this problem.

Sure, adding enough locking to ensure repository consistency at all
times would be nice but it also needs to be granular enough to not harm
performance. I can't say I understand the git repository layout and
usage well enough to design that.

Thanks

Michal