Allow plugins to override boot failure view

[timja]

See JENKINS-XXXXX.

Currently it does not seem possible to provide your own BootFailure subclass for a startup error, Jenkins just displays a huge stacktrace with an error and you can't provide a nice error.

Before

After

Testing done

Manually with jenkinsci/configuration-as-code-plugin#2352

Created a file called jenkins.yaml in the work directory with these contents:

invalid:
  systemMessage2: Hello World

I have not been able to figure out how to write a unit test for this, at least with JTH the process is destroyed forcefully if Jenkins does not start and there's no way to get the information.

Proposed changelog entries

• BootFailure subclasses can now override the Jenkins failed to startup page

Proposed upgrade guidelines

N/A

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[timja]

@jenkinsci/core anyone mind taking a look please?

[jglick]

with JTH the process is destroyed forcefully if Jenkins does not start and there's no way to get the information

Might be a good addition to RealJenkinsRule: a variant of startJenkins that does not wait for the ping endpoint to response, letting the test code use e.g. HtmlUnit to query the Jenkins UI as it is.

[jglick]

Logic could probably be simplified, but looks OK as it is.

[jglick]

Could be simplified to return @CheckForNull BootFailure.

[timja]

with JTH the process is destroyed forcefully if Jenkins does not start and there's no way to get the information

Might be a good addition to RealJenkinsRule: a variant of startJenkins that does not wait for the ping endpoint to response, letting the test code use e.g. HtmlUnit to query the Jenkins UI as it is.

maybe but the use-case seems very limited.

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/label ready-for-merge

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

Thanks!