Introduce Mutation Testing quality check GitHub Action (#790)

* Introduce Mutation Testing quality check GitHub Action

This workflow will identify code within a PR which isn't fully
covered with tests using a process called Mutation testing.

This process isn't perfect, but hopefully the feedback it does
identify can start a conversation about where tests are missing.

* Remove config folder which crashes pitest

The pitest git integration tries to automatically find the git repo
root directory. In doing so, the presence of a directory called config
causes it to crash.

Author: Adrian Clay

.github/workflows/mutationtesting.yml

@@ -0,0 +1,26 @@
+name: Mutation Testing
+on:
+  pull_request:
+
+jobs:
+  pitest:
+    # Only run on PRs from the repo. PRs from forks will fail due to lack of permissions and
+    # must use a two stage process
+    if: github.event.pull_request.head.repo.full_name == github.repository
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout project
+        uses: actions/checkout@v4
+        with:
+          # important to set a fetch depth of 2. By default the checkout action make no history available
+          fetch-depth: 2
+      - name: Setup Java JDK
+        uses: actions/[email protected]
+        with:
+          distribution: 'temurin'
+          java-version: 21
+      - name: run pitest
+        working-directory: ./service
+        run: ./gradlew pitest-github
+        env:
+          REPO_TOKEN: ${{ secrets.GITHUB_TOKEN }}

arcmutate-licence.txt

@@ -0,0 +1,7 @@
+#Licence file for pitest extensions
+#Tue Jul 16 09:22:03 BST 2024
+expires=16/07/2026
+keyVersion=1
+packages=uk.nhs.adaptors.*
+signature=LGGLekLoKiQ4JOPs2iLFzr9Xm62elcZHhKwOAtcxXlU5ofB+aoEpRkSuQt/VH0kYi44eqeAGnkcrZ2knY5OMwu2YYUxySXb+J1IXvxLpQwXNy0L7DZuTz85w6RecvyfKXIYXD/DrS3gfq+o3jwCrWGqYYajVuwl5Q3ZqFGVYk1w\=
+type=OSSS

service/build.gradle

@@ -6,6 +6,10 @@ plugins {
 	id "com.github.spotbugs" version "6.0.20"
 	id "io.freefair.lombok" version "8.6"
 	id 'jacoco'
+
+	// Mutation testing
+	id 'info.solidsoft.pitest' version '1.15.0'
+	id 'com.arcmutate.github' version '1.2.1'
 }
 
 apply plugin: 'java'
@@ -21,6 +25,7 @@ sourceCompatibility = '21'
 
 checkstyle {
 	toolVersion '8.38'
+	configDirectory = 'checkstyle' as File
 }
 
 repositories {
@@ -66,6 +71,9 @@ dependencies {
 	testImplementation 'org.wiremock:wiremock-standalone:3.9.1'
 	testImplementation 'com.squareup.okhttp3:okhttp:4.12.0'
 	testImplementation 'com.squareup.okhttp3:mockwebserver:4.12.0'
+
+	pitest 'com.arcmutate:base:1.3.1'
+	pitest 'com.arcmutate:pitest-git-plugin:1.3.2'
 }
 
 test {
@@ -132,10 +140,23 @@ spotbugsMain {
 		xml {
 			enabled = true
 		}
-		excludeFilter.set(file("config/spotbugs/exclude.xml"))
+		excludeFilter.set(file("spotbugs/exclude.xml"))
 	}
 }
 
+pitest {
+	pitestVersion = '1.16.1'
+	junit5PluginVersion = '1.2.1'
+	outputFormats = ['gitci']
+
+	// git feature limits analysis to contents of PR only
+	features = ["+GIT(from[HEAD~1])"]
+
+	mutators = ['STRONGER', 'EXTENDED_ALL']
+
+	threads = project.getGradle().getStartParameter().getMaxWorkerCount()
+}
+
 bootJar {
 	exclude("**/TransformJsonToXml*")
 }