[BROOKLYN-285] Dangling reference in Propagator.producer: can cause subscription to self, and infinite subscription loop

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.9.0
• Fix Version/s: 0.10.0
• Assignee: Unassigned
• Reporter: Aled Sage
• Created: 31/May/16 08:10
• Updated: 31/May/16 08:30
• Resolved: 31/May/16 08:30

Description

When the "producer" entity of a Propagator enricher is unmanaged, it can cause rebind to resolve the dangling reference value as null. In 0.9.0, that is treated as the producer being "self". This can cause an infinite loop, where the Propagator subscribes to its own sensor, and every time its sensor changes it resets the sensor value.

To reproduce, deploy this app:

location: localhost
services:
- type: org.apache.brooklyn.entity.stock.BasicApplication
  brooklyn.children:
  - type: org.apache.brooklyn.entity.machine.MachineEntity

And programmatically add a Propagator enricher to the top-level app, such as:

appId = "u4VDoIOa";
childId = "LPYT4g27";
sensor = org.apache.brooklyn.core.sensor.Sensors.newSensor(String.class, "host.name");
app = mgmt.getEntityManager().getEntity(appId);
child = mgmt.getEntityManager().getEntity(childId);
app.enrichers().add(org.apache.brooklyn.api.sensor.EnricherSpec.create(org.apache.brooklyn.enricher.stock.Propagator.class)
    .configure("enricher.producer", child)
    .configure("enricher.propagating.inclusions", com.google.common.collect.ImmutableList.of(sensor)));

Then unmanange the child entity (e.g. via the advanced tab in the web-console), and restart Brooklyn.

The CPU goes through the roof, and the debug log shows log messages like:

2016-05-31 09:02:22,599 DEBUG o.a.b.c.m.i.LocalSubscriptionManager [brooklyn-execmanager-gfOvVoKq-3]: 388000 events for subscriber Subscription[qF9YFEBA;BasicApplicationImpl{id=u4VDoIOa}@u4VDoIOa:host.name]