[JENKINS-72107] Introduce Loadable interface

[Vlatombe]

CloudBees CI uses load() method to reload selectively parts of the Jenkins object model from disk, without reloading everything.

This formalizes the interface that gets used for this purpose, even if most of the Saveable implementations already had a load() method by convention.

I could not find any use case in the community that could rely on this mecanism. For example, configuration as code uses data binding from casc configuration to live objects, but doesn't need to interact with disk.

See JENKINS-72107.

Testing done

Proposed changelog entries

• Developer: Formalize an interface for objects that can be loaded from disk.

Proposed upgrade guidelines

N/A

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[jtnord]

as this is opening up the load could it be the case that a plugin has made a previous assumption that this would only be called once at Jenkins startup. Also that it may not be thread safe?

Should this be synchronized or have you checked for classes that overload this, or made those synchronized where they need to be (there may be none, given this is deprecated)

[Vlatombe]

I could find the following OSS overloads.

• https://github.com/jenkinsci/global-build-stats-plugin/blob/9867e364d6719cc1e01fe974d3499e29733cf138/src/main/java/hudson/plugins/global_build_stats/GlobalBuildStatsPlugin.java#L47 makes it public
• https://github.com/jenkinsci/docker-traceability-plugin/blob/9d197cc6da0e641e78944441f14ce022f244f550/docker-traceability-plugin/src/main/java/org/jenkinsci/plugins/docker/traceability/DockerTraceabilityPlugin.java#L84-L90 does seem like useless
• https://github.com/jenkinsci/jira-issues-versioning-plugin/blob/73d4a878fcb0bf62c1fd7f75a59dd64f23432be8/hudson-plugin/src/main/java/org/jvnet/hudson/plugins/jira/issueversioning/plugin/hudson/PluginImpl.java#L14 useless

There doesn't seem to be an intensive usage of it (thankfully). Make it synchronized as well as save() could be considered (same as Descriptor) though there seems to be a general point to synchronizing configuration change whether in configure(..) load() or save()

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!