NIFI-9688 Improve Logback shutdown handling

[exceptionfactory]

Description of PR

NIFI-9688 Improves Logback shutdown handling, avoiding premature closure of the Logback context during application shutdown, and enabling log messages to be written during application termination.

The default LogbackServletContainerInitializer registers a LogbackServletContainerListener that closes the Logback context on web application shutdown. When Jetty registers NiFi web applications, it configures the Logback initializer using the standard Java ServiceLoader. With this behavior, Jetty calls contextDestroyed on the Logback Listener before NiFi termination has completed. This results is missing a number of log messages when shutting down NiFi.

Setting the logbackDisableServletContainerInitializer environment variable to true disables the default Logback listener registration, avoiding the premature context shutdown. Adding the DelayingShutdownHook to the logback.xml configuration allows Logback to perform a clean shutdown after NiFi termination has finished.

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[greyp9]

These changes look good. I wanted to document that the use of nifi.sh run and Ctrl+C may not take advantage of this improvement. nifi.sh start and nifi.sh stop do show the shutdown logging. Also tested an invocation of one of the system tests (with a modified bootstrap.conf).

[greyp9]

It might be good to document alternatives to the use of the environment variable to activate this behavior. I noticed this issue in the context of working on the system tests. There is a bootstrap.conf available to set system properties, for example.

[exceptionfactory]

Thanks for feedback @greyp9! The Logback documentation describes the configuration options under the WebShutdownHook section. Will update this pull request to include changes to bootstrap.conf in the system tests.

[markap14]

Thanks @exceptionfactory ! Definitely a great catch. I was really confused just the other day because i wasn't seeing some logs that I expected on shutdown. And then I found this Jira. Thanks for fixing!