[FLINK-23696][connectors/rabbitmq] Fix RMQSourceTest.testRedeliveredSessionIDsAck

[cmick]

What is the purpose of the change

Fix RMQSourceTest.testRedeliveredSessionIDsAck that fails on azure due to a race condition, described in FLINK-23696

Brief change log

Fix the unit test by moving the asserts into a synchronized code block.

Verifying this change

This change is already covered by existing tests.

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• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• c3683b9 Azure: SUCCESS

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Last check on commit 1fe63c9 (Mon Nov 08 00:36:52 UTC 2021)

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[cmick]

@flinkbot run azure

[fapaul]

Thanks for the quick response and fix. It looks reasonable to me but I was trying to run the test locally multiple with jUnit and it seems the test does not properly reset its state.

When running the test repeatedly only the first execution succeeds and all following fail because I guess some of the internal DummyContext state is not reset. This makes testing the hardening of the test difficult.

Can you also take a look at this?

[cmick]

@fapaul Thanks, You're absolutely right here. I've added resetting the DummyContext state in 01a81f7. Now it can be run multiple times - I've executed it locally a thousand times with a success (though the issue described here is more likely to occur on virtualized environments).

[fapaul]

I also ran the test a couple 1000 times locally and it always passed. LGTM 👍

Can you also create the backport to 1.14/1.13?

[cmick]

@fapaul Thanks for the review. Sure, backports added in #17740 and #17741

[fapaul]

@flinkbot run azure