[JENKINS-70558] Fix TopicAssociation not set after Jenkins restart

[ckreisl]

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The PR #481 introduced a small bug that the TopicAssociation field is not set after a Jenkins restart.

Description from Jenkins issue: https://issues.jenkins.io/browse/JENKINS-70558

In the job configuration if TopicAssociation is enabled and Jenkins is restarted the option is not set anymore.
The reason is that the readResolve() function checks the config value
private transient boolean enableTopicAssociation = Config.DEFAULT_ENABLE_TOPIC_ASSOCIATION;
(GerritTrigger.java) which is always false. Since this condition is always the case the topicAssociation value in the GerritTrigger class is set to "null".

[ckreisl]

I think that's the way how we want it. If the old build configuration xml is read it will set the value already and create the new TopicAssociation class.

[rsandell]

config.xml deserialization doesn't care about @DataBoundSetter et.al. Those are only relevant for form submission and CasC.

[rsandell]

You can write a test using JenkinsSessionRule or RealJenkinsRule to test that the configuration actually survives a restart.

[rsandell]

config.xml deserialization doesn't care about @DataBoundSetter et.al. Those are only relevant for form submission and CasC.

[ckreisl]

You can write a test using JenkinsSessionRule or RealJenkinsRule to test that the configuration actually survives a restart.

Thanks for the pointer, added a test now and removed the @DataBoundSetter.