launcher: support system java when it is a shim

[yaauie]

Release notes

• [rn:skip] does not affect bundled-JDK artifacts

What does this PR do?

Help the vendored jruby.sh loader to correctly resolve a JAVA_HOME from the JAVACMD we provide it by asking java what its home is when the one on our path isn't inside of a directory structure that looks like a valid java home.

This enables our launchers to work with a "system java" that is actually a shim like the one maintained by jenv, while still trusting a resolved java executable that is inside of an approprtiately-shaped java home.

This has no affect on bundled-JDK artifacts, since the presence of a bundled JDK preempts our attempts to use a system JDK.

Why is it important/What is the impact to the user?

As a dev, I use jenv to manage my javas, and in local dev entrypoints like bin/logstash-plugin lose track of the java version declared in the project's .java-version and fall through to my system default, which is unsupported.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
  - [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

1. ensure command -v java resolves to a jenv shim (e.g., ~/.jenv/shims/java)
2. ensure that the global jenv resolves to an unsupported java (like my OS's default 1.8)
3. invoke bin/logstash-plugin list --verbose
4. observe that it actually works with this patch, or fails with Detected Java version: 1.8 without this patch

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @yaauie? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[infra-vault-gh-plugin-prod]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: bf1b54f

Failed CI Steps

• :rspec: Ruby unit tests
• 🥼 x-pack integration

History

• 💛 Build #5200 was flaky be0fc5a