On Tue, Dec 1, 2015 at 7:43 AM Tim Penhey <tim.pen...@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 > > -- > Juju-dev mailing list > Juju-dev@lists.ubuntu.com > Modify settings or unsubscribe at: > https://lists.ubuntu.com/mailman/listinfo/juju-dev >
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