Report mutation test coverage of proposed changes (#346)

Thanks to a free Arcmutate OSS license, GitHub Actions now runs Pitest against
files changed in the context of a PR. 

While there, update Pitest's configuration to ignore Refaster rule collection
classes, as mutations of those classes will not impact the associated unit
tests.

See:
- https://www.arcmutate.com
- https://pitest.org

.github/workflows/pitest-analyze-pr.yml

@@ -0,0 +1,37 @@
+# Performs mutation testing analysis on the files changed by a pull request and
+# uploads the results. The associated PR is subsequently updated by the
+# `pitest-update-pr.yml` workflow. See https://blog.pitest.org/oss-pitest-pr/
+# for details.
+name: "Mutation testing"
+on:
+  pull_request:
+permissions:
+  contents: read
+jobs:
+  analyze-pr:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3.1.0
+        with:
+          fetch-depth: 2
+      - name: Set up JDK
+        uses: actions/setup-java@v3.6.0
+        with:
+          java-version: 17.0.4
+          distribution: temurin
+          cache: maven
+      - name: Run Pitest
+        # By running with features `+GIT(from[HEAD~1]), +gitci`, Pitest only
+        # analyzes lines changed in the associated pull request, as GitHub
+        # exposes the changes unique to the PR as a single commit on top of the
+        # target branch. See https://blog.pitest.org/pitest-pr-setup for
+        # details.
+        run: mvn test pitest:mutationCoverage -DargLine.xmx=2048m -Dverification.skip -Dfeatures="+GIT(from[HEAD~1]), +gitci"
+      - name: Aggregate Pitest reports
+        run: mvn pitest-git:aggregate -DkilledEmoji=":tada:" -DmutantEmoji=":zombie:" -DtrailingText="Mutation testing report by [Pitest](https://pitest.org/). Review any surviving mutants by inspecting the line comments under [_Files changed_](${{ github.event.number }}/files)."
+      - name: Upload Pitest reports as artifact
+        uses: actions/upload-artifact@v3.1.1
+        with:
+          name: pitest-reports
+          path: ./target/pit-reports-ci

.github/workflows/pitest-update-pr.yml

@@ -0,0 +1,35 @@
+# Updates a pull request based on the corresponding mutation testing analysis
+# performed by the `pitest-analyze-pr.yml` workflow. See
+# https://blog.pitest.org/oss-pitest-pr/ for details.
+name: "Mutation testing: post results"
+on:
+  workflow_run:
+    workflows: ["Mutation testing"]
+    types:
+      - completed
+permissions:
+  actions: read
+  checks: write
+  contents: read
+  pull-requests: write
+jobs:
+  update-pr:
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3.1.0
+      - name: Set up JDK
+        uses: actions/setup-java@v3.6.0
+        with:
+          java-version: 17.0.4
+          distribution: temurin
+          cache: maven
+      - name: Download Pitest analysis artifact
+        uses: dawidd6/action-download-artifact@v2.24.2
+        with:
+          workflow: ${{ github.event.workflow_run.workflow_id }}
+          name: pitest-reports
+          path: ./target/pit-reports-ci
+      - name: Update PR
+        run: mvn -DrepoToken="${{ secrets.GITHUB_TOKEN }}" pitest-github:updatePR

README.md

@@ -166,7 +166,7 @@ Some other commands one may find relevant:
 
 - `mvn fmt:format` formats the code using
   [`google-java-format`][google-java-format].
-- `./run-mutation-tests.sh` runs mutation tests using [PIT][pitest]. The
+- `./run-mutation-tests.sh` runs mutation tests using [Pitest][pitest]. The
   results can be reviewed by opening the respective
   `target/pit-reports/index.html` files. For more information check the [PIT
   Maven plugin][pitest-maven].

cdg-pitest-licence.txt

@@ -0,0 +1,7 @@
+# Arcmutate license for Error Prone Support, requested by sending an email to
+# support@arcmutate.com.
+expires=07/11/2023
+keyVersion=1
+signature=MhZxMbnO6UovNfllM0JuVWkZyvRT3/G5o/uT0Mm36c7200VpZNVu03gTAGivnl9W5RzvZhfpIHccuQ5ctjQkrqhsFSrl4fyqPqu3y5V2fsHIdFXP/G72EGj6Kay9ndLpaEHalqE0bEwxdnHMzEYq5y3O9vUPv8MhUl57xk+rvBo\=
+packages=tech.picnic.errorprone.*
+type=OSSS

pom.xml

@@ -157,6 +157,7 @@
         <version.mockito>4.9.0</version.mockito>
         <version.nopen-checker>1.0.1</version.nopen-checker>
         <version.nullaway>0.10.5</version.nullaway>
+        <version.pitest-git>1.0.1</version.pitest-git>
         <version.surefire>2.22.2</version.surefire>
     </properties>
 
@@ -428,6 +429,16 @@
                         </execution>
                     </executions>
                 </plugin>
+                <plugin>
+                    <groupId>com.groupcdg</groupId>
+                    <artifactId>pitest-git-maven-plugin</artifactId>
+                    <version>${version.pitest-git}</version>
+                </plugin>
+                <plugin>
+                    <groupId>com.groupcdg</groupId>
+                    <artifactId>pitest-github-maven-plugin</artifactId>
+                    <version>${version.pitest-git}</version>
+                </plugin>
                 <plugin>
                     <groupId>com.spotify.fmt</groupId>
                     <artifactId>fmt-maven-plugin</artifactId>
@@ -1231,8 +1242,12 @@
                     <version>1.9.11</version>
                     <configuration>
                         <excludedClasses>
+                            <!-- AutoValue generated classes. -->
                             <excludedClass>*.AutoValue_*</excludedClass>
+                            <!-- Refaster rule collections. -->
+                            <excludedClass>*.refaster*.*Rules*</excludedClass>
                         </excludedClasses>
+                        <failWhenNoMutations>false</failWhenNoMutations>
                         <!-- Use multiple threads to speed things up. Extend
                         timeouts to prevent false positives as a result of
                         contention. -->
@@ -1241,6 +1256,11 @@
                         <timestampedReports>false</timestampedReports>
                     </configuration>
                     <dependencies>
+                        <dependency>
+                            <groupId>com.groupcdg</groupId>
+                            <artifactId>pitest-git-plugin</artifactId>
+                            <version>${version.pitest-git}</version>
+                        </dependency>
                         <dependency>
                             <groupId>org.pitest</groupId>
                             <artifactId>pitest-junit5-plugin</artifactId>