ci: Cancel older builders on Jenkins

When a new build is triggered cancel any older builds to conserve
resources.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

Jenkinsfile

@@ -1,4 +1,17 @@
 pipeline{
+	agent none
+	stages {
+		stage ('Master build') {
+			agent { node { label 'master' } }
+			stages {
+				stage('Cancel older builds') {
+					steps {
+						cancelPreviousBuilds()
+					}
+				}
+			}
+		}
+		stage ('Worker build') {
 			agent { node { label 'bionic' } }
 			options {
 				timeout(time: 1, unit: 'HOURS')
@@ -44,3 +57,20 @@ pipeline{
 				}
 			}
 		}
+	}
+}
+
+def cancelPreviousBuilds() {
+	// Check for other instances of this particular build, cancel any that are older than the current one
+	def jobName = env.JOB_NAME
+		def currentBuildNumber = env.BUILD_NUMBER.toInteger()
+		def currentJob = Jenkins.instance.getItemByFullName(jobName)
+
+		// Loop through all instances of this particular job/branch
+		for (def build : currentJob.builds) {
+			if (build.isBuilding() && (build.number.toInteger() < currentBuildNumber)) {
+				echo "Older build still queued. Sending kill signal to build number: ${build.number}"
+				build.doStop()
+			}
+		}
+}