[activemq] branch activemq-5.16.x updated: AMQ-6660 - Use a separate lock for lazy scheduler creation to prevent deadlocks

[cs...@apache.org]

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cshannon pushed a commit to branch activemq-5.16.x
in repository https://gitbox.apache.org/repos/asf/activemq.git


The following commit(s) were added to refs/heads/activemq-5.16.x by this push:
     new dc786ed  AMQ-6660 - Use a separate lock for lazy scheduler creation to prevent deadlocks
dc786ed is described below

commit dc786edeccb55cc16facb73b48466b3ef55263fa
Author: Christopher L. Shannon (cshannon) <ch...@gmail.com>
AuthorDate: Fri Jun 25 10:54:36 2021 -0400

    AMQ-6660 - Use a separate lock for lazy scheduler creation to prevent
    deadlocks
    
    Switch to using a dedicated lock to initialize the Scheduler object
    inside of ActiveMQConnection to prevent a deadlock scenario that occurs
    when using the intrinsic ActiveMQConnection lock. This is using double
    checked locking so this lock will only be contended with during lazy
    creation and then will rely on the volatile reference for future reads
    and when stopping/closing.
    
    (cherry picked from commit 3f5abd4433d3dca911ac418d899346e7dd675b7b)
---
 .../src/main/java/org/apache/activemq/ActiveMQConnection.java          | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java b/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java
index 6d8ce63..5d11fb6 100644
--- a/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java
+++ b/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java
@@ -200,6 +200,7 @@ public class ActiveMQConnection implements Connection, TopicConnection, QueueCon
     protected AtomicInteger transportInterruptionProcessingComplete = new AtomicInteger(0);
     private long consumerFailoverRedeliveryWaitPeriod;
     private volatile Scheduler scheduler;
+    private final Object schedulerLock = new Object();
     private boolean messagePrioritySupported = false;
     private boolean transactedIndividualAck = false;
     private boolean nonBlockingRedelivery = false;
@@ -2383,7 +2384,7 @@ public class ActiveMQConnection implements Connection, TopicConnection, QueueCon
                 // without lock contention report the closing state
                 throw new ConnectionClosedException();
             }
-            synchronized (this) {
+            synchronized (schedulerLock) {
                 result = scheduler;
                 if (result == null) {
                     checkClosed();