Jenkins file diffs with declarative pipeline and multibranch - jenkins

Question

I have a scenario where I manage two pipelines within the same repo, with two different Jenkinsfiles.
I have set up two Jenkins multibranch pipelines to handle the two different Jenkinsfiles, by path discovery and set up github webhooks to trigger a build when a PR on particular branches is created.
I have not found a way to get changes in files for a particular PR so I thought of leveraging git by doing git diff --name-status origin/master...HEAD but it fails, since Jenkins only checks out the target branch.
The logstash:
using credential github-user-token-uname-pwd
Fetching changes from the remote Git repository
Fetching without tags
> git rev-parse --is-inside-work-tree # timeout=10
> git config remote.origin.url https://github.com/myreponame # timeout=10
Fetching upstream changes from https://github.com/myreponame
> git --version # timeout=10
using GIT_ASKPASS to set credentials Github token in uname-pwd form used by jenkins to register and manage webhooks
> git fetch --no-tags --force --progress https://github.com/myreponame +refs/heads/BRANCH-X:refs/remotes/origin/BRANCH-X
Checking out Revision 440df9b08667458fa4ade4be57ecbf59a4 (BRANCH-X)
Commit message: "move post build where it belongs"
> git config core.sparsecheckout # timeout=10
> git checkout -f 440df9b08667458fa4ade4be57ecbf59a4
> git rev-list --no-walk ab28e843c0fc7807c4cbd2d6f612e5d76b # timeout=10
[Pipeline] }
[Pipeline] // stage
[Pipeline] withEnv
[Pipeline] {
[Pipeline] withCredentials
Masking supported pattern matches of $SECRET_ACCESS_KEY or $ACCESS_KEY_ID
[Pipeline] {
[Pipeline] withEnv
[Pipeline] {
[Pipeline] stage
[Pipeline] sh
+ git diff --name-status origin/master...HEAD
and the error I get:
fatal: ambiguous argument 'origin/master...HEAD': unknown revision or path not in the working tree.
Is there a way to retrieve the files' changes of a PR within multibranch pipelines or get Jenkins to be able to discover the source branch of said PR?

Answer 1

You could try to fetch origin and then diff with FETCH_HEAD
{ git fetch origin master
git diff —name-status FETCH_HEAD...HEAD
}

Answer 2

So here is what I had to do.
The error I was facing was due to Jenkins default checkout behaviour with multibranch pipelines: it checks out and track a branch and that branch only, it does not fetch other branches.
I eventually added
options {
skipDefaultCheckout true
}
and a stage that checks out all the origin remote branches
stage('Checking out repo'){
steps {
script {
checkout(
[
$class: 'GitSCM',
branches: [[name: 'origin/FEATUREBRANCH*']],
doGenerateSubmoduleConfigurations: false,
extensions: [[$class: 'LocalBranch'],
[$class: 'CleanBeforeCheckout']],
submoduleCfg: [],
userRemoteConfigs: [[credentialsId: 'github-deploy-key',
url: 'git#github.com:myawesomerepo.git']]
]
)
}
}
}