Merge pull request #306 from nschaefe/master

fix issue 305: CodeConverter.replaceArrayAccess leads to java.util.ConcurrentModificationException

4 files changed, 794 insertions, 11 deletions

diff --git a/src/main/javassist/bytecode/analysis/Type.java b/src/main/javassist/bytecode/analysis/Type.java
index db02df35..33dc81f3 100644
--- a/src/main/javassist/bytecode/analysis/Type.java
+++ b/src/main/javassist/bytecode/analysis/Type.java
@@ -15,9 +15,7 @@
  */
 package javassist.bytecode.analysis;
 
-import java.util.HashMap;
-import java.util.IdentityHashMap;
-import java.util.Map;
+import java.util.*;
 
 import javassist.ClassPool;
 import javassist.CtClass;
@@ -496,24 +494,26 @@ public class Type {
         if (typeMap == null||typeMap.isEmpty())
             alterMap.clear();
 
-        for (String name:alterMap.keySet())
+        Iterator<String> it = alterMap.keySet().iterator();
+        while (it.hasNext()) {
+            String name = it.next();
             if (!typeMap.containsKey(name))
-                alterMap.remove(name);
+                it.remove();
+        }
 
         // Reduce to subinterfaces
         // This does not need to be recursive since we make a copy,
         // and that copy contains all super types for the whole hierarchy
-        for (CtClass intf:alterMap.values()) {
-            CtClass[] interfaces;
+        Collection<CtClass> interfaces = new ArrayList<>();
+        for (CtClass intf : alterMap.values()) {
             try {
-                interfaces = intf.getInterfaces();
+                interfaces.addAll(Arrays.asList(intf.getInterfaces()));
             } catch (NotFoundException e) {
                 throw new RuntimeException(e);
             }
-
-            for (CtClass c:interfaces)
-                alterMap.remove(c.getName());
         }
+        for (CtClass c : interfaces)
+            alterMap.remove(c.getName());
 
         return alterMap;
     }
diff --git a/src/test/javassist/JvstTest.java b/src/test/javassist/JvstTest.java
index aab0b907..47a61083 100644
--- a/src/test/javassist/JvstTest.java
+++ b/src/test/javassist/JvstTest.java
@@ -1179,6 +1179,7 @@ public class JvstTest extends JvstTestRoot {
         suite.addTestSuite(test.javassist.proxy.ProxySimpleTest.class);
         suite.addTestSuite(test.javassist.bytecode.analysis.AnalyzerTest.class);
         suite.addTestSuite(test.javassist.convert.ArrayAccessReplaceTest.class);
+        suite.addTestSuite(test.javassist.convert.ArrayAccessReplaceTest2.class);
         suite.addTestSuite(test.javassist.bytecode.analysis.DomTreeTest.class);
         return suite;
     }
diff --git a/src/test/test/javassist/convert/ArrayAccessReplaceTest2.java b/src/test/test/javassist/convert/ArrayAccessReplaceTest2.java
new file mode 100644
index 00000000..5fb6401f
--- /dev/null
+++ b/src/test/test/javassist/convert/ArrayAccessReplaceTest2.java
@@ -0,0 +1,21 @@
+package test.javassist.convert;
+
+import javassist.ClassPool;
+import javassist.CodeConverter;
+import javassist.CtClass;
+import junit.framework.TestCase;
+
+public class ArrayAccessReplaceTest2 extends TestCase {
+
+    public void testAdvancedInstrumentation() throws Exception {
+        ClassPool pool = new ClassPool(true);
+        CtClass monitoringClass = pool.get(ArrayAccessReplaceTest2.class.getName());
+        CtClass targetClass = pool.get(InstrumentationTarget.class.getName());
+        CodeConverter converter = new CodeConverter();
+        // we just test if the instrumentation works, the monitoring class does not need to actually contain the replacer methods
+        // what is only relevant when code gets executed
+        converter.replaceArrayAccess(monitoringClass, new CodeConverter.DefaultArrayAccessReplacementMethodNames());
+        targetClass.instrument(converter);
+    }
+
+}
\ No newline at end of file
diff --git a/src/test/test/javassist/convert/InstrumentationTarget.java b/src/test/test/javassist/convert/InstrumentationTarget.java
new file mode 100644
index 00000000..82ac7af0
--- /dev/null
+++ b/src/test/test/javassist/convert/InstrumentationTarget.java
@@ -0,0 +1,761 @@
+package test.javassist.convert;
+
+import java.util.AbstractQueue;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
+
+
+/**
+ * A clone of java.util.concurrent.ArrayBlockingQueue as target for testing
+ */
+public class InstrumentationTarget<E> extends AbstractQueue<E>
+        implements BlockingQueue<E>, java.io.Serializable {
+    /**
+     * Serialization ID. This class relies on default serialization
+     * even for the items array, which is default-serialized, even if
+     * it is empty. Otherwise it could not be declared final, which is
+     * necessary here.
+     */
+    private static final long serialVersionUID = -817911632652898426L;
+
+    /**
+     * The queued items
+     */
+    final Object[] items;
+
+    /**
+     * items index for next take, poll, peek or remove
+     */
+    int takeIndex;
+
+    /**
+     * items index for next put, offer, or add
+     */
+    int putIndex;
+
+    /**
+     * Number of elements in the queue
+     */
+    int count;
+
+    /*
+     * Concurrency control uses the classic two-condition algorithm
+     * found in any textbook.
+     */
+
+    /**
+     * Main lock guarding all access
+     */
+    final ReentrantLock lock;
+    /**
+     * Condition for waiting takes
+     */
+    private final Condition notEmpty;
+    /**
+     * Condition for waiting puts
+     */
+    private final Condition notFull;
+
+    // Internal helper methods
+
+    /**
+     * Circularly increment i.
+     */
+    final int inc(int i) {
+        return (++i == items.length) ? 0 : i;
+    }
+
+    /**
+     * Circularly decrement i.
+     */
+    final int dec(int i) {
+        return ((i == 0) ? items.length : i) - 1;
+    }
+
+    @SuppressWarnings("unchecked")
+    static <E> E cast(Object item) {
+        return (E) item;
+    }
+
+    /**
+     * Returns item at index i.
+     */
+    final E itemAt(int i) {
+        return this.<E>cast(items[i]);
+    }
+
+    /**
+     * Throws NullPointerException if argument is null.
+     *
+     * @param v the element
+     */
+    private static void checkNotNull(Object v) {
+        if (v == null)
+            throw new NullPointerException();
+    }
+
+    /**
+     * Inserts element at current put position, advances, and signals.
+     * Call only when holding lock.
+     */
+    private void insert(E x) {
+        items[putIndex] = x;
+        putIndex = inc(putIndex);
+        ++count;
+        notEmpty.signal();
+    }
+
+    /**
+     * Extracts element at current take position, advances, and signals.
+     * Call only when holding lock.
+     */
+    private E extract() {
+        final Object[] items = this.items;
+        E x = this.<E>cast(items[takeIndex]);
+        items[takeIndex] = null;
+        takeIndex = inc(takeIndex);
+        --count;
+        notFull.signal();
+        return x;
+    }
+
+    /**
+     * Deletes item at position i.
+     * Utility for remove and iterator.remove.
+     * Call only when holding lock.
+     */
+    void removeAt(int i) {
+        final Object[] items = this.items;
+        // if removing front item, just advance
+        if (i == takeIndex) {
+            items[takeIndex] = null;
+            takeIndex = inc(takeIndex);
+        } else {
+            // slide over all others up through putIndex.
+            for (; ; ) {
+                int nexti = inc(i);
+                if (nexti != putIndex) {
+                    items[i] = items[nexti];
+                    i = nexti;
+                } else {
+                    items[i] = null;
+                    putIndex = i;
+                    break;
+                }
+            }
+        }
+        --count;
+        notFull.signal();
+    }
+
+    /**
+     * Creates an {@code Target} with the given (fixed)
+     * capacity and default access policy.
+     *
+     * @param capacity the capacity of this queue
+     * @throws IllegalArgumentException if {@code capacity < 1}
+     */
+    public InstrumentationTarget(int capacity) {
+        this(capacity, false);
+    }
+
+    /**
+     * Creates an {@code Target} with the given (fixed)
+     * capacity and the specified access policy.
+     *
+     * @param capacity the capacity of this queue
+     * @param fair     if {@code true} then queue accesses for threads blocked
+     *                 on insertion or removal, are processed in FIFO order;
+     *                 if {@code false} the access order is unspecified.
+     * @throws IllegalArgumentException if {@code capacity < 1}
+     */
+    public InstrumentationTarget(int capacity, boolean fair) {
+        if (capacity <= 0)
+            throw new IllegalArgumentException();
+        this.items = new Object[capacity];
+        lock = new ReentrantLock(fair);
+        notEmpty = lock.newCondition();
+        notFull = lock.newCondition();
+    }
+
+    /**
+     * Creates an {@code Target} with the given (fixed)
+     * capacity, the specified access policy and initially containing the
+     * elements of the given collection,
+     * added in traversal order of the collection's iterator.
+     *
+     * @param capacity the capacity of this queue
+     * @param fair     if {@code true} then queue accesses for threads blocked
+     *                 on insertion or removal, are processed in FIFO order;
+     *                 if {@code false} the access order is unspecified.
+     * @param c        the collection of elements to initially contain
+     * @throws IllegalArgumentException if {@code capacity} is less than
+     *                                  {@code c.size()}, or less than 1.
+     * @throws NullPointerException     if the specified collection or any
+     *                                  of its elements are null
+     */
+    public InstrumentationTarget(int capacity, boolean fair,
+                                 Collection<? extends E> c) {
+        this(capacity, fair);
+
+        final ReentrantLock lock = this.lock;
+        lock.lock(); // Lock only for visibility, not mutual exclusion
+        try {
+            int i = 0;
+            try {
+                for (E e : c) {
+                    checkNotNull(e);
+                    items[i++] = e;
+                }
+            } catch (ArrayIndexOutOfBoundsException ex) {
+                throw new IllegalArgumentException();
+            }
+            count = i;
+            putIndex = (i == capacity) ? 0 : i;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue if it is
+     * possible to do so immediately without exceeding the queue's capacity,
+     * returning {@code true} upon success and throwing an
+     * {@code IllegalStateException} if this queue is full.
+     *
+     * @param e the element to add
+     * @return {@code true} (as specified by {@link Collection#add})
+     * @throws IllegalStateException if this queue is full
+     * @throws NullPointerException  if the specified element is null
+     */
+    public boolean add(E e) {
+        return super.add(e);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue if it is
+     * possible to do so immediately without exceeding the queue's capacity,
+     * returning {@code true} upon success and {@code false} if this queue
+     * is full.  This method is generally preferable to method {@link #add},
+     * which can fail to insert an element only by throwing an exception.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        checkNotNull(e);
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            if (count == items.length)
+                return false;
+            else {
+                insert(e);
+                return true;
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting
+     * for space to become available if the queue is full.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public void put(E e) throws InterruptedException {
+        checkNotNull(e);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            while (count == items.length)
+                notFull.await();
+            insert(e);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting
+     * up to the specified wait time for space to become available if
+     * the queue is full.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit)
+            throws InterruptedException {
+
+        checkNotNull(e);
+        long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            while (count == items.length) {
+                if (nanos <= 0)
+                    return false;
+                nanos = notFull.awaitNanos(nanos);
+            }
+            insert(e);
+            return true;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E poll() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return (count == 0) ? null : extract();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E take() throws InterruptedException {
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            while (count == 0)
+                notEmpty.await();
+            return extract();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            while (count == 0) {
+                if (nanos <= 0)
+                    return null;
+                nanos = notEmpty.awaitNanos(nanos);
+            }
+            return extract();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E peek() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return (count == 0) ? null : itemAt(takeIndex);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // this doc comment is overridden to remove the reference to collections
+    // greater in size than Integer.MAX_VALUE
+
+    /**
+     * Returns the number of elements in this queue.
+     *
+     * @return the number of elements in this queue
+     */
+    public int size() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // this doc comment is a modified copy of the inherited doc comment,
+    // without the reference to unlimited queues.
+
+    /**
+     * Returns the number of additional elements that this queue can ideally
+     * (in the absence of memory or resource constraints) accept without
+     * blocking. This is always equal to the initial capacity of this queue
+     * less the current {@code size} of this queue.
+     *
+     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+     * an element will succeed by inspecting {@code remainingCapacity}
+     * because it may be the case that another thread is about to
+     * insert or remove an element.
+     */
+    public int remainingCapacity() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return items.length - count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element {@code e} such
+     * that {@code o.equals(e)}, if this queue contains one or more such
+     * elements.
+     * Returns {@code true} if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * <p>Removal of interior elements in circular array based queues
+     * is an intrinsically slow and disruptive operation, so should
+     * be undertaken only in exceptional circumstances, ideally
+     * only when the queue is known not to be accessible by other
+     * threads.
+     *
+     * @param o element to be removed from this queue, if present
+     * @return {@code true} if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        if (o == null) return false;
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
+                if (o.equals(items[i])) {
+                    removeAt(i);
+                    return true;
+                }
+            }
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns {@code true} if this queue contains the specified element.
+     * More formally, returns {@code true} if and only if this queue contains
+     * at least one element {@code e} such that {@code o.equals(e)}.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return {@code true} if this queue contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
+                if (o.equals(items[i]))
+                    return true;
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence.
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this queue.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this queue
+     */
+    public Object[] toArray() {
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            final int count = this.count;
+            Object[] a = new Object[count];
+            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
+                a[k] = items[i];
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence; the runtime type of the returned array is that of
+     * the specified array.  If the queue fits in the specified array, it
+     * is returned therein.  Otherwise, a new array is allocated with the
+     * runtime type of the specified array and the size of this queue.
+     *
+     * <p>If this queue fits in the specified array with room to spare
+     * (i.e., the array has more elements than this queue), the element in
+     * the array immediately following the end of the queue is set to
+     * {@code null}.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose {@code x} is a queue known to contain only strings.
+     * The following code can be used to dump the queue into a newly
+     * allocated array of {@code String}:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     * <p>
+     * Note that {@code toArray(new Object[0])} is identical in function to
+     * {@code toArray()}.
+     *
+     * @param a the array into which the elements of the queue are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this queue
+     * @throws ArrayStoreException  if the runtime type of the specified array
+     *                              is not a supertype of the runtime type of every element in
+     *                              this queue
+     * @throws NullPointerException if the specified array is null
+     */
+    @SuppressWarnings("unchecked")
+    public <T> T[] toArray(T[] a) {
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            final int count = this.count;
+            final int len = a.length;
+            if (len < count)
+                a = (T[]) java.lang.reflect.Array.newInstance(
+                        a.getClass().getComponentType(), count);
+            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
+                a[k] = (T) items[i];
+            if (len > count)
+                a[count] = null;
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public String toString() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int k = count;
+            if (k == 0)
+                return "[]";
+
+            StringBuilder sb = new StringBuilder();
+            sb.append('[');
+            for (int i = takeIndex; ; i = inc(i)) {
+                Object e = items[i];
+                sb.append(e == this ? "(this Collection)" : e);
+                if (--k == 0)
+                    return sb.append(']').toString();
+                sb.append(',').append(' ');
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this queue.
+     * The queue will be empty after this call returns.
+     */
+    public void clear() {
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
+                items[i] = null;
+            count = 0;
+            putIndex = 0;
+            takeIndex = 0;
+            notFull.signalAll();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        checkNotNull(c);
+        if (c == this)
+            throw new IllegalArgumentException();
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int n = 0;
+            int max = count;
+            while (n < max) {
+                c.add(this.<E>cast(items[i]));
+                items[i] = null;
+                i = inc(i);
+                ++n;
+            }
+            if (n > 0) {
+                count = 0;
+                putIndex = 0;
+                takeIndex = 0;
+                notFull.signalAll();
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        checkNotNull(c);
+        if (c == this)
+            throw new IllegalArgumentException();
+        if (maxElements <= 0)
+            return 0;
+        final Object[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int n = 0;
+            int max = (maxElements < count) ? maxElements : count;
+            while (n < max) {
+                c.add(this.<E>cast(items[i]));
+                items[i] = null;
+                i = inc(i);
+                ++n;
+            }
+            if (n > 0) {
+                count -= n;
+                takeIndex = i;
+                notFull.signalAll();
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an iterator over the elements in this queue in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     *
+     * <p>The returned {@code Iterator} is a "weakly consistent" iterator that
+     * will never throw {@link java.util.ConcurrentModificationException
+     * ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue in proper sequence
+     */
+    public Iterator<E> iterator() {
+        return new Itr();
+    }
+
+    /**
+     * Iterator for Target. To maintain weak consistency
+     * with respect to puts and takes, we (1) read ahead one slot, so
+     * as to not report hasNext true but then not have an element to
+     * return -- however we later recheck this slot to use the most
+     * current value; (2) ensure that each array slot is traversed at
+     * most once (by tracking "remaining" elements); (3) skip over
+     * null slots, which can occur if takes race ahead of iterators.
+     * However, for circular array-based queues, we cannot rely on any
+     * well established definition of what it means to be weakly
+     * consistent with respect to interior removes since these may
+     * require slot overwrites in the process of sliding elements to
+     * cover gaps. So we settle for resiliency, operating on
+     * established apparent nexts, which may miss some elements that
+     * have moved between calls to next.
+     */
+    private class Itr implements Iterator<E> {
+        private int remaining; // Number of elements yet to be returned
+        private int nextIndex; // Index of element to be returned by next
+        private E nextItem;    // Element to be returned by next call to next
+        private E lastItem;    // Element returned by last call to next
+        private int lastRet;   // Index of last element returned, or -1 if none
+
+        Itr() {
+            final ReentrantLock lock = InstrumentationTarget.this.lock;
+            lock.lock();
+            try {
+                lastRet = -1;
+                if ((remaining = count) > 0)
+                    nextItem = itemAt(nextIndex = takeIndex);
+            } finally {
+                lock.unlock();
+            }
+        }
+
+        public boolean hasNext() {
+            return remaining > 0;
+        }
+
+        public E next() {
+            final ReentrantLock lock = InstrumentationTarget.this.lock;
+            lock.lock();
+            try {
+                if (remaining <= 0)
+                    throw new NoSuchElementException();
+                lastRet = nextIndex;
+                E x = itemAt(nextIndex);  // check for fresher value
+                if (x == null) {
+                    x = nextItem;         // we are forced to report old value
+                    lastItem = null;      // but ensure remove fails
+                } else
+                    lastItem = x;
+                while (--remaining > 0 && // skip over nulls
+                        (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
+                    ;
+                return x;
+            } finally {
+                lock.unlock();
+            }
+        }
+
+        public void remove() {
+            final ReentrantLock lock = InstrumentationTarget.this.lock;
+            lock.lock();
+            try {
+                int i = lastRet;
+                if (i == -1)
+                    throw new IllegalStateException();
+                lastRet = -1;
+                E x = lastItem;
+                lastItem = null;
+                // only remove if item still at index
+                if (x != null && x == items[i]) {
+                    boolean removingHead = (i == takeIndex);
+                    removeAt(i);
+                    if (!removingHead)
+                        nextIndex = dec(nextIndex);
+                }
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+
+}
+