Plugins: Ensure additionalSettings() do not conflict

Plugins can preovide additional settings to be added to the settings
provided in elasticsearch.yml. However, if two different plugins supply
the same setting key, the last one to be loaded wins, which is
indeterminate. This change enforces plugins cannot have conflicting
settings, at startup time. As a followup, we should do this when
installing plugins as well, to give earlier errors when two plugins
collide.

core/src/main/java/org/elasticsearch/plugins/PluginsService.java

@@ -202,9 +202,18 @@ public class PluginsService extends AbstractComponent {
     }
 
     public Settings updatedSettings() {
+        Map<String, String> foundSettings = new HashMap<>();
         final Settings.Builder builder = Settings.settingsBuilder();
         for (Tuple<PluginInfo, Plugin> plugin : plugins) {
-            builder.put(plugin.v2().additionalSettings());
+            Settings settings = plugin.v2().additionalSettings();
+            for (String setting : settings.getAsMap().keySet()) {
+                String oldPlugin = foundSettings.put(setting, plugin.v1().getName());
+                if (oldPlugin != null) {
+                    throw new IllegalArgumentException("Cannot have additional setting [" + setting + "] " +
+                        "in plugin " + plugin.v1().getName() + ", already added in plugin " + oldPlugin);
+                }
+            }
+            builder.put(settings);
         }
         return builder.put(this.settings).build();
     }

core/src/test/java/org/elasticsearch/plugins/PluginsServiceTests.java

@@ -0,0 +1,85 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.plugins;
+
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.env.Environment;
+import org.elasticsearch.index.store.IndexStoreModule;
+import org.elasticsearch.test.ESTestCase;
+
+public class PluginsServiceTests extends ESTestCase {
+    public static class AdditionalSettingsPlugin1 extends AbstractPlugin {
+        @Override
+        public String name() {
+            return "additional-settings1";
+        }
+        @Override
+        public String description() {
+            return "adds additional setting 'foo.bar'";
+        }
+        @Override
+        public Settings additionalSettings() {
+            return Settings.builder().put("foo.bar", "1").put(IndexStoreModule.STORE_TYPE, IndexStoreModule.Type.MMAPFS.getSettingsKey()).build();
+        }
+    }
+    public static class AdditionalSettingsPlugin2 extends AbstractPlugin {
+        @Override
+        public String name() {
+            return "additional-settings2";
+        }
+        @Override
+        public String description() {
+            return "adds additional setting 'foo.bar'";
+        }
+        @Override
+        public Settings additionalSettings() {
+            return Settings.builder().put("foo.bar", "2").build();
+        }
+    }
+
+    public void testAdditionalSettings() {
+        Settings settings = Settings.builder()
+            .put("path.home", createTempDir())
+            .put("my.setting", "test")
+            .put(IndexStoreModule.STORE_TYPE, IndexStoreModule.Type.SIMPLEFS.getSettingsKey())
+            .putArray("plugin.types", AdditionalSettingsPlugin1.class.getName()).build();
+        PluginsService service = new PluginsService(settings, new Environment(settings));
+        Settings newSettings = service.updatedSettings();
+        assertEquals("test", newSettings.get("my.setting")); // previous settings still exist
+        assertEquals("1", newSettings.get("foo.bar")); // added setting exists
+        assertEquals(IndexStoreModule.Type.SIMPLEFS.getSettingsKey(), newSettings.get(IndexStoreModule.STORE_TYPE)); // does not override pre existing settings
+    }
+
+    public void testAdditionalSettingsClash() {
+        Settings settings = Settings.builder()
+            .put("path.home", createTempDir())
+            .putArray("plugin.types", AdditionalSettingsPlugin1.class.getName(), AdditionalSettingsPlugin2.class.getName()).build();
+        PluginsService service = new PluginsService(settings, new Environment(settings));
+        try {
+            service.updatedSettings();
+            fail("Expected exception when building updated settings");
+        } catch (IllegalArgumentException e) {
+            String msg = e.getMessage();
+            assertTrue(msg, msg.contains("Cannot have additional setting [foo.bar]"));
+            assertTrue(msg, msg.contains("plugin additional-settings1"));
+            assertTrue(msg, msg.contains("plugin additional-settings2"));
+        }
+    }
+}