Ensure ordering of plugin initialization (#93882)

As Node collects and initializes various parts of the system from
plugins, it does so by calling helper methods on PluginsService to
iterate the loaded plugins and call the desired method. These methods
rely on the internal list of plugins that were loaded, but the order of
that list is not stable. This commit changes that list to be the order
plugins were loaded in, so that initialization of services of dependent
plugins can be relied on.

Author: rjernst

docs/changelog/93882.yaml

@@ -0,0 +1,6 @@
+pr: 93882
+summary: Ensure ordering of plugin initialization
+area: Infra/Plugins
+type: bug
+issues:
+ - 93851

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

@@ -50,6 +50,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
@@ -102,7 +103,8 @@ record LoadedPlugin(PluginDescriptor descriptor, Plugin instance, ClassLoader lo
     private final Path configPath;
 
     /**
-     * We keep around a list of plugins and modules
+     * We keep around a list of plugins and modules. The order of
+     * this list is that which the plugins and modules were loaded in.
      */
     private final List<LoadedPlugin> plugins;
     private final PluginsAndModules info;
@@ -156,7 +158,7 @@ public PluginsService(Settings settings, Path configPath, Path modulesDirectory,
             }
         }
 
-        Map<String, LoadedPlugin> loadedPlugins = loadBundles(seenBundles);
+        LinkedHashMap<String, LoadedPlugin> loadedPlugins = loadBundles(seenBundles);
 
         var inspector = PluginIntrospector.getInstance();
         this.info = new PluginsAndModules(getRuntimeInfos(inspector, pluginsList, loadedPlugins), modulesList);
@@ -280,8 +282,8 @@ protected List<LoadedPlugin> plugins() {
         return this.plugins;
     }
 
-    private Map<String, LoadedPlugin> loadBundles(Set<PluginBundle> bundles) {
-        Map<String, LoadedPlugin> loaded = new HashMap<>();
+    private LinkedHashMap<String, LoadedPlugin> loadBundles(Set<PluginBundle> bundles) {
+        LinkedHashMap<String, LoadedPlugin> loaded = new LinkedHashMap<>();
         Map<String, Set<URL>> transitiveUrls = new HashMap<>();
         List<PluginBundle> sortedBundles = PluginsUtils.sortBundles(bundles);
         Set<URL> systemLoaderURLs = JarHell.parseModulesAndClassPath();