[SLING-7447] Race condition in ThreadLocalCleaner initialization code

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Fix Version/s: Commons Threads 3.2.16
• Component/s: Commons
• Assignee: Robert Munteanu
• Reporter: Robert Munteanu
• Created: 25/Jan/18 21:51
• Updated: 29/Jan/18 09:04
• Resolved: 25/Jan/18 22:25

Description

There is a race condition in the ThreadLocalCleaner initialization code that can lead to the fields not being fully initialized when read.

The stack trace is similar to

Exception in thread "sling-oak-observation-2" java.lang.NullPointerException
        at org.apache.sling.commons.threads.impl.ThreadLocalCleaner.copy(ThreadLocalCleaner.java:110)
        at org.apache.sling.commons.threads.impl.ThreadLocalCleaner.saveOldThreadLocals(ThreadLocalCleaner.java:230)
        at org.apache.sling.commons.threads.impl.ThreadLocalCleaner.<init>(ThreadLocalCleaner.java:155)
        at org.apache.sling.commons.threads.impl.ThreadPoolExecutorCleaningThreadLocals.beforeExecute(ThreadPoolExecutorCleaningThreadLocals.java:58)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1139)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:748)

Assume that two threads, T1 and T2 enter the constructor at more or less the same time. The following events can then occur, leading to the above invalid access

T1	T2
check threadLocalsField (is null)
invoke initReflectionFields, load threadLocalsField
	check threadLocalsField (is not null)
	invoke saveOldTreadLocals
	invoke copy

At this point, the tableField field is null since initReflectionFields has not yet completed, which leads to the above NPE.

edit: formatting

Issue Links

• is related to: SLING-7320 NPE in sling commons threads