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From Lorenz Quack <quack.lor...@gmail.com>
Subject Re: Accumulation of Links from global shared durable subscriptions (JMS 2.0)
Date Tue, 07 Nov 2017 11:22:38 GMT
On Tue, 2017-11-07 at 10:43 +0000, Rob Godfrey wrote:
> On 7 November 2017 at 10:26, Lorenz Quack <quack.lorenz@gmail.com> wrote:
> 
> > 
> > On Tue, 2017-11-07 at 09:51 +0000, Robbie Gemmell wrote:
> > > 
> > > 
> > > Hi Lorenz,
> > > 
> > > This sounds/looks good to me overall, though I wonder around whether
> > > the 'more than 1 left' checking + subsequent calls are racey (e.g
> > > across different connections with subscribers detaching at the same
> > > time) and if so about the consequences.
> > > 
> > > Robbie
> > Hi Robbie,
> > 
> > Well spotted. It is known to be racey.
> > In the worst case the last two links would unsubscribe concurrently and
> > both be removed from the link registry.  Upon unsubscribe the link would
> > not be found in the registry and the attach would be rejected with
> > "not-found". The subscription queue would remain on the broker.
> > There are other races connected to shared subscriptions / link registry.
> > That does not make it better but we really want to get the release out
> > so we created QPID-7996 to address these issues in 7.0.1.
> > 
> > We are going to back port the change an spin a RC2 today.
> > 
> > 
> It'd be ugly as hell - but couldn't you just wrap the search for links /
> link close statement in a synchronized block synchronizing on the vhost or
> the link registry?
> 
> -- Rob
> 

I guess. The LinkRegistry is not exposed but I just checked and there does
not seem to be anything else that is synchronized on the VirtualHost so we
could use that and should not be a lot of contention or potential for
deadlocks.


Kind regards,
Lorenz

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