Jenkins’ pipelines represent a totally different direction from traditional, script-based jobs. Instead of specifying your SCM configuration and other build semantics in your job, you mostly script them out via a pipeline (“Jenkinsfile”) file, which is a heterogeneous script/declarative mess. Although you can be purely declarative, this is sometimes too strict to be useful, e.g. not being able to have traditional variable assignments in order to pass information between steps. Even though there are drawbacks, your whole workflow is largely version-controlled.

One of the drawbacks is the complexity of managing shared-library dependencies that you might have in order to make some of your Java/Groovy logic reusable. You can define these in your project (or, the case of multibranch pipelines, the folder) or at the admin level. You can also define these on the fly in the code.

Gerrit change-requests are applied essentially by fetching on a pseudo-refspec location (refs/changes/), and then cherry-picking it in. Therefore, in order to use one, you need to 1) clone, 2) fetch, and 3) either cherry-pick or checkout (or a couple of other methods). Although you can do this with a little effort with your actual Jenkinsfile (which is configured in the job; you can take the refspec from the environment during a verification and then use “FETCH_HEAD” as your branch), these are not intuitively available for the shared-libraries that you might be importing into your pipeline.

It turns out that you can massage the on-the-fly library loader to do this for you.

if (env.GERRIT_PATCHSET_REVISION) {
  echo("Using shared-library for verification.")

  library([
    identifier: 'myLibrary@' + env.GERRIT_PATCHSET_REVISION,
    retriever: modernSCM([
      $class: 'GitSCMSource',
      remote: 'https://repo.host/pipeline/library',
      traits: [
        [$class: 'jenkins.plugins.git.traits.BranchDiscoveryTrait'],
        [
          $class: 'RefSpecsSCMSourceTrait',
          templates: [
            [value: '+refs/heads/*:refs/remotes/@{remote}/*'], 
            [value: "+refs/changes/*:refs/remotes/@{remote}/*"]
          ]
        ]
      ]
    ])
  ])
} else {
  echo("Using shared-library from branch (not a verification).")

  library("myLibrary@" + env.BRANCH_NAME)
}

The principal things to notice are:

1. We are telling it to bring all of the change-requests into scope (“+refs/changes/:refs/remotes/@{remote}/“).
2. We are telling Jenkins to import exactly the library version tied to the change (“‘myLibrary@’ + env.GERRIT_PATCHSET_REVISION”). This wouldn’t be accessible without (1).

It works great.

I generated the original version of the code by using the Snippet Generator with the “library” step and then modifying according to the above.

Note that this pipeline can be used both in a multibranch pipeline job context as well as in the normal [single-branch] pipeline job used for verification (because we would only want to kick-off verification jobs just for the branch of the change). env.BRANCH_NAME will automatically be defined in the multibranch context.