Register OptionHandlers through META-INF/services/annotations and Annotation Indexer rather than META-INF/services and Commons Discovery

[basil]

CLICommand has two mechanisms for registering OptionHandlers:

1. jenkins/core/src/main/java/hudson/cli/CLICommand.java

   Lines 530 to 542 in ab2ae8b

   	/**
   	* Auto-discovers {@link OptionHandler}s and add them to the given command line parser.
   	*/
   	protected void registerOptionHandlers() {
   	try {
   	for (Class c : Index.list(OptionHandlerExtension.class, Jenkins.get().pluginManager.uberClassLoader, Class.class)) {
   	Type t = Types.getBaseClass(c, OptionHandler.class);
   	CmdLineParser.registerHandler(Types.erasure(Types.getTypeArgument(t, 0)), c);
   	}
   	} catch (IOException e) {
   	throw new Error(e);
   	}
   	}

   which uses the Annotation Indexer to index (based on an @OptionHandlerExtension annotation) and load the handlers.

2. jenkins/core/src/main/java/hudson/cli/CLICommand.java

   Lines 564 to 582 in 6d17999

   	static {
   	// register option handlers that are defined
   	ClassLoaders cls = new ClassLoaders();
   	Jenkins j = Jenkins.getInstanceOrNull();
   	if (j != null) { // only when running on the controller
   	cls.put(j.getPluginManager().uberClassLoader);
   	ResourceNameIterator servicesIter =
   	new DiscoverServiceNames(cls).findResourceNames(OptionHandler.class.getName());
   	final ResourceClassIterator itr =
   	new DiscoverClasses(cls).findResourceClasses(servicesIter);
   	while (itr.hasNext()) {
   	Class h = itr.nextResourceClass().loadClass();
   	Class c = Types.erasure(Types.getTypeArgument(Types.getBaseClass(h, OptionHandler.class), 0));
   	CmdLineParser.registerHandler(c, h);
   	}
   	}
   	}

   which uses a combination of Kohsuke's META-INF/services generator to index (based on a @MetaInfServices(OptionHandler.class) annotation) the handlers and Commons Discovery to load the handlers.

There is a desire to reduce and eventually remove usages of Commons Discovery, since it is an abandoned and unmaintained library. The second of these two code paths cannot be replaced with ServiceLoader, since ServiceLoader insists on having a zero-argument public constructor for each service, and OptionHandler violates this contract. So if getting rid of Commons Discovery is desired, then the second code path must be deprecated in favor of the first. Once all usages of the second code path in core and plugins have been converted to the first code path, and once those releases have been sufficiently deployed, then the second code path can be deleted.

This PR migrates from the second pattern to the first pattern in Jenkins core. I am aware of two cases where the second pattern is used in plugins:

• https://github.com/jenkinsci/credentials-plugin/blob/f51f5d5b0ef3ad8f7a3701d1fb4bdb095cee2c4f/src/main/java/com/cloudbees/plugins/credentials/cli/CredentialsStoreOptionHandler.java#L41
• https://github.com/jenkinsci/purge-job-history-plugin/blob/edaa4d430d7d9359ea4fa1376a3abb212eb1ae8a/src/main/java/jenkins/plugins/purgejobhistory/JobOptionHandler.java#L35

I am not aware of any usages in proprietary plugins.

Implementation

The second code path was being invoked at class loading time (a single time) and loading the bulk of the handlers in practice there. The first code path was being invoked when each CLI command was invoked, which was simultaneously too frequent (a handler only needs to be registered once) and too late: HelpCommandTest started failing because we could not show the help until the handler had been loaded, and the handler was not loaded until the command was first invoked. We solved the latter problem by unifying the invocation of these two code paths from the same place: initialization of the class. In practice, this means that the existing handlers are loaded after this PR at the same time (and frequency) as they were before this PR, minimizing regressions and increasing consistency.

Testing done

mvn clean verify -Dtest=hudson.cli.AddJobToViewCommandTest,hudson.cli.BuildCommandTest,hudson.cli.CancelQuietDownCommandTest,hudson.cli.ClearQueueCommandTest,hudson.cli.ComputerStateTest,hudson.cli.ConnectNodeCommandTest,hudson.cli.ConsoleCommandTest,hudson.cli.CopyJobCommandTest,hudson.cli.CreateJobCommandTest,hudson.cli.CreateNodeCommandTest,hudson.cli.CreateViewCommandTest,hudson.cli.DeleteBuildsCommandTest,hudson.cli.DeleteJobCommandTest,hudson.cli.DeleteNodeCommandTest,hudson.cli.DeleteViewCommandTest,hudson.cli.DisablePluginCommandTest,hudson.cli.DisconnectNodeCommandTest,hudson.cli.EnableJobCommandTest,hudson.cli.EnablePluginCommandTest,hudson.cli.GetJobCommandTest,hudson.cli.GetNodeCommandTest,hudson.cli.GetViewCommandTest,hudson.cli.GroovyshCommandTest,hudson.cli.HelpCommandTest,hudson.cli.InstallPluginCommandTest,hudson.cli.ListJobsCommandTest,hudson.cli.ListPluginsCommandTest,hudson.cli.OfflineNodeCommandTest,hudson.cli.OnlineNodeCommandTest,hudson.cli.QuietDownCommandTest,hudson.cli.ReloadConfigurationCommandTest,hudson.cli.ReloadJobCommandTest,hudson.cli.RemoveJobFromViewCommandTest,hudson.cli.RunRangeCommand2Test,hudson.cli.RunRangeCommandTest,hudson.cli.SetBuildDescriptionCommandTest,hudson.cli.SetBuildDisplayNameCommandTest,hudson.cli.UpdateNodeCommandTest,hudson.cli.UpdateViewCommandTest,hudson.cli.ViewManipulationTestBase,hudson.cli.WaitNodeOfflineCommandTest,hudson.cli.WaitNodeOnlineCommandTest,hudson.model.ComputerSetTest,hudson.model.ItemsTest,hudson.model.listeners.ItemListenerTest,hudson.util.RobustReflectionConverterTest,jenkins.cli.StopBuildsCommandTest,jenkins.model.JenkinsManagePermissionTest,jenkins.model.NodeListenerTest,jenkins.model.RunIdMigratorTest,jenkins.model.ScriptListenerTest,jenkins.security.Security3314Test,jenkins.security.Security3448Test,lib.form.PasswordTest

Proposed changelog entries

N/A

Proposed upgrade guidelines

N/A

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[basil]

Moved from lines 535-537, so please do not review this as original code authored by me.

[basil]

Answer: it is actually necessary if we want to remove Commons Discovery someday, as ServiceLoader cannot work with the OptionHandler classes that have no public zero-argument constructor.