MySQL Bugs: #97730: xdev api: ConcurrentModificationException at Session.close

Bug #97730	xdev api: ConcurrentModificationException at Session.close
Submitted:	21 Nov 2019 11:15	Modified:	28 May 2021 16:11
Reporter:	Bernd Wahlen	Email Updates:
Status:	Closed	Impact on me:	None
Category:	Connector / J	Severity:	S3 (Non-critical)
Version:	8.0.18	OS:	Linux
Assigned to:	Filipe Silva	CPU Architecture:	x86

Description:
It looks like session map is not Thread safe:

java.util.ConcurrentModificationException (pool-13-thread-4)
java.util.ConcurrentModificationException: null
        at java.util.HashMap$HashIterator.nextNode(HashMap.java:1495) ~[?:?]
        at java.util.HashMap$KeyIterator.next(HashMap.java:1518) ~[?:?]
        at com.mysql.cj.xdevapi.ClientImpl.idleProtocol(ClientImpl.java:345) ~[mysql-connector-java-8.0.18.jar:8.0.18]
        at com.mysql.cj.xdevapi.ClientImpl$PooledXProtocol.close(ClientImpl.java:376) ~[mysql-connector-java-8.0.18.jar:8.0.18]
        at com.mysql.cj.MysqlxSession.quit(MysqlxSession.java:90) ~[mysql-connector-java-8.0.18.jar:?]
        at com.mysql.cj.xdevapi.SessionImpl.close(SessionImpl.java:217) ~[mysql-connector-java-8.0.18.jar:8.0.18]

I have to syncronize it myself outside like this:
    private final Object sessionMapLock = new Object();

    public Session getSession() {
        synchronized (sessionMapLock) {
            return client.getSession();
        }
    }

    public void closeSession(Session s) {
        synchronized (sessionMapLock) {
            s.close();
        }
    }

How to repeat:
use ThreadPoolExecutor and run multiple small Operations which open&closes many sessions.

Suggested fix:
use ConcurrentHashMap intead of HashMap

Hi Bernd,

Thanks for this bug report and your interest in Connector/J.

You are right. It seems we missed a spot in the synchronization here.

Bug#99102 is a duplicate of this bug report.

Posted by developer:
 
Added the following entry to the C/J 8.0.24 changelog: 

" Concurrently getting and closing multiple sessions from the same X DevAPI Client object might result in a ConcurrentModificationException thrown by Connector/J at the closing of a session."