1.x: eager concatMap to choose safe or unsafe queue based on platform.

I forgot to add the choice because 2.x SpscArrayQueue doesn't use
Unsafe. I copied the SpscAtomicArrayQueue from ReactiveX#3169 and I hope it won't
conflict.

Author: akarnokd

src/main/java/rx/internal/operators/OperatorEagerConcatMap.java

@@ -24,7 +24,8 @@
 import rx.Observable.Operator;
 import rx.exceptions.Exceptions;
 import rx.functions.*;
-import rx.internal.util.unsafe.SpscArrayQueue;
+import rx.internal.util.atomic.SpscAtomicArrayQueue;
+import rx.internal.util.unsafe.*;
 import rx.subscriptions.Subscriptions;
 
 public final class OperatorEagerConcatMap<T, R> implements Operator<R, T> {
@@ -278,7 +279,13 @@ static final class EagerInnerSubscriber<T> extends Subscriber<T> {
         public EagerInnerSubscriber(EagerOuterSubscriber<?, T> parent, int bufferSize) {
             super();
             this.parent = parent;
-            this.queue = new SpscArrayQueue<T>(bufferSize);
+            Queue<T> q;
+            if (UnsafeAccess.isUnsafeAvailable()) {
+                q = new SpscArrayQueue<T>(bufferSize);
+            } else {
+                q = new SpscAtomicArrayQueue<T>(bufferSize);
+            }
+            this.queue = q;
             request(bufferSize);
         }
 

src/main/java/rx/internal/util/atomic/AtomicReferenceArrayQueue.java

@@ -0,0 +1,75 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * 
+ * Original License: https://github.com/JCTools/JCTools/blob/master/LICENSE
+ * Original location: https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic/AtomicReferenceArrayQueue.java
+ */
+package rx.internal.util.atomic;
+
+import java.util.*;
+import java.util.concurrent.atomic.AtomicReferenceArray;
+
+import rx.internal.util.unsafe.Pow2;
+
+abstract class AtomicReferenceArrayQueue<E> extends AbstractQueue<E> {
+    protected final AtomicReferenceArray<E> buffer;
+    protected final int mask;
+    public AtomicReferenceArrayQueue(int capacity) {
+        int actualCapacity = Pow2.roundToPowerOfTwo(capacity);
+        this.mask = actualCapacity - 1;
+        this.buffer = new AtomicReferenceArray<E>(actualCapacity);
+    }
+    @Override
+    public Iterator<E> iterator() {
+        throw new UnsupportedOperationException();
+    }
+    @Override
+    public void clear() {
+        // we have to test isEmpty because of the weaker poll() guarantee
+        while (poll() != null || !isEmpty())
+            ;
+    }
+    protected final int calcElementOffset(long index, int mask) {
+        return (int)index & mask;
+    }
+    protected final int calcElementOffset(long index) {
+        return (int)index & mask;
+    }
+    protected final E lvElement(AtomicReferenceArray<E> buffer, int offset) {
+        return buffer.get(offset);
+    }
+    protected final E lpElement(AtomicReferenceArray<E> buffer, int offset) {
+        return buffer.get(offset); // no weaker form available
+    }
+    protected final E lpElement(int offset) {
+        return buffer.get(offset); // no weaker form available
+    }
+    protected final void spElement(AtomicReferenceArray<E> buffer, int offset, E value) {
+        buffer.lazySet(offset, value);  // no weaker form available
+    }
+    protected final void spElement(int offset, E value) {
+        buffer.lazySet(offset, value);  // no weaker form available
+    }
+    protected final void soElement(AtomicReferenceArray<E> buffer, int offset, E value) {
+        buffer.lazySet(offset, value);
+    }
+    protected final void soElement(int offset, E value) {
+        buffer.lazySet(offset, value);
+    }
+    protected final void svElement(AtomicReferenceArray<E> buffer, int offset, E value) {
+        buffer.set(offset, value);
+    }
+    protected final E lvElement(int offset) {
+        return lvElement(buffer, offset);
+    }
+}

src/main/java/rx/internal/util/atomic/SpscAtomicArrayQueue.java

@@ -0,0 +1,124 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * 
+ * Original License: https://github.com/JCTools/JCTools/blob/master/LICENSE
+ * Original location: https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic/SpscAtomicArrayQueue.java
+ */
+package rx.internal.util.atomic;
+
+import java.util.concurrent.atomic.*;
+
+/**
+ * A Single-Producer-Single-Consumer queue backed by a pre-allocated buffer.
+ * <p>
+ * This implementation is a mashup of the <a href="http://sourceforge.net/projects/mc-fastflow/">Fast Flow</a>
+ * algorithm with an optimization of the offer method taken from the <a
+ * href="http://staff.ustc.edu.cn/~bhua/publications/IJPP_draft.pdf">BQueue</a> algorithm (a variation on Fast
+ * Flow), and adjusted to comply with Queue.offer semantics with regards to capacity.<br>
+ * For convenience the relevant papers are available in the resources folder:<br>
+ * <i>2010 - Pisa - SPSC Queues on Shared Cache Multi-Core Systems.pdf<br>
+ * 2012 - Junchang- BQueue- Efficient and Practical Queuing.pdf <br>
+ * </i> This implementation is wait free.
+ * 
+ * @param <E>
+ */
+public final class SpscAtomicArrayQueue<E> extends AtomicReferenceArrayQueue<E> {
+    private static final Integer MAX_LOOK_AHEAD_STEP = Integer.getInteger("jctools.spsc.max.lookahead.step", 4096);
+    final AtomicLong producerIndex;
+    protected long producerLookAhead;
+    final AtomicLong consumerIndex;
+    final int lookAheadStep;
+    public SpscAtomicArrayQueue(int capacity) {
+        super(capacity);
+        this.producerIndex = new AtomicLong();
+        this.consumerIndex = new AtomicLong();
+        lookAheadStep = Math.min(capacity / 4, MAX_LOOK_AHEAD_STEP);
+    }
+
+    @Override
+    public boolean offer(E e) {
+        if (null == e) {
+            throw new NullPointerException("Null is not a valid element");
+        }
+        // local load of field to avoid repeated loads after volatile reads
+        final AtomicReferenceArray<E> buffer = this.buffer;
+        final int mask = this.mask;
+        final long index = producerIndex.get();
+        final int offset = calcElementOffset(index, mask);
+        if (index >= producerLookAhead) {
+            int step = lookAheadStep;
+            if (null == lvElement(buffer, calcElementOffset(index + step, mask))) {// LoadLoad
+                producerLookAhead = index + step;
+            }
+            else if (null != lvElement(buffer, offset)){
+                return false;
+            }
+        }
+        soProducerIndex(index + 1); // ordered store -> atomic and ordered for size()
+        soElement(buffer, offset, e); // StoreStore
+        return true;
+    }
+
+    @Override
+    public E poll() {
+        final long index = consumerIndex.get();
+        final int offset = calcElementOffset(index);
+        // local load of field to avoid repeated loads after volatile reads
+        final AtomicReferenceArray<E> lElementBuffer = buffer;
+        final E e = lvElement(lElementBuffer, offset);// LoadLoad
+        if (null == e) {
+            return null;
+        }
+        soConsumerIndex(index + 1); // ordered store -> atomic and ordered for size()
+        soElement(lElementBuffer, offset, null);// StoreStore
+        return e;
+    }
+
+    @Override
+    public E peek() {
+        return lvElement(calcElementOffset(consumerIndex.get()));
+    }
+
+    @Override
+    public int size() {
+        /*
+         * It is possible for a thread to be interrupted or reschedule between the read of the producer and consumer
+         * indices, therefore protection is required to ensure size is within valid range. In the event of concurrent
+         * polls/offers to this method the size is OVER estimated as we read consumer index BEFORE the producer index.
+         */
+        long after = lvConsumerIndex();
+        while (true) {
+            final long before = after;
+            final long currentProducerIndex = lvProducerIndex();
+            after = lvConsumerIndex();
+            if (before == after) {
+                return (int) (currentProducerIndex - after);
+            }
+        }
+    }
+
+    private void soProducerIndex(long newIndex) {
+        producerIndex.lazySet(newIndex);
+    }
+
+    private void soConsumerIndex(long newIndex) {
+        consumerIndex.lazySet(newIndex);
+    }
+    
+    private long lvConsumerIndex() {
+        return consumerIndex.get();
+    }
+    private long lvProducerIndex() {
+        return producerIndex.get();
+    }
+}