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https://issues.apache.org/jira/browse/AMQ-4160?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Stirling Chow reopened AMQ-4160:
--------------------------------
Regression: Unit Test Broken
Reopening as I've found two problems with the patch as applied:
+ // Should we try to connect to that URI?
+ if (activeEvents.putIfAbsent(uri, event) != null) {
+ LOG.debug("Discovery agent generated a duplicate onServiceAdd
event for: "+uri );
+ }
---> there should be a "return" statement after the LOG.debug
+ // Only remove bridge if this is the active discovery event for
the URL.
+ if (activeEvents.remove(url, event)) {
+ synchronized (bridges) {
+ bridges.remove(uri);
+ }
---> the activeEvents.remove(url, event)) should be activeEvents.remove(uri,
event) (i.e., uri instead of url)
The existing AMQ4160Test.java will not pass with these updates since it
demonstrates the original bug, which was based on concurrent attempts being
allowed to occur. The original patch was still allowing concurrent attempts.
With these updated changes, concurrent attempts are disallowed so
AMQ4160Test.java will fail --- I'll have to provide an updated unit test to
account for this behaviour.
> DiscoveryNetworkConnector can lose 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
> Assignee: Timothy Bish
> Priority: Critical
> Fix For: 5.8.0
>
> Attachments: AMQ4160.patch, AMQ4160Test.java
>
>
> 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.
> This patch is a band aid to the design flaws in
> {{DiscoveryNetworkConnector}}, and a refactoring of the connector should be
> considered. In particular, there is a tight-coupling between
> {{DiscoveryNetworkConnector}} and {{SimpleDiscoveryAgent}} that is not
> evident through their interfaces. For example, {{DiscoveryNetworkConnector}}
> assumes that the call to {{discoveryAgent.serviceFailed(...)}} will result in
> a call back to {{DiscoveryNetworkConnector.onServiceRemove(...)}}. The call
> to {{onServiceRemove(...)}} is necessary to clean up the resources used by
> the bridge, but that requirement is not explicit, and therefore easily missed.
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