Can I augment scm in Jenkinsfile?

It's taken me ages to understand what checkout scm really means in Jenkinsfile (checkout is a function and scm is a default global variable by the way).

Now that I've understood it, I want to augment scm for example to increase the timeout for a particular checkout or to set sparseCheckoutPaths. Is this possible? If so, how?

For Git, checkout scm is basically equivalent to :

checkout([
     $class: 'GitSCM',
     branches: scm.branches,
     doGenerateSubmoduleConfigurations: scm.doGenerateSubmoduleConfigurations,
     extensions: scm.extensions,
     userRemoteConfigs: scm.userRemoteConfigs
])

If you want to add sparse checkout to the existing scm, what you would do is something like:

checkout([
     $class: 'GitSCM',
     branches: scm.branches,
     doGenerateSubmoduleConfigurations: scm.doGenerateSubmoduleConfigurations,
     extensions: scm.extensions + [$class: 'SparseCheckoutPaths',  sparseCheckoutPaths:[[$class:'SparseCheckoutPath', path:'path/to/file.xml']]],
     userRemoteConfigs: scm.userRemoteConfigs
])

Even better, you can define a custom step, sparseCheckout in a shared library.

def call(scm, files) {
    if (scm.class.simpleName == 'GitSCM') {
        def filesAsPaths = files.collect {
            [path: it]
        }

        return checkout([$class                           : 'GitSCM',
                         branches                         : scm.branches,
                         doGenerateSubmoduleConfigurations: scm.doGenerateSubmoduleConfigurations,
                         extensions                       : scm.extensions +
                                 [[$class: 'SparseCheckoutPaths', sparseCheckoutPaths: filesAsPaths]],
                         submoduleCfg                     : scm.submoduleCfg,
                         userRemoteConfigs                : scm.userRemoteConfigs
        ])
    } else {
        // fallback to checkout everything by default
        return checkout(scm)
    }
}

Then you call it with:

sparseCheckout(scm, ['path/to/file.xml'])