Add test to ensure IllegalArgumentException is thrown

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

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

@@ -0,0 +1,73 @@
+/*
+ * 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.common.settings.Settings;
+import org.opensearch.indices.SystemIndexDescriptor;
+import org.opensearch.test.OpenSearchIntegTestCase;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Consumer;
+
+import static org.hamcrest.Matchers.containsString;
+
+@OpenSearchIntegTestCase.ClusterScope(scope = OpenSearchIntegTestCase.Scope.TEST, numDataNodes = 0, numClientNodes = 0)
+public class SystemIndexPluginIT extends OpenSearchIntegTestCase {
+
+    @Override
+    protected Collection<Class<? extends Plugin>> nodePlugins() {
+        return List.of(SystemIndexPlugin1.class, SystemIndexPlugin2.class);
+    }
+
+    public void test2SystemIndexPluginsImplementOnSystemIndices_shouldFail() throws Exception {
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> internalCluster().startNode());
+        assertThat(e.getMessage(), containsString("Cannot have more than one plugin implementing onSystemIndex"));
+    }
+
+}
+
+final class SystemIndexPlugin1 extends Plugin implements SystemIndexPlugin {
+
+    private Map<String, Set<String>> systemIndices;
+
+    public SystemIndexPlugin1() {}
+
+    @Override
+    public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings settings) {
+        final SystemIndexDescriptor systemIndexDescriptor = new SystemIndexDescriptor(".system-index1", "System index 1");
+        return Collections.singletonList(systemIndexDescriptor);
+    }
+
+    @Override
+    public Consumer<Map<String, Set<String>>> onSystemIndices() {
+        return (systemIndices) -> { this.systemIndices = systemIndices; };
+    }
+}
+
+class SystemIndexPlugin2 extends Plugin implements SystemIndexPlugin {
+
+    private Map<String, Set<String>> systemIndices;
+
+    public SystemIndexPlugin2() {}
+
+    @Override
+    public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings settings) {
+        final SystemIndexDescriptor systemIndexDescriptor = new SystemIndexDescriptor(".system-index2", "System index 2");
+        return Collections.singletonList(systemIndexDescriptor);
+    }
+
+    @Override
+    public Consumer<Map<String, Set<String>>> onSystemIndices() {
+        return (systemIndices) -> { this.systemIndices = systemIndices; };
+    }
+}