package com.atlassian.sal.core.lifecycle;
   
   import java.util.ArrayList;
   import java.util.Collection;
   import java.util.Collections;
   import java.util.HashSet;
   import java.util.List;
   import java.util.Set;
   
  import javax.annotation.PostConstruct;
  import javax.annotation.PreDestroy;
  
  import com.atlassian.plugin.PluginAccessor;
  import com.atlassian.plugin.event.PluginEventListener;
  import com.atlassian.plugin.event.PluginEventManager;
  import com.atlassian.plugin.event.events.PluginEnabledEvent;
  import com.atlassian.plugin.event.events.PluginFrameworkStartedEvent;
  import com.atlassian.sal.api.lifecycle.LifecycleAware;
  import com.atlassian.sal.api.lifecycle.LifecycleManager;
  
  import org.osgi.framework.Bundle;
  import org.osgi.framework.BundleContext;
  import org.osgi.framework.ServiceEvent;
  import org.osgi.framework.ServiceListener;
  import org.osgi.framework.ServiceReference;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import static com.atlassian.plugin.osgi.util.OsgiHeaderUtil.getPluginKey;
  import static org.osgi.framework.Constants.OBJECTCLASS;
  
  /**
   * Manage the tracking of {@link LifecycleAware} services registered to OSGi and call their {@link LifecycleAware#onStart} when
   * required.
   * <p/>
   * The {@code onStart} method is called when the application is setup according to {@link LifecycleManager#isApplicationSetUp}, the
   * plugin is enabled, and the plugin framework has started.
   * <p/>
   * This class has concurrency controls for the case of concurrent service registration/deregistration with plugin and application
   * state activity. It does not account for concurrency of plugin notifications against themselves. This does not happen during
   * normal usage.
   */
  public abstract class DefaultLifecycleManager implements LifecycleManager
  {
      private static final Logger log = LoggerFactory.getLogger(DefaultLifecycleManager.class);
  
      private final PluginEventManager pluginEventManager;
      private final PluginAccessor pluginAccessor;
      private final BundleContext bundleContext;
  
      /**
       * The LifecycleAware services which we know about via OSGi notifications, but which we have not yet called onStart() on.
       * <p/>
       * The documentation for {@link org.osgi.framework.ServiceReference} is quite explicit that while a framework may return
       * multiple ServiceReference objects referring to the same ServiceRegistration, all such ServiceReference objects are equals()
       * and have the same hashCode(), so it is safe to use them in a set like this.
       * <p/>
       * Inspection of Felix code (as at 4.2.1 of this writing) shows that in fact Felix is careful to have only a single
       * ServiceReference for each ServiceRegistration. If we move to a framework where this is not so, or if this causes problems,
       * we'd need to replace this with a map from the ServiceReference.getProperty(SERVICE_ID) to ServiceReference so we can key
       * service tracking off SERVICE_ID.
       */
      private final Set<ServiceReference<LifecycleAware>> pendingLifecycleAwares;
      private final ServiceListener serviceListener;
  
      private boolean started;
  
      public DefaultLifecycleManager(
              final PluginEventManager pluginEventManager,
              final PluginAccessor pluginAccessor,
              final BundleContext bundleContext)
      {
          this.pluginEventManager = pluginEventManager;
          this.pluginAccessor = pluginAccessor;
          this.bundleContext = bundleContext;
          this.pendingLifecycleAwares = Collections.synchronizedSet(new HashSet<ServiceReference<LifecycleAware>>());
          this.serviceListener = new LifecycleAwareServiceListener();
          this.started = false;
      }
  
      @PostConstruct
      public void postConstruct() throws Exception
      {
          pluginEventManager.register(this);
          // Gather all the current LifecycleAware instances, and watch for new ones
          final String filter = "(" + OBJECTCLASS + "=" + LifecycleAware.class.getName() + ")";
          // Since OSGi doesn't (afaics) provide an atomic way to add a ServiceListener and get brought up to date, we need to be a
          // bit careful here. If we miss an add, we won't call an onStart() we should, and if we miss a remove we'll possibly leak.
          // So we starting listening, add everything we can see, and discard anything invalid.
  
          // An an aside, add everything, add listener, add everything doesn't work - a service that is removed after the snapshot
          // for the second add everything is readded when this snapshot is added, and so you end up missing a remove.
  
          // Services that come and go here before this point are irrelevant
          bundleContext.addServiceListener(serviceListener, filter);
          // Anything added from now on is captured, so we don't lose services
          final Collection<ServiceReference<LifecycleAware>> services =
                  bundleContext.getServiceReferences(LifecycleAware.class, null);
          // Our snapshot may contain services that have already made it to pending. This means we might get double adds (which the
         // set takes care of), or we might re-add something in our snapshot that gets removed in this window. Such a service might
         // have turned up before we started listening or after, so the listener might think the remove is spurious or normal, but
         // the service is in our snapshot.
         pendingLifecycleAwares.addAll(services);
         // Any remove from now on is correctly processed - we've gathered everything and put it it pendingLifecycleAwares. However,
         // we might have re-added some service that was removed post snapshot but before we updated pendingLifecycleAwares. To deal
         // with these stale services, we do a sweep and chuck them out.  We can sweep the services from the getServiceReferences
         // snapshot, because those that arrived via the listener will be present when removed. A service added before the listener
         // is added but removed afterwards, but before the snapshot, is just ignored as a spurious remove.
         for (final ServiceReference<LifecycleAware> service : services)
         {
             // The documentation for getBundle() says this is a sanctioned way to detect deregistration
             if (null == service.getBundle())
             {
                 pendingLifecycleAwares.remove(service);
             }
         }
     }
 
     @PreDestroy
     public void preDestroy() throws Exception
     {
         bundleContext.removeServiceListener(serviceListener);
         // We  clear out our pendingLifecycleAwares, to assist garbage collection. This is just a heuristic - if this code moves
         // this is *not* sufficient to prevent leaks, since a thread could be in serviceListener now, having arrived before we
         // removed the listener but not yet finished. Since this is a @PreDestroy and things are going away, the GC will save us.
         // If we do move this code, we should probably move pendingLifecycleAwares to the serviceListener and have their lifetimes
         // match, but I'm not going to pre-emptively solve this synchronization problem without knowledge of the lifetimes and where
         // things are moving to.
         pendingLifecycleAwares.clear();
         pluginEventManager.unregister(this);
     }
 
     @PluginEventListener
     public void onPluginFrameworkStarted(final PluginFrameworkStartedEvent event)
     {
         startIfApplicationSetup();
     }
 
     @PluginEventListener
     public void onPluginEnabled(final PluginEnabledEvent event)
     {
         // I don't think it's actually worth restricting to the "right" plugin here - if a whole lot of stuff is enabled when
         // we get the event, we may as well start them all in one hit. This will stop lots of doubling handling of outstanding
         // LifecycleAware instances. However, this is called a lot before startup, so we hoist the started check.
         if (started)
         {
             notifyStartableLifecycleAwares();
         }
     }
 
     public void start()
     {
         startIfApplicationSetup();
     }
 
     private void startIfApplicationSetup()
     {
         final boolean doSetup = !started && isApplicationSetUp();
         if (doSetup)
         {
             started = true;
             // We could call the internal notify irrespective of doSetup, since it guards internally, but since we need to guard
             // for subclasses, we may as well hoist this in here and save a bit of effort in the no-op case.
             notifyStartableLifecycleAwares();
             notifyOnStart();
         }
     }
 
     /**
      * An override point for subclasses that require callback notification after the initial start of the system.
      */
     protected void notifyOnStart()
     {
     }
 
     private void notifyStartableLifecycleAwares()
     {
         // Grab a copy to operate on, so we don't call onStart() holding a lock. We're using toArray here to keep the
         // synchronization clear at the cost of the unchecked cast below. If we went via ImmutableList.copyOf, we'd need to either
         // synchronize externally or depend on the implementation of copyOf - see Collections.synchronizedSet re iteration.
         final Object[] pending = pendingLifecycleAwares.toArray();
 
         // Notify those we can, remembering what we handled.
         final List<ServiceReference<LifecycleAware>> completed = new ArrayList<ServiceReference<LifecycleAware>>(pending.length);
         for (final Object serviceRaw : pending)
         {
             @SuppressWarnings("unchecked")
             // serviceRaw comes from Object[] pending which is a snapshot of pendingLifecycleAwares so the cast is safe
             final ServiceReference<LifecycleAware> service = (ServiceReference<LifecycleAware>) serviceRaw;
             if (notifyLifecycleAwareIfStartedAndEnabled(service))
             {
                 completed.add(service);
             }
         }
         // Now remove those we handled.
         pendingLifecycleAwares.removeAll(completed);
     }
 
     /**
      * Notify a LifecycleAware if the system is started and the plugin publishing the service is enabled.
      *
      * @param service the service to attempt to start
      * @return true iff the service was started or is stale, false if it should be retained for later processing.
      */
     private boolean notifyLifecycleAwareIfStartedAndEnabled(final ServiceReference<LifecycleAware> service)
     {
         if (started)
         {
             final Bundle bundle = service.getBundle();
             final LifecycleAware lifecycleAware = bundleContext.getService(service);
             try
             {
                 // Either of bundle or lifecycleAware can be null if the service isn't registered.
                 if ((null != bundle) && (null != lifecycleAware))
                 {
                     final String pluginKey = getPluginKey(bundle);
                     if (pluginAccessor.isPluginEnabled(pluginKey))
                     {
                         try
                         {
                             log.debug("Calling LifecycleAware.onStart() '{}' from plugin '{}'", lifecycleAware, pluginKey);
                             lifecycleAware.onStart();
                         }
                         catch (final RuntimeException ex)
                         {
                             log.error("LifecycleAware.onStart() failed for component with class '{}' from plugin '{}'",
                                     new Object[] { lifecycleAware.getClass().getName(), pluginKey, ex });
                         }
                         // We have called onStart
                         return true;
                     }
                     else
                     {
                         // We have not called onStart and need to later (when the plugin enables)
                         return false;
                     }
                 }
                 else
                 {
                     // This can happen if we take our snapshot of services to notify, and then one is unregistered. In this case,
                     // likely both bundle and lifecycleAware are null, so there's not much we can get in the message here, and it means
                     // niceties like trying to safely dig data out of them is probably not worth a whole lot.
                     log.warn("Discarding onStart() for stale LifecycleAware");
                     // This reference is stale, so we report true as it can be discarded
                     return true;
                 }
             }
             finally
             {
                 if (null != lifecycleAware)
                 {
                     bundleContext.ungetService(service);
                 }
             }
         }
         else
         {
             // We have not called onStart and need to later (when the system is started)
             return false;
         }
     }
 
     private class LifecycleAwareServiceListener implements ServiceListener
     {
         /**
          * Handle a service changed by updating the list of pendingLifecycleAwares or dispatching onStart as necessary.
          *
          * @param serviceEvent details of the changed service
          */
         @Override
         public void serviceChanged(final ServiceEvent serviceEvent)
         {
             @SuppressWarnings("unchecked")
             // The cast here is safe since we filter by this object class when adding the service listener
             final ServiceReference<LifecycleAware> service = (ServiceReference<LifecycleAware>) serviceEvent.getServiceReference();
             switch (serviceEvent.getType())
             {
                 case ServiceEvent.REGISTERED:
                 {
                     if (!notifyLifecycleAwareIfStartedAndEnabled(service))
                     {
                         // We didn't satisfy notification conditions, so hang on to the service for later.
                         pendingLifecycleAwares.add(service);
                     }
                     // else we tried to onStart it (we're started and the plugin is enabled), or it's stale. Either way, we're done.
                     break;
                 }
                 case ServiceEvent.UNREGISTERING:
                 {
                     // It's possible we still have a reference to the service, for example if the plugin got as far as
                     // registering the service before failing, and hence never became enabled. See the comment on the
                     // pendingLifecycleAwares field for an explanation of why removing the ServiceReference directly is safe.
                     pendingLifecycleAwares.remove(service);
                     break;
                 }
                 default:
                 {
                     // We filter on objectClass, which org.osgi.framework.ServiceRegistration.setProperties() says cannot be
                     // changed dynamically, so we don't need to handle MODIFIED or MODIFIED_ENDMATCH.
                     break;
                 }
             }
         }
     }
 }