Replace ExecutionList with linked list of RunnableExecutionPair

Patch does CAS loop and XCHG instead of synchronized which should
be faster.

Patch also reduces per-future memory consumption.

Author: stepancheg

client/src/main/java/org/asynchttpclient/future/AbstractListenableFuture.java

@@ -28,13 +28,15 @@
 
 package org.asynchttpclient.future;
 
+import java.util.ArrayList;
 import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
 import org.asynchttpclient.ListenableFuture;
 
 /**
- * An abstract base implementation of the listener support provided by {@link ListenableFuture}. This class uses an {@link ExecutionList} to guarantee that all registered listeners
- * will be executed. Listener/Executor pairs are stored in the execution list and executed in the order in which they were added, but because of thread scheduling issues there is
+ * An abstract base implementation of the listener support provided by {@link ListenableFuture}.
+ * Listener/Executor pairs are stored in the {@link RunnableExecutorPair} linked list in the order in which they were added, but because of thread scheduling issues there is
  * no guarantee that the JVM will execute them in order. In addition, listeners added after the task is complete will be executed immediately, even if some previously added
  * listeners have not yet been executed.
  *
@@ -43,41 +45,48 @@
  */
 public abstract class AbstractListenableFuture<V> implements ListenableFuture<V> {
 
-    private volatile boolean hasRun;
-    private volatile boolean executionListInitialized;
-    private volatile ExecutionList executionList;
+    /**
+     * Marks that execution is already done, and new runnables
+     * should be executed right away instead of begin added to the list.
+     */
+    private static final RunnableExecutorPair executedMarker = new RunnableExecutorPair(null, null, null);
 
-    private ExecutionList executionList() {
-        ExecutionList localExecutionList = executionList;
-        if (localExecutionList == null) {
-            synchronized (this) {
-                localExecutionList = executionList;
-                if (localExecutionList == null) {
-                    localExecutionList = new ExecutionList();
-                    executionList = localExecutionList;
-                    executionListInitialized = true;
-                }
-            }
-        }
-        return localExecutionList;
-    }
+    /**
+     * Linked list of executions or a {@link #executedMarker}.
+     */
+    private volatile RunnableExecutorPair executionList;
+    private static final AtomicReferenceFieldUpdater<AbstractListenableFuture, RunnableExecutorPair> executionListField =
+            AtomicReferenceFieldUpdater.newUpdater(AbstractListenableFuture.class, RunnableExecutorPair.class, "executionList");
 
     @Override
     public ListenableFuture<V> addListener(Runnable listener, Executor exec) {
-        executionList().add(listener, exec);
-        if (hasRun) {
-            runListeners();
+        for (;;) {
+            RunnableExecutorPair executionListLocal = this.executionList;
+            if (executionListLocal == executedMarker) {
+                RunnableExecutorPair.executeListener(listener, exec);
+                return this;
+            }
+
+            RunnableExecutorPair pair = new RunnableExecutorPair(listener, exec, executionListLocal);
+            if (executionListField.compareAndSet(this, executionListLocal, pair)) {
+                return this;
+            }
         }
-        return this;
     }
 
     /**
      * Execute the execution list.
      */
     protected void runListeners() {
-        hasRun = true;
-        if (executionListInitialized) {
-            executionList().execute();
+        RunnableExecutorPair execution = executionListField.getAndSet(this, executedMarker);
+        if (execution == executedMarker) {
+            return;
+        }
+
+        RunnableExecutorPair reversedList = RunnableExecutorPair.reverseList(execution);
+
+        while (reversedList != null) {
+            RunnableExecutorPair.executeListener(reversedList.runnable, reversedList.executor);
         }
     }
 }

client/src/main/java/org/asynchttpclient/future/ExecutionList.java

@@ -0,0 +1,57 @@
+package org.asynchttpclient.future;
+
+import java.util.concurrent.Executor;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * Linked list of runnables with executors.
+ */
+final class RunnableExecutorPair {
+    private static final Logger log = Logger.getLogger(RunnableExecutorPair.class.getPackage().getName());
+
+    final Runnable runnable;
+    final Executor executor;
+    RunnableExecutorPair next;
+
+    RunnableExecutorPair(Runnable runnable, Executor executor, RunnableExecutorPair next) {
+        this.runnable = runnable;
+        this.executor = executor;
+        this.next = next;
+    }
+
+    /**
+     * Submits the given runnable to the given {@link Executor} catching and logging all {@linkplain RuntimeException runtime exceptions} thrown by the executor.
+     */
+    static void executeListener(Runnable runnable, Executor executor) {
+        try {
+            executor.execute(runnable);
+        } catch (RuntimeException e) {
+            // Log it and keep going, bad runnable and/or executor. Don't punish the other runnables if
+            // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
+            // up.
+            log.log(Level.SEVERE, "RuntimeException while executing runnable " + runnable + " with executor " + executor, e);
+        }
+    }
+
+    static RunnableExecutorPair reverseList(RunnableExecutorPair list) {
+        // The pairs in the stack are in the opposite order from how they were added
+        // so we need to reverse the list to fulfill our contract.
+        // This is somewhat annoying, but turns out to be very fast in practice. Alternatively, we
+        // could drop the contract on the method that enforces this queue like behavior since depending
+        // on it is likely to be a bug anyway.
+
+        // N.B. All writes to the list and the next pointers must have happened before the above
+        // synchronized block, so we can iterate the list without the lock held here.
+        RunnableExecutorPair prev = null;
+
+        while (list != null) {
+            RunnableExecutorPair next = list.next;
+            list.next = prev;
+            prev = list;
+            list = next;
+        }
+
+        return prev;
+    }
+}

client/src/main/java/org/asynchttpclient/future/RunnableExecutorPair.java

@@ -0,0 +1,36 @@
+package org.asynchttpclient.future;
+
+import java.util.ArrayList;
+
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+/**
+ * @author Stepan Koltsov
+ */
+public class RunnableExecutorPairTest {
+
+    @Test
+    public void testReverseList() {
+        // empty
+        {
+            Assert.assertNull(RunnableExecutorPair.reverseList(null));
+        }
+
+        for (int len = 1; len < 5; ++len) {
+            ArrayList<RunnableExecutorPair> list = new ArrayList<>();
+            for (int i = 0; i < len; ++i) {
+                RunnableExecutorPair prev = i != 0 ? list.get(i - 1) : null;
+                list.add(new RunnableExecutorPair(() -> {}, null, prev));
+            }
+
+            RunnableExecutorPair reversed = RunnableExecutorPair.reverseList(list.get(list.size() - 1));
+            for (int i = 0; i < len; ++i) {
+                Assert.assertSame(reversed, list.get(i));
+                Assert.assertSame(i != len - 1 ? list.get(i + 1) : null, reversed.next);
+                reversed = reversed.next;
+            }
+        }
+    }
+
+}