Build job when certain files have been updated - jenkins

Question

I am writing a Jenkinsfile to do a number of checks on a repo. The check runs everytime a PR has been created. The goal of my check is to make sure that in my PR, 2 particularly files (a.txt) and (b.txt) have been changed.
The issue is that these 2 files always must be changed when a PR is issued and I dont want my developers to forget about updating these files. So I want to write a check that fails the build if these 2 files have not been updated. I tried something like this
stage('Check if file changed'){
when {
allOf {
changeset "a.txt"
changeset "b.txt"
}
}
steps {
// Do something
}
}
But the issue with this is that it wants the change to happen in the commit. A developer could create a PR with one file updated and then add another commit to update the other file but this check will always want both files updated in the latest commit. Can someone suggest a way for me to accomplish what I need ? Also in the above case, it doest fail the build if a file is not updated. It just skips the stage. I want the build to fail. Any suggestions ?
Locally running gives me the following output
git diff --name-status master
M CHANGELOG.md
M Jenkinsfile
I want to use this in my jenkins build but there I get the error
+ git diff --name-status master
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
script returned exit code 128

Answer 1

The pattern you have used in allOf{} inside when{} DSL is wrong. It would be something like this
pipeline {
agent any;
stages {
stage('build') {
when {
allOf{
changeset "README.md"; changeset "README-one.md"
}
}
steps {
echo "from build"
}
}
}
}
note : both changeset has to be seperate with ; not , or space. I can see In Jenkins documentation they have mention , - might be thats a mistake