Stirling Chow created AMQ-4160:
----------------------------------
Summary: DiscoveryNetworkConnector can loss track of active
bridges, resulting in permanent bridge failure or continued attempts to
re-connect existing bridges
Key: AMQ-4160
URL: https://issues.apache.org/jira/browse/AMQ-4160
Project: ActiveMQ
Issue Type: Bug
Reporter: Stirling Chow
Priority: Critical
Fix For: 5.8.0
Symptom
=======
{{DiscoveryNetworkConnector}} is not thread-safe, and as a result, race
conditions allow the {{bridges}} data structure to become corrupt and not
accurately represent the bridges that exist.
Because {{bridges}} is used to control whether a discovery event will result in
a bridge creation attempt, if it is not accurate, two results are possible:
# A discovery event will be ignored even though its corresponding bridge is not
active; as a result, the bridge will never be established
# A discovery event will be processed even though its corresponding bridge is
active; as a result, the bridge creation will fail (because of duplicate
connections), and be indefinitely retried, generating many WARN/ERROR log
messages
Cause
=====
{{DiscoveryNetworkConnector}} updates a {{bridges}} hashmap whenever a bridge
is created or removed:
{code:title=DiscoveryNetworkConnector.java}
public void onServiceAdd(DiscoveryEvent event) {
...
// Should we try to connect to that URI?
synchronized (bridges) {
if( bridges.containsKey(uri) ) {
if (LOG.isDebugEnabled()) {
LOG.debug("Discovery agent generated a duplicate
onServiceAdd event for: "+uri );
}
return;
}
}
...
NetworkBridge bridge = createBridge(localTransport, remoteTransport,
event);
try {
bridge.start();
synchronized (bridges) {
bridges.put(uri, bridge);
}
...
}
public void onServiceRemove(DiscoveryEvent event) {
String url = event.getServiceName();
if (url != null) {
URI uri;
try {
uri = new URI(url);
} catch (URISyntaxException e) {
LOG.warn("Could not connect to remote URI: " + url + " due to bad
URI syntax: " + e, e);
return;
}
synchronized (bridges) {
bridges.remove(uri);
}
}
}
{code}
There are a number of problems:
# The check-and-set for adding an entry {{bridges}} is not atomic. As a
result, concurrent attempts to add a bridge to the same URL can be allowed to
proceed to bridge creation. Only one of the bridges will be established (the
other will fail when it attempts to add duplicate connections); however, the
failure of the second bridge will result in a call to {{onServiceRemove(...)}}
that will remove the single/shared {{bridges}} entry.
# The bridge is started before an entry is added to {{bridges}}. Since bridges
are multi-threaded, it is possible that an exception will be handled by a
different thread at some time between {{bridge.start()}} and {{bridges.put(uri,
bridge}}. In processing this exception, the thread will call
{{onServiceRemove(...}}, which will remove the (non-existent) {{bridges}}
entry. Subseqently, {{bridges.put(uri, bridge)}} is executed, and adds the
entry to {{bridges}} even though it is now stale and represents a failed
bridge. Subsequent attempts to restore the bridge will be ignored, and no
active bridge will be created.
The lack of thread-safety in {{DiscoveryNetworkConnector}} is exacerbated by
AMQ-4159, which can result in many concurrent attempts to establish a bridge to
the same URL. AMQ-4159 also described how multiple calls can be made to
{{onServiceRemove(...)}}, which can result in mal-behaviour similar to the
second case described above (i.e., unexpected removal of a bridge that is
active).
Solution
========
The attached patch attempts to restore some thread-safety to
{{DiscoveryNetworkConnector}} by making the check-and-set atomic and adding the
entry to {{bridges}} prior to starting the bridge. An additional structure,
{{activeEvents}}, keeps track of the event that represents the current attempt
to establish a bridge to a given remote URL --- this is used to prevent
multiple calls to {{onServiceRemove(...)}} from removing a {{bridges}} entry
that was *not* added by the corresponding discovery event.
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