Improve the inheritFrom functionality to better cover containers and volumes.

[iocanel]

This pull request allows a PodTemplate to define a parent template that will be used as a base.

This allows the user to remove repeating/boilerplate code from pipeline and move it in the plugin configuration.

The main benefits are reusability (e.g. don't add the jnlp container everywhere), readability, and more importantly decoupling the pipelines from specific configurations, that might change between environments.

For example: There are environments that may or may not allow the use of hostPath mounts, privileged containers etc. So instead of maintaining two different sets of Jenkinsfile inside a project one could just move those environment sensitive details in the parent pod, that could even be defined inside the jenkins docker image.

[iocanel]

Apparently, this is dublicating existing code....

[carlossg]

Added some comments and needs rebase

[carlossg]

typo ARGUMETS

[carlossg]

I think this needs some help text in help-inheritFrom.html, and some docs in the README

[iocanel]

@carlossg: I will apply your comments, rebase and squash.

Since, I added this pull request I've been playing a lot with this feature and I am starting to think that it would make sense to be able to inheritFrom multiple parents. So that I can easily compose a pod out of prexisting templates (I think of it as maven profiles). So one could define in the config.xml a template for 'maven', 'golang' etc and then just use that. So the example we have at the readme could then look like:

podTemplate(label: 'mypod', inheritFrom: ['maven', 'golang'],
volumes: [secretVolume(secretName: 'shared-secrets', mountPath: '/etc/shared-secrets')]) {
node ('mypod') {
    stage 'Get a Maven project'
    git 'https://github.com/jenkinsci/kubernetes-plugin.git'
    container('maven') {
        stage 'Build a Maven project'
        sh 'mvn clean install'
    }

    stage 'Get a Golang project'
    git url: 'https://github.com/hashicorp/terraform.git'
    container('golang') {
        stage 'Build a Go project'
        sh """
        mkdir -p /go/src/github.com/hashicorp
        ln -s `pwd` /go/src/github.com/hashicorp/terraform
        cd /go/src/github.com/hashicorp/terraform && make core-dev
        """
    }
  }
}

Would you like to add that feature too?

[carlossg]

wouldn't that be more like

containers: [ 'maven', 'golang' ] 

or something like that?
because is the pod inheriting another pod or each container template?

[iocanel]

The problem with using something like: containers: [ 'maven', 'golang' ] is that you can just reference a containerTemplate by name, since its not an independent entity (they are always nested under podTemplates).

Of course, this is something we could change if we wanted, but by inheriting podTemplates also allows us to inherit volumes and I can see that being useful too.

[carlossg]

right, so we'd need a description somewhere about what gets inherited from podTemplate inheritFrom and what's the precedence (volumes, containers,...)

[iocanel]

Fixed typo, added docs, rebased, squashed and added support for multiple parent pod template via inheritFrom.