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From "Felix Meschberger (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FELIX-3456) Component ignores required static service addition when in Activating state
Date Fri, 13 Apr 2012 18:20:18 GMT

    [ https://issues.apache.org/jira/browse/FELIX-3456?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13253605#comment-13253605
] 

Felix Meschberger commented on FELIX-3456:
------------------------------------------

Thanks for reporting. This is in fact an issue.

I am not against replacing the size int by an AtomicInteger; in the interest of concurrency
stability this might make sense. (though this might prevent SCR from being used in pre-Java
5 setups; we still consider this requirement valid for SCR)

But I am not convinced with the state lock. This going to be dangerous for deadlocks because
during activate and deactive services may be registered or unregistered, which may cause deadlocks:
We currently have one single situation where we synchronize (creation of delayed component
service instances) and we realize occasional deadlocks.

Thinking the other way around: We have some transient states during which events may happen
which may get lost. One such transient state is the activating state. And this is such an
event which may get lost. Another event may be a configuration update.

I have been contemplating some kind of event queuing and some way of processing such a queue
when leaving the transient state.

In the concrete example:

T1 registers Service A, causes component into the ACTIVATING state
T2 registers Service B, enqueues event since component is ACTIVATING
T1 terminates ACTIVATING, checks queue and handles event.

The queue could be as simple as a list of Runnable on each component manager.
                
> Component ignores required static service addition when in Activating state
> ---------------------------------------------------------------------------
>
>                 Key: FELIX-3456
>                 URL: https://issues.apache.org/jira/browse/FELIX-3456
>             Project: Felix
>          Issue Type: Bug
>          Components: Declarative Services (SCR)
>    Affects Versions:  scr-1.6.0
>         Environment: Using org.apache.felix.scr svn rev 1298268 on Mac
>            Reporter: Richard Ellis
>            Priority: Critical
>         Attachments: FELIX-3456-1.diff
>
>
> I have a component with two required static service references (A and B). In my scenario
A and B are registered nearly simultaneously on different threads and this causes the DependencyManager
to ignore the addition of one of these two services (B). This causes the component to remain
unsatisfied and never activate, since the service that was ignored is not re-registered at
any time and nothing subsequently causes the component to re-activate.
> This happens as follows:
> 12:30:59:317 Thread 1 - Registers Service B/257
> 12:30:59:320 Thread 2 - Registers Service A/258
> 12:30:59:320 Thread 2 - Dependency Manager: Adding Service A/258
> 12:30:59:321 Thread 2 - Dependency Manager: Service serviceA registered, activate component
> 12:30:59:321 Thread 2 - State transition : Unsatisfied -> Activating
> 12:30:59:321 Thread 2 - Activating component
> 12:30:59:321 Thread 1 - Dependency Manager: Adding Service B/257
> 12:30:59:321 Thread 2 - Dependency not satisfied: serviceB
> 12:30:59:321 Thread 1 - Dependency Manager: Added service serviceB is ignored for static
reference <--- I believe we end up here because Thread 2 has moved the component from Unsatisfied
to Activating and the reference is a static reference
> 12:30:59:321 Thread 2 - Not all dependencies satisified, cannot activate
> 12:30:59:321 Thread 2 - State transition : Activating -> Unsatisfied
> Because the addition of Service B has been ignored and serviceB is a required dependency
my component then never activates even though my reqiured service is present.
> There is a comment in DependencyManager#serviceAdded method:
> // FELIX-1413: if the dependency is static and the component is
> // satisfied (active) added services are not considered until
> // the component is reactivated for other reasons.
> This suggests that the static service should only be ignored if the component is satisfied(active),
which would be correct, but in this case the component is only activating (and will fail to
activate because one of the two dependencies is not yet satisfied) and there is no check of
state at this time.
> A simple fix would be to check the state of the component as well as if the service is
static e.g.
> replace if ( m_dependencyMetadata.isStatic() )
> with if ( m_dependencyMetadata.isStatic() && m_componentManager.getState() ==
AbstractComponentManager.STATE_ACTIVE )
> This is an easy fix, but I guess may leave a small window where a static reference could
get replaced while a component was still activating if another instance of the same service
was registered on a different thread.
> There are other fixes that could be done by synchronizing more around service additions.
> Is anyone willing to make this fix or does anyone have any thoughts about this issue?
> Thanks

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