Hello,

Would be nice to add a when conditions to pipeline post actions. Here is an example:

pipeline {
    agent any;
    stages { ... }
    // run only for master branch
    post {
        when { branch 'master' }
        failure {
            // send slack notification about failure build
        }
    }
    // run without any additional conditions (it works now)
    post {
        success { ... }
        cleanup { ... }
    }
}

I don't have a good idea how to implement this without breaking-changes. In this example
there is an idea that we can add multiple pipeline post blocks (one for every when condition)

Motivation:
I would like to send notification about failed builds, but only for releases braches. We don't want to receive notifications about failed pull requests builds.