[7.x] Fix serialization of evaluation response. (#47557)

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/EvaluateDataFrameAction.java

@@ -217,7 +217,7 @@ public List<EvaluationMetricResult> getMetrics() {
         @Override
         public void writeTo(StreamOutput out) throws IOException {
             out.writeString(evaluationName);
-            out.writeList(metrics);
+            out.writeNamedWriteableList(metrics);
         }
 
         @Override

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/action/EvaluateDataFrameActionResponseTests.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+package org.elasticsearch.xpack.core.ml.action;
+
+import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
+import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.test.AbstractWireSerializingTestCase;
+import org.elasticsearch.xpack.core.ml.action.EvaluateDataFrameAction.Response;
+import org.elasticsearch.xpack.core.ml.dataframe.evaluation.EvaluationMetricResult;
+import org.elasticsearch.xpack.core.ml.dataframe.evaluation.MlEvaluationNamedXContentProvider;
+import org.elasticsearch.xpack.core.ml.dataframe.evaluation.regression.MeanSquaredError;
+import org.elasticsearch.xpack.core.ml.dataframe.evaluation.regression.RSquared;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class EvaluateDataFrameActionResponseTests extends AbstractWireSerializingTestCase<Response> {
+
+    @Override
+    protected NamedWriteableRegistry getNamedWriteableRegistry() {
+        return new NamedWriteableRegistry(new MlEvaluationNamedXContentProvider().getNamedWriteables());
+    }
+
+    @Override
+    protected Response createTestInstance() {
+        String evaluationName = randomAlphaOfLength(10);
+        List<EvaluationMetricResult> metrics =
+            Arrays.asList(
+                new MeanSquaredError.Result(randomDouble()),
+                new RSquared.Result(randomDouble()));
+        int numMetrics = randomIntBetween(0, metrics.size());
+        return new Response(evaluationName, metrics.subList(0, numMetrics));
+    }
+
+    @Override
+    protected Writeable.Reader<Response> instanceReader() {
+        return Response::new;
+    }
+}