Add a big test of the new code

server/src/test/java/org/elasticsearch/ingest/PipelineConfigurationTests.java

@@ -26,13 +26,45 @@
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.Map;
 import java.util.function.Predicate;
 
 import static org.hamcrest.Matchers.anEmptyMap;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
+import static org.hamcrest.Matchers.sameInstance;
 
 public class PipelineConfigurationTests extends AbstractXContentTestCase<PipelineConfiguration> {
 
+    public void testConfigInvariants() {
+        Map<String, Object> original = Map.of("a", 1);
+        Map<String, Object> mutable = new HashMap<>(original);
+        PipelineConfiguration configuration = new PipelineConfiguration("1", mutable);
+        // the config is equal to the original & mutable map, regardless of how you get a reference to it
+        assertThat(configuration.getConfig(), equalTo(original));
+        assertThat(configuration.getConfig(), equalTo(mutable));
+        assertThat(configuration.getConfig(), equalTo(configuration.getConfig(false)));
+        assertThat(configuration.getConfig(), equalTo(configuration.getConfig(true)));
+        // the config is the same instance as itself when unmodifiable is true
+        assertThat(configuration.getConfig(), sameInstance(configuration.getConfig()));
+        assertThat(configuration.getConfig(), sameInstance(configuration.getConfig(true)));
+        // but it's not the same instance as the original mutable map, nor if unmodifiable is false
+        assertThat(configuration.getConfig(), not(sameInstance(mutable)));
+        assertThat(configuration.getConfig(), not(sameInstance(configuration.getConfig(false))));
+
+        // changing the mutable map doesn't alter the pipeline's configuration
+        mutable.put("b", 2);
+        assertThat(configuration.getConfig(), equalTo(original));
+
+        // the modifiable map can be modified
+        Map<String, Object> modifiable = configuration.getConfig(false);
+        modifiable.put("c", 3); // this doesn't throw an exception
+        assertThat(modifiable.get("c"), equalTo(3));
+        // but the next modifiable copy is a new fresh copy, and doesn't reflect those changes
+        assertThat(configuration.getConfig(), equalTo(configuration.getConfig(false)));
+    }
+
     public void testSerialization() throws IOException {
         PipelineConfiguration configuration = new PipelineConfiguration(
             "1",