[#3675] Fix livelock issue in MpscLinkedQueue

Motivation:

All read operations should be safe to execute from multiple threads which was not the case and so could produce a livelock.

Modifications:

Modify methods so these are safe to be called from multiple threads.

Result:

No more livelock.

common/src/main/java/io/netty/util/internal/MpscLinkedQueue.java

@@ -21,10 +21,10 @@ package io.netty.util.internal;
 import java.io.IOException;
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
-import java.lang.reflect.Array;
-import java.util.Arrays;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.List;
 import java.util.NoSuchElementException;
 import java.util.Queue;
 
@@ -165,11 +165,19 @@ final class MpscLinkedQueue<E> extends MpscLinkedQueueTailRef<E> implements Queu
         int count = 0;
         MpscLinkedQueueNode<E> n = peekNode();
          for (;;) {
-            if (n == null) {
+            // If value == null it means that clearMaybe() was called on the MpscLinkedQueueNode.
+            if (n == null || n.value() == null) {
+                break;
+            }
+            MpscLinkedQueueNode<E> next = n.next();
+            if (n == next) {
+                break;
+            }
+            n = next;
+            if (++ count == Integer.MAX_VALUE) {
+                // Guard against overflow of integer.
                 break;
             }
-            count ++;
-            n = n.next();
         }
         return count;
     }
@@ -186,40 +194,26 @@ final class MpscLinkedQueue<E> extends MpscLinkedQueueTailRef<E> implements Queu
             if (n == null) {
                 break;
             }
-            if (n.value() == o) {
+            E value = n.value();
+            // If value == null it means that clearMaybe() was called on the MpscLinkedQueueNode.
+            if (value == null) {
+                return false;
+            }
+            if (value == o) {
                 return true;
             }
-            n = n.next();
+            MpscLinkedQueueNode<E> next = n.next();
+            if (n == next) {
+                break;
+            }
+            n = next;
         }
         return false;
     }
 
     @Override
     public Iterator<E> iterator() {
-        return new Iterator<E>() {
-            private MpscLinkedQueueNode<E> node = peekNode();
-
-            @Override
-            public boolean hasNext() {
-                return node != null;
-            }
-
-            @Override
-            public E next() {
-                MpscLinkedQueueNode<E> node = this.node;
-                if (node == null) {
-                    throw new NoSuchElementException();
-                }
-                E value = node.value();
-                this.node = node.next();
-                return value;
-            }
-
-            @Override
-            public void remove() {
-                throw new UnsupportedOperationException();
-            }
-        };
+        return new ReadOnlyIterator<E>(toList().iterator());
     }
 
     @Override
@@ -248,53 +242,46 @@ final class MpscLinkedQueue<E> extends MpscLinkedQueueTailRef<E> implements Queu
         throw new NoSuchElementException();
     }
 
+    private List<E> toList(int initialCapacity) {
+        return toList(new ArrayList<E>(initialCapacity));
+    }
+
+    private List<E> toList() {
+        return toList(new ArrayList<E>());
+    }
+
+    private List<E> toList(List<E> elements) {
+        MpscLinkedQueueNode<E> n = peekNode();
+        for (;;) {
+            if (n == null) {
+                break;
+            }
+            E value = n.value();
+            if (value == null) {
+                break;
+            }
+            if (!elements.add(value)) {
+                // Seems like there is no space left, break here.
+                break;
+            }
+            MpscLinkedQueueNode<E> next = n.next();
+            if (n == next) {
+                break;
+            }
+            n = next;
+        }
+        return elements;
+    }
+
     @Override
     public Object[] toArray() {
-        final Object[] array = new Object[size()];
-        final Iterator<E> it = iterator();
-        for (int i = 0; i < array.length; i ++) {
-            if (it.hasNext()) {
-                array[i] = it.next();
-            } else {
-                return Arrays.copyOf(array, i);
-            }
-        }
-        return array;
+        return toList().toArray();
     }
 
     @Override
     @SuppressWarnings("unchecked")
     public <T> T[] toArray(T[] a) {
-        final int size = size();
-        final T[] array;
-        if (a.length >= size) {
-            array = a;
-        } else {
-            array = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
-        }
-
-        final Iterator<E> it = iterator();
-        for (int i = 0; i < array.length; i++) {
-            if (it.hasNext()) {
-                array[i] = (T) it.next();
-            } else {
-                if (a == array) {
-                    array[i] = null;
-                    return array;
-                }
-
-                if (a.length < i) {
-                    return Arrays.copyOf(array, i);
-                }
-
-                System.arraycopy(array, 0, a, 0, i);
-                if (a.length > i) {
-                    a[i] = null;
-                }
-                return a;
-            }
-        }
-        return array;
+        return toList(a.length).toArray(a);
     }
 
     @Override