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

[Phillip Wood <phillip.wood@talktalk.net>]

Hi Michal/Duy

On 21/02/2019 13:50, Michal Suchánek wrote:
> On Thu, 21 Feb 2019 17:50:38 +0700
> Duy Nguyen <pclouds@gmail.com> wrote:
> 
>> On Tue, Feb 19, 2019 at 12:05 AM Michal Suchanek <msuchanek@suse.de> wrote:
>>>
>>> When adding wotktrees git can die in get_common_dir_noenv while
>>> examining existing worktrees because the commondir file does not exist.
>>> Rather than testing if the file exists before reading it handle ENOENT.
>>
>> I don't think we could go around fixing every access to incomplete
>> worktrees like this. If this is because of racy 'worktree add', then
>> perhaps a better solution is make it absolutely clear it's not ready
>> for anybody to access.
>>
>> For example, we can suffix the worktree directory name with ".lock"
>> and make sure get_worktrees() ignores entries ending with ".lock".
>> That should protect other commands while 'worktree add' is still
>> running. Only when the worktree is complete that 'worktree add' should
>> rename the directory to lose ".lock" and run external commands like
>> git-checkout to populate the worktree.
> 
> 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.

Best Wishes

Phillip

> Thanks
> 
> Michal
>