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https://issues.apache.org/jira/browse/FELIX-3456?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13254080#comment-13254080
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Felix Meschberger commented on FELIX-3456:
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> A colleague tried a completely asynchronous model for service events in scr
> and although start seemed to work,
> stopping resulted in a lot of work on stopped bundles and component objects
Been there done that ... SCR was fully asynchronous at one point in time.
Stopping failed miserably because services have been gone when the component
was about to be stopped and expected statically bound services to still be
available.
Startup worked correctly and the spec does not favor one or the other (I think
there is nothing about synchronicity in the spec). I converted to synchronous
startup to have better control during system startup (namely to have all
components activated when the FRAMEWORK_STARTED event is fired by the
framework). In fact, it is quite nice to have all components fuly handled once
the bundle's STARTED or STOPPED event is fired. (This is one of my concerns
with blueprint which is asynchronous and makes it hard to reason about system
state).
Overall and in hindsight it looks like synchronous startup is preferable to
asynchronous startup.
BTW: The last time I checked, Equinox DS was also fully synchronous.
> We might have an internal and external state.
In your proposal thread 2 waits for the lock and thus thread 1 and thread 2 are
susceptive to deadlocks. Particularly if you take another service into account
.... The reason for this is, that when a service is supplied we are called back
from the framework and thus have to be extremely carefull since the framework
may still hold locks (for example bundle locks during bundle start)
> 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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