Add test for classpath plugin extended plugin loading

Signed-off-by: Craig Perkins <[email protected]>

server/src/internalClusterTest/java/org/opensearch/plugins/ClasspathPluginIT.java

@@ -0,0 +1,52 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.plugins;
+
+import org.opensearch.test.OpenSearchIntegTestCase;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.hamcrest.Matchers.equalTo;
+
+@OpenSearchIntegTestCase.ClusterScope(scope = OpenSearchIntegTestCase.Scope.TEST, numDataNodes = 0)
+public class ClasspathPluginIT extends OpenSearchIntegTestCase {
+
+    public interface SampleExtension {}
+
+    public static class SampleExtensiblePlugin extends Plugin implements ExtensiblePlugin {
+        public SampleExtensiblePlugin() {}
+
+        @Override
+        public void loadExtensions(ExtensiblePlugin.ExtensionLoader loader) {
+            int nLoaded = 0;
+            for (SampleExtension e : loader.loadExtensions(SampleExtension.class)) {
+                nLoaded++;
+            }
+
+            assertThat(nLoaded, equalTo(1));
+        }
+    }
+
+    public static class SampleExtendingPlugin extends Plugin implements SampleExtension {
+        public SampleExtendingPlugin() {}
+    };
+
+    @Override
+    protected Collection<Class<? extends Plugin>> nodePlugins() {
+        return Stream.concat(super.nodePlugins().stream(), Stream.of(SampleExtensiblePlugin.class, SampleExtendingPlugin.class))
+            .collect(Collectors.toList());
+    }
+
+    public void testPluginExtensionWithClasspathPlugins() throws IOException {
+        internalCluster().startNode();
+    }
+}

server/src/internalClusterTest/resources/META-INF/services/org.opensearch.plugins.ClasspathPluginIT$SampleExtension

@@ -0,0 +1,8 @@
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+# The OpenSearch Contributors require contributions made to
+# this file be licensed under the Apache-2.0 license or a
+# compatible open source license.
+
+org.opensearch.plugins.ClasspathPluginIT$SampleExtendingPlugin