ConcurrentModificationException in Datafeed loop

[symphony-youri]

Bug Report

A ConcurrentModificationException can happen in the Datafeed (DF) loop upon startup.

The access to the listeners collection in the DF loop is not protected and accessed by multiple threads. Synchronizing the list should help to that matter.

Steps to Reproduce:

This is not always happening but restarting a bot using the DF loop and receiving events while starting should raise the issue.

Expected Result:

No exception.

Actual Result:

2021-03-15T17:28:38,530Z DEBUG [SymphonyBdk_DatafeedThread-111] c.s.b.c.r.RetryWithRecovery : Exception recovered
java.util.ConcurrentModificationException: null
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911) ~[?:1.8.0_272]
	at java.util.ArrayList$Itr.next(ArrayList.java:861) ~[?:1.8.0_272]
	at com.symphony.bdk.core.service.datafeed.impl.AbstractDatafeedLoop.handleV4EventList(AbstractDatafeedLoop.java:82) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.core.service.datafeed.impl.DatafeedLoopV1.readAndHandleEvents(DatafeedLoopV1.java:109) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.core.retry.RetryWithRecovery.executeOnce(RetryWithRecovery.java:95) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at io.github.resilience4j.retry.Retry.lambda$decorateCheckedSupplier$3f69f149$1(Retry.java:137) ~[resilience4j-retry-1.5.0.jar!/:1.5.0]
	at io.github.resilience4j.retry.Retry.executeCheckedSupplier(Retry.java:419) ~[resilience4j-retry-1.5.0.jar!/:1.5.0]
	at com.symphony.bdk.core.retry.resilience4j.Resilience4jRetryWithRecovery.execute(Resilience4jRetryWithRecovery.java:65) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.core.service.datafeed.impl.DatafeedLoopV1.readDatafeed(DatafeedLoopV1.java:121) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.core.service.datafeed.impl.DatafeedLoopV1.start(DatafeedLoopV1.java:86) ~[symphony-bdk-core-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.spring.service.DatafeedAsyncLauncherService.uncheckedStart(DatafeedAsyncLauncherService.java:88) ~[symphony-bdk-core-spring-boot-starter-2.1.1.jar!/:2.1.1]
	at com.symphony.bdk.http.api.tracing.MDCUtils$MdcRunnable.run(MDCUtils.java:59) [symphony-bdk-http-api-2.1.1.jar!/:2.1.1]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_272]