utils/fslock needs to DIAF

[Andrew Wilkins]

On Tue, Dec 1, 2015 at 7:43 AM Tim Penhey <tim.penhey at canonical.com> wrote:

> Hi folks,
>
> The fslock was a mistake that I added to the codebase some time back. It
> provided an overly simplistic solution to a more complex problem.
>
> Really the filesystem shouldn't be used as a locking mechanism.
>
> Most of the code that exists for the fslock now is working around its
> deficiencies. Instead we should be looking for a better replacement.
>
> Some "features" that were added to fslock were added to work around the
> issue that the lock did not die with the process that created it, so
> some mechanism was needed to determine whether the lock should be broken
> or not.
>
> What we really need is a good OS agnostic abstraction that provides the
> ability to create a "named" lock, acquire the lock, release the lock,
> and make sure that the lock dies when the process dies, so another
> process that is waiting can acquire the lock. This way no "BreakLock"
> functionality is required, nor do we need to try and do think like
> remember which process owns the lock.
>
> So...
>
> We have three current operating systems we need to support:
>
> Linux - Ubuntu and CentOS
> MacOS - client only - bit the CLI uses a lock for the local cache
> Windows
>
> For Linux, and possibly MacOS, flock is a possibility, but can we do
> better? Is there something that is better suited?
>
> For Windows, while you can create global semaphores or mutex instances,
> I'm not sure of entities that die with the process. Can people recommend
> solutions?
>

I've been wanting to do this for a long time (I think I've whinged in your
vicinity about it before), but I've held off because of uncertainty about
compatibility with NFS (which is probably a rare scenario, and only for the
client).

I think it was jam that brought up the heritage of directory locking in
bzr, where flock doesn't work reliably over NFS. I think that's old news
though. The manpage for flock discusses the NFS/kernel limitations of
flock; since we don't go back past precise, we should be fine.

I think flock (fcntl F_SETLK/F_GETLK) is an appropriate option for Linux
and OS X. Windows has FileLock (
https://msdn.microsoft.com/en-us/library/windows/desktop/aa365202(v=vs.85).aspx),
and LockFileEx (for more control). Just as with F_SETLK, if the process
dies, the lock is released.

Cheers,
Andrew

Cheers,
> Tim
>
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