Add information about async tasks to CrashReports. Addresses BUKKIT-2491

Async tasks are notorious for causing CMEs and corrupted data when
accessing the API. This change makes a linked list to track recent tasks
that may no longer be running. It is accessed via the toString method on
the scheduler. This behavior is not guaranteed, but it is accessible as
such currently.

Although toString is located in the scheduler, its contract does not
guarantee an accurate or up to date call when accessed from a second
thread.

src/main/java/org/bukkit/craftbukkit/CraftCrashReport.java

@@ -28,7 +28,7 @@ public class CraftCrashReport implements Callable {
             for (Map.Entry<Thread, ? extends Object[]> entry : Thread.getAllStackTraces().entrySet()) {
                 value.append(' ').append(entry.getKey().getState().name()).append(' ').append(entry.getKey().getName()).append(": ").append(Arrays.toString(entry.getValue())).append(',');
             }
-            value.append('}');
+            value.append("}\n   ").append(Bukkit.getScheduler().toString());
         } catch (Throwable t) {
             value.append("\n   Failed to handle CraftCrashReport:\n");
             PrintWriter writer = new PrintWriter(value);

src/main/java/org/bukkit/craftbukkit/scheduler/CraftAsyncDebugger.java

@@ -0,0 +1,37 @@
+package org.bukkit.craftbukkit.scheduler;
+
+import org.bukkit.plugin.Plugin;
+
+
+class CraftAsyncDebugger {
+    private CraftAsyncDebugger next = null;
+    private final int expiry;
+    private final Plugin plugin;
+    private final Class<? extends Runnable> clazz;
+
+    CraftAsyncDebugger(final int expiry, final  Plugin plugin, final Class<? extends Runnable> clazz) {
+        this.expiry = expiry;
+        this.plugin = plugin;
+        this.clazz = clazz;
+
+    }
+
+    final CraftAsyncDebugger getNextHead(final int time) {
+        CraftAsyncDebugger next, current = this;
+        while (time > current.expiry && (next = current.next) != null) {
+            current = next;
+        }
+        return current;
+    }
+
+    final CraftAsyncDebugger setNext(final CraftAsyncDebugger next) {
+        return this.next = next;
+    }
+
+    StringBuilder debugTo(final StringBuilder string) {
+        for (CraftAsyncDebugger next = this; next != null; next = next.next) {
+            string.append(plugin.getDescription().getName()).append(':').append(clazz.getName()).append('@').append(expiry).append(',');
+        }
+        return string;
+    }
+}

src/main/java/org/bukkit/craftbukkit/scheduler/CraftScheduler.java

@@ -71,6 +71,13 @@ public class CraftScheduler implements BukkitScheduler {
     private final ConcurrentHashMap<Integer, CraftTask> runners = new ConcurrentHashMap<Integer, CraftTask>();
     private volatile int currentTick = -1;
     private final Executor executor = Executors.newCachedThreadPool();
+    private CraftAsyncDebugger debugHead = new CraftAsyncDebugger(-1, null, null) {@Override StringBuilder debugTo(StringBuilder string) {return string;}};
+    private CraftAsyncDebugger debugTail = debugHead;
+    private static final int RECENT_TICKS;
+
+    static {
+        RECENT_TICKS = 30;
+    }
 
     public int scheduleSyncDelayedTask(final Plugin plugin, final Runnable task) {
         return this.scheduleSyncDelayedTask(plugin, task, 0l);
@@ -325,6 +332,7 @@ public class CraftScheduler implements BukkitScheduler {
                 }
                 parsePending();
             } else {
+                debugTail = debugTail.setNext(new CraftAsyncDebugger(currentTick + RECENT_TICKS, task.getOwner(), task.getTaskClass()));
                 executor.execute(task);
                 // We don't need to parse pending
                 // (async tasks must live with race-conditions if they attempt to cancel between these few lines of code)
@@ -339,6 +347,7 @@ public class CraftScheduler implements BukkitScheduler {
         }
         pending.addAll(temp);
         temp.clear();
+        debugHead = debugHead.getNextHead(currentTick);
     }
 
     private void addTask(final CraftTask task) {
@@ -418,4 +427,12 @@ public class CraftScheduler implements BukkitScheduler {
         }
         return true;
     }
+
+    @Override
+    public String toString() {
+        int debugTick = currentTick;
+        StringBuilder string = new StringBuilder("Recent tasks from ").append(debugTick - RECENT_TICKS).append('-').append(debugTick).append('{');
+        debugHead.debugTo(string);
+        return string.append('}').toString();
+    }
 }

src/main/java/org/bukkit/craftbukkit/scheduler/CraftTask.java

@@ -75,4 +75,8 @@ class CraftTask implements BukkitTask, Runnable {
     void setNext(CraftTask next) {
         this.next = next;
     }
+
+    Class<? extends Runnable> getTaskClass() {
+        return task.getClass();
+    }
 }