Fix plugin load order

The old algorithm failed, because the tree below lead to the issue, that
the scm-branchwp-plugin was loaded before the scm-review-plugin was
ready.

This commit changes the order in the way, that leafs are loaded last.

+- scm-editor-plugin d
   +- scm-branchwp-plugin a
+- scm-mail-plugin c
   +- scm-review-plugin b
      +- scm-branchwp-plugin a
   +- scm-branchwp-plugin a

scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java

@@ -191,11 +191,11 @@ public final class PluginProcessor
 
     logger.info("install plugin tree:\n{}", pluginTree);
 
-    List<PluginNode> rootNodes = pluginTree.getRootNodes();
+    List<PluginNode> leafLastNodes = pluginTree.getLeafLastNodes();
 
     logger.trace("create plugin wrappers and build classloaders");
 
-    Set<InstalledPlugin> wrappers = createPluginWrappers(classLoader, rootNodes);
+    Set<InstalledPlugin> wrappers = createPluginWrappers(classLoader, leafLastNodes);
 
     logger.debug("collected {} plugins", wrappers.size());
 
@@ -259,33 +259,6 @@ public final class PluginProcessor
     }
   }
 
-  /**
-   * Method description
-   *
-   *
-   * @param plugins
-   * @param classLoader
-   * @param nodes
-   *
-   * @throws IOException
-   */
-  private void appendPluginWrappers(Set<InstalledPlugin> plugins,
-    ClassLoader classLoader, List<PluginNode> nodes)
-    throws IOException
-  {
-
-    // TODO fix plugin loading order
-    for (PluginNode node : nodes)
-    {
-      appendPluginWrapper(plugins, classLoader, node);
-    }
-
-    for (PluginNode node : nodes)
-    {
-      appendPluginWrappers(plugins, classLoader, node.getChildren());
-    }
-  }
-
   /**
    * Method description
    *
@@ -484,19 +457,22 @@ public final class PluginProcessor
    *
    *
    * @param classLoader
-   * @param rootNodes
+   * @param nodes
    *
    * @return
    *
    * @throws IOException
    */
   private Set<InstalledPlugin> createPluginWrappers(ClassLoader classLoader,
-                                                    List<PluginNode> rootNodes)
+                                                    List<PluginNode> nodes)
     throws IOException
   {
     Set<InstalledPlugin> plugins = Sets.newHashSet();
 
-    appendPluginWrappers(plugins, classLoader, rootNodes);
+    for (PluginNode node : nodes)
+    {
+      appendPluginWrapper(plugins, classLoader, node);
+    }
 
     return plugins;
   }

scm-webapp/src/main/java/sonia/scm/plugin/PluginTree.java

@@ -39,6 +39,8 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.util.Arrays;
+import java.util.LinkedHashSet;
+import java.util.LinkedList;
 import java.util.List;
 
 //~--- JDK imports ------------------------------------------------------------
@@ -134,13 +136,26 @@ public final class PluginTree
    *
    * @return
    */
-  public List<PluginNode> getRootNodes()
+  public List<PluginNode> getLeafLastNodes()
   {
-    return rootNodes;
+    LinkedHashSet<PluginNodeHashWrapper> leafFirst = new LinkedHashSet<>();
+
+    rootNodes.forEach(node -> appendLeafFirst(leafFirst, node));
+
+    LinkedList<PluginNode> leafLast = new LinkedList<>();
+
+    leafFirst.stream().map(PluginNodeHashWrapper::getPluginNode).forEach(leafLast::addFirst);
+
+    return leafLast;
   }
 
-  //~--- methods --------------------------------------------------------------
+  private void appendLeafFirst(LinkedHashSet<PluginNodeHashWrapper> leafFirst, PluginNode node) {
+    node.getChildren().forEach(child -> appendLeafFirst(leafFirst, child));
+    leafFirst.add(new PluginNodeHashWrapper(node));
+  }
 
+
+  //~--- methods --------------------------------------------------------------
   /**
    * Method description
    *
@@ -235,8 +250,32 @@ public final class PluginTree
       append(buffer, indent + "   ", child);
     }
   }
-//~--- fields ---------------------------------------------------------------
+
+  //~--- fields ---------------------------------------------------------------
 
   /** Field description */
   private final List<PluginNode> rootNodes = Lists.newArrayList();
+
+  private static class PluginNodeHashWrapper {
+    private final PluginNode pluginNode;
+
+    private PluginNodeHashWrapper(PluginNode pluginNode) {
+      this.pluginNode = pluginNode;
+    }
+
+    public PluginNode getPluginNode() {
+      return pluginNode;
+    }
+
+    @Override
+    public int hashCode() {
+      return pluginNode.getId().hashCode();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      return obj instanceof PluginNodeHashWrapper
+        && ((PluginNodeHashWrapper) obj).pluginNode.getId().equals(this.pluginNode.getId());
+    }
+  }
 }