[JENKINS-51820] Removing Java Web Start support

[jglick]

See JENKINS-51820.

Amends #3766.

Preparation for dropping Java 8 support, since we did not support JWS on newer Java versions anyway.

• Changes to the agent JAR as noted in upstream [JENKINS-51820] Removing Java Web Start support remoting#532.
• Removing the JWS button from the GUI displayed on Java 8.
• Stripping out parts of jenkins-agent.jnlp which were interpreted by the real javaws but ignored by the java -jar agent.jar -jnlpUrl … parser.
• Removing agent installer modules as they only work in GUI ~ JWS mode, and among other concerns add overhead to every agent connection even when unused. These repositories could then be archived, or optionally redesigned to run as standalone tools (especially the Windows one which is the most likely to be used):
  • https://github.com/jenkinsci/slave-installer-module
  • https://github.com/jenkinsci/windows-slave-installer-module
  • https://github.com/jenkinsci/systemd-slave-installer-module
  • https://github.com/jenkinsci/launchd-slave-installer-module
  • https://github.com/jenkinsci/upstart-slave-installer-module
• Removing

  jenkins/core/src/main/resources/hudson/slaves/JNLPLauncher/help-vmargs.html

  Lines 2 to 4 in db4d8b4

  	If the agent JVM should be launched with additional VM arguments, such as "-Xmx256m",
  	specify those here. List of all the options are available
  	<a href="https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html">here</a>.

  since the only remaining usage was a prompt for you to run Java yourself:

  jenkins/core/src/main/resources/hudson/slaves/JNLPLauncher/main.jelly

  Line 41 in db4d8b4

  <pre>java${it.launcher.vmargs == null ? '' : ' ' + it.launcher.vmargs} -jar <a href="${rootURL}/jnlpJars/agent.jar">agent.jar</a> -jnlpUrl ${h.inferHudsonURL(request)}${it.url}jenkins-agent.jnlp ${it.launcher.getWorkDirOptions(it)}</pre>

  Basically the only remaining function of the vmargs field would have been as a “note to self” field where you can write down arguments you would want to pass next time you connect, if you are having a hard time remembering them or something. (Previously it actually controlled what JVM args javaws would pass to the agent. We cannot do that when you run java -jar agent.jar because the JVM is already started.)

Sanity tested on Linux using inbound TCP and WebSocket agents running simple builds, with and without security enabled, with both Java 8 and 11 for both controller and agent.

Proposed changelog entries

• Remove Java Web Start support for launching inbound agents, along with the GUI mode, the platform-specific agent installers and the JAR signature.

Proposed upgrade guidelines

Jenkins no longer supports attaching a static inbound agent by clicking the ☕ Launch button from an agent machine’s web browser when running the controller on Java 8, as Java Web Start has been removed from newer versions of most distributions. You should instead download the agent JAR file from the provided link and run the supplied command (java -jar agent.jar -jnlpUrl …) on the agent machine. The JVM options field was removed from inbound launcher configuration as it would no longer have any effect other than adjusting the suggested command.

The control window displayed when using a Java Web Start agent is also removed. Its main function was to be closed (now done by simply terminating the shell process). It also offered a menu with platform-specific agent installers that would offer to create system services to make the agent permanent; you can accomplish something similar in many ways without this GUI, according to your operating system.

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[basil]

Very nice! Presumably https://github.com/jenkinsci/netx can be archived once this PR is merged. I also created #6545 and jenkinsci/extras-executable-war#52 with some additional JNLP-related cleanup.

[jeffret-b]

Basically the only remaining function of the vmargs field is as a “note to self” field where you can write down arguments you would want to pass next time you connect, if you are having a hard time remembering them or something. (Previously it actually controlled what JVM args javaws would pass to the agent. We cannot do that when you run java -jar agent.jar because the JVM is already started.) Perhaps it would make sense to delete this now.

I would go ahead and delete this now.

[jeffret-b]

I am strongly in favor of this improvement. I tried a couple of times previously to push this removal through, but wasn't able then to overcome the resistance. There really is no reason to continue supporting JWS, especially as we move forward with Java versions.

This PR covers all of the different areas related to JWS that I can recall. It looks like it handles the changes correctly.

[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!