Add the ability to specify constraints for the order in which shutdowners should run.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5185 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/presents/server/ShutdownManager.java

@@ -21,6 +21,9 @@
 
 package com.threerings.presents.server;
 
+import java.util.ArrayList;
+import java.util.HashMap;
+
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 
@@ -46,6 +49,8 @@ public class ShutdownManager
         public void shutdown ();
     }
 
+    public static enum Constraint { RUNS_BEFORE, RUNS_AFTER };
+
     @Inject ShutdownManager (@EventQueue RunQueue dobjq)
     {
         _dobjq = dobjq;
@@ -67,6 +72,20 @@ public class ShutdownManager
         _downers.remove(downer);
     }
 
+    /**
+     * Adds a constraint that a certain shutdowner must be run before another.
+     */
+    public void addConstraint (Shutdowner lhs, Constraint constraint, Shutdowner rhs)
+        throws IllegalArgumentException
+    {
+        if (lhs == null || rhs == null) {
+            throw new IllegalArgumentException("Cannot add constraint about null shutdowner.");
+        }
+        Shutdowner before = (constraint == Constraint.RUNS_BEFORE) ? lhs : rhs;
+        Shutdowner after = (constraint == Constraint.RUNS_BEFORE) ? rhs : lhs;
+        _downers.addDependency(after, before);
+    }
+
     /**
      * Queues up a request to shutdown on the dobjmgr thread. This method may be safely called from
      * any thread.
@@ -85,11 +104,17 @@ public class ShutdownManager
      */
     public void shutdown ()
     {
-        ObserverList<Shutdowner> downers = _downers;
-        if (downers == null) {
+        if (_downers == null) {
             log.warning("Refusing repeat shutdown request.");
             return;
         }
+
+        ObserverList<Shutdowner> downers = ObserverList.newSafeInOrder();
+
+        while (!_downers.isEmpty()) {
+            downers.add(_downers.removeAvailableElement());
+        }
+
         _downers = null;
 
         // shut down all shutdown participants
@@ -101,9 +126,153 @@ public class ShutdownManager
         });
     }
 
+    /**
+     * We maintain a bidirectional graph to manage the order that the items are removed.  Children
+     *  must wait until their parents are accessed - thus removing an available element means that
+     *  a node without parents (an orphan) is removed and returned.
+     * @param <T>
+     */
+    protected class DependencyGraph<T>
+    {
+        /**
+         * Adds an element with no initial dependencies from the graph.
+         */
+        public void add (T element)
+        {
+            DependencyNode<T> node = new DependencyNode<T>(element);
+            _nodes.put(element, node);
+            _orphans.add(element);
+        }
+
+        /**
+         * Removes an element and its dependencies from the graph.
+         */
+        public void remove (T element)
+        {
+            DependencyNode<T> node = _nodes.remove(element);
+            _orphans.remove(element);
+
+            // Remove ourselves as a child of our parents.
+            for (DependencyNode<T> parent : node.parents) {
+                parent.children.remove(node);
+            }
+
+            // Remove ourselves as a parent of our children, possibly orphaning them.
+            for (DependencyNode<T> child : node.children) {
+                child.parents.remove(node);
+                if (child.parents.isEmpty()) {
+                    _orphans.add(child.content);
+                }
+            }
+        }
+
+        /**
+         * Removes and returns an element which is available, meaning not dependent upon any other
+         *  still in the graph.
+         */
+        public T removeAvailableElement ()
+        {
+            T elem = _orphans.get(0);
+            DependencyNode<T> node = _nodes.get(elem);
+            remove(elem);
+
+            return elem;
+        }
+
+        /**
+         * Returns the number of elements in the graph.
+         */
+        public int size ()
+        {
+            return _nodes.size();
+        }
+
+        /**
+         * Returns whether there are no more elements in the graph.
+         */
+        public boolean isEmpty ()
+        {
+            return size() == 0;
+        }
+
+        /**
+         * Records a new dependency of the dependant upon the dependee.
+         */
+        public void addDependency (T dependant, T dependee)
+            throws IllegalArgumentException
+        {
+            _orphans.remove(dependant);
+            DependencyNode<T> dependantNode = _nodes.get(dependant);
+            DependencyNode<T> dependeeNode = _nodes.get(dependee);
+
+            if (dependsOn(dependee, dependant)) {
+                throw new IllegalArgumentException("Refusing to create circular dependency.");
+            }
+            dependantNode.parents.add(dependeeNode);
+            dependeeNode.children.add(dependantNode);
+
+        }
+
+        /**
+         * Returns whether elem1 is designated to depend on elem2.
+         */
+        public boolean dependsOn (T elem1, T elem2)
+        {
+            DependencyNode<T> node1 = _nodes.get(elem1);
+            DependencyNode<T> node2 = _nodes.get(elem2);
+
+            ArrayList<DependencyNode<T>> nodesToCheck = new ArrayList<DependencyNode<T>>();
+            ArrayList<DependencyNode<T>> nodesAlreadyChecked = new ArrayList<DependencyNode<T>>();
+            nodesToCheck.addAll(node1.parents);
+
+            // We prevent circular dependencies when we add dependencies.  Otherwise, this'd be
+            //  potentially non-terminating.
+            while (!nodesToCheck.isEmpty()) {
+                // We take it off the end since we don't care about order and this is faster.
+                DependencyNode<T> checkNode = nodesToCheck.remove(nodesToCheck.size() - 1);
+
+                if (nodesAlreadyChecked.contains(checkNode)) {
+                    // We've seen him before, no need to check again.
+                    continue;
+
+                } else if (checkNode == node2) {
+                    // We've found our dependency
+                    return true;
+
+                } else  {
+                    nodesAlreadyChecked.add(checkNode);
+                    nodesToCheck.addAll(checkNode.parents);
+                }
+            }
+
+            return false;
+        }
+
+        /** All the nodes included in the graph. */
+        protected HashMap<T, DependencyNode<T>> _nodes = new HashMap<T, DependencyNode<T>>();
+
+        /** Nodes in the graph with no parents/dependencies. */
+        protected ArrayList<T> _orphans = new ArrayList<T>();
+
+        protected class DependencyNode<T>
+        {
+            public T content;
+            public ArrayList<DependencyNode<T>> parents;
+            public ArrayList<DependencyNode<T>> children;
+
+            public DependencyNode (T contents)
+            {
+                this.content = contents;
+                this.parents = new ArrayList<DependencyNode<T>>();
+                this.children = new ArrayList<DependencyNode<T>>();
+            }
+        }
+    }
+
+    /** All of the registered shutdowners along with related constraints. */
+    protected DependencyGraph<Shutdowner> _downers = new DependencyGraph<Shutdowner>();
+
     /** The queue we'll use to get onto the dobjmgr thread before shutting down. */
     protected RunQueue _dobjq;
 
-    /** A list of shutdown participants. */
-    protected static ObserverList<Shutdowner> _downers = ObserverList.newSafeInOrder();
 }