https://issues.jenkins-ci.org/browse/JENKINS-34268 was closed implementing the possibility to lock a resources using a label in pipelines.
But there is no syntax for knowing what the actually locked resource name is when using a pipeline and locking on a label.
Specifically the proposed syntax from https://github.com/jenkinsci/lockable-resources-plugin/pull/36 : ", variable: 'MY_VAR'" is missing.

Our use case is that we wan't to specify a label containing node names for legacy builders that can't run the slave.jar (java7) anymore. We want to lock a resource (host name) from the label and then ssh to that machine and execute some build steps. This works with the FreestyleJob functionality of "Reserved resources variable name".

Solution

It is now possible to access the locked resource via an configurable environment variable.

lock(label: 'some_resource', variable: 'MY_VAR') {
  echo env.MY_VAR
}