Issue #12017: Kill surviving mutations in AbstractFileSetCheck relate…

…d to fileExtensions

.ci/pitest-suppressions/pitest-api-suppressions.xml

@@ -9,15 +9,6 @@
     <lineContent>return Collections.unmodifiableSet(tokens);</lineContent>
   </mutation>
 
-  <mutation unstable="false">
-    <sourceFile>AbstractFileSetCheck.java</sourceFile>
-    <mutatedClass>com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck</mutatedClass>
-    <mutatedMethod>getFileExtensions</mutatedMethod>
-    <mutator>org.pitest.mutationtest.engine.gregor.mutators.experimental.ArgumentPropagationMutator</mutator>
-    <description>replaced call to java/util/Arrays::copyOf with argument</description>
-    <lineContent>return Arrays.copyOf(fileExtensions, fileExtensions.length);</lineContent>
-  </mutation>
-
   <mutation unstable="false">
     <sourceFile>AbstractFileSetCheck.java</sourceFile>
     <mutatedClass>com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck</mutatedClass>

src/test/java/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheckTest.java

@@ -263,6 +263,20 @@ public void testMultiFileFireErrors() throws Exception {
                 .isEqualTo(2);
     }
 
+    /**
+     * S2384 - Mutable members should not be stored or returned directly.
+     * Inspection is valid, a pure unit test is required as this condition can't be recreated in
+     * a test with checks and input file as none of the checks try to modify the fileExtensions.
+     */
+    @Test
+    public void testCopiedArrayIsReturned() {
+        final DummyFileSetCheck check = new DummyFileSetCheck();
+        check.setFileExtensions(".tmp");
+        assertWithMessage("Extensions should be copied")
+            .that(check.getFileExtensions())
+            .isNotSameInstanceAs(check.getFileExtensions());
+    }
+
     public static class DummyFileSetCheck extends AbstractFileSetCheck {
 
         private static final String MSG_KEY = "File should not be empty.";