utils/fslock needs to DIAF

[David Cheney]

On Tue, Dec 1, 2015 at 11:00 AM, Andrew Wilkins
<andrew.wilkins at canonical.com> wrote:
> On Tue, Dec 1, 2015 at 7:57 AM David Cheney <david.cheney at canonical.com>
> wrote:
>>
>> Doesn't look like there is windows support, and it uses fcntl (flock)
>> under the hood, which is what we have now.
>
>
> flock isn't the problematic thing Tim is talking about. utils/fslock
> attempts to create a directory in a known location, and if it succeeds, it
> "holds the lock". Unlocking means removing the directory.

The problem is if the process dies/exits/goes mental while holding the
lock we get into this existential gridlock where we're not sure if the
process _is_ alive, so we shouldn't remove the lock, or the process is
dead, so we should remove the lock.

abstract unix domain sockets do not have this problem on windows; kill
the process, file is removed from the abstract namespace.

>
> We would have to contribute Windows support, but it's not hard, I've done it
> before.
>
>>
>> On Tue, Dec 1, 2015 at 10:55 AM, Casey Marshall
>> <casey.marshall at canonical.com> wrote:
>> > How about github.com/camlistore/lock ?
>> >
>> > On Mon, Nov 30, 2015 at 5:43 PM, 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?
>> >>
>> >> Cheers,
>> >> Tim
>> >>
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>> >
>> >
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