2.x: Fix the extra retention problem in ReplaySubject (#5892)

* 2.x: Fix the extra retention problem in ReplaySubject

* Cover the already-trimmed case.

Author: akarnokd

src/main/java/io/reactivex/subjects/ReplaySubject.java

@@ -13,15 +13,14 @@
 
 package io.reactivex.subjects;
 
-import io.reactivex.annotations.Nullable;
 import java.lang.reflect.Array;
 import java.util.*;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.*;
 
 import io.reactivex.Observer;
 import io.reactivex.Scheduler;
-import io.reactivex.annotations.CheckReturnValue;
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.internal.functions.ObjectHelper;
 import io.reactivex.internal.util.NotificationLite;
@@ -94,8 +93,9 @@
  * in a non-blocking and thread-safe manner via {@link #hasValue()}, {@link #getValue()},
  * {@link #getValues()} or {@link #getValues(Object[])}.
  * <p>
- * Note that due to concurrency requirements, a size-bounded {@code ReplaySubject} may hold strong references to more
- * source emissions than specified.
+ * Note that due to concurrency requirements, a size- and time-bounded {@code ReplaySubject} may hold strong references to more
+ * source emissions than specified while it isn't terminated yet. Use the {@link #cleanupBuffer()} to allow
+ * such inaccessible items to be cleaned up by GC once no consumer references it anymore.
  * <dl>
  *  <dt><b>Scheduler:</b></dt>
  *  <dd>{@code ReplaySubject} does not operate by default on a particular {@link io.reactivex.Scheduler} and
@@ -415,6 +415,24 @@ public T getValue() {
         return buffer.getValue();
     }
 
+    /**
+     * Makes sure the item cached by the head node in a bounded
+     * ReplaySubject is released (as it is never part of a replay).
+     * <p>
+     * By default, live bounded buffers will remember one item before
+     * the currently receivable one to ensure subscribers can always
+     * receive a continuous sequence of items. A terminated ReplaySubject
+     * automatically releases this inaccessible item.
+     * <p>
+     * The method must be called sequentially, similar to the standard
+     * {@code onXXX} methods.
+     * @since 2.1.11 - experimental
+     */
+    @Experimental
+    public void cleanupBuffer() {
+        buffer.trimHead();
+    }
+
     /** An empty array to avoid allocation in getValues(). */
     private static final Object[] EMPTY_ARRAY = new Object[0];
 
@@ -563,6 +581,12 @@ interface ReplayBuffer<T> {
          * @return true if successful
          */
         boolean compareAndSet(Object expected, Object next);
+
+        /**
+         * Make sure an old inaccessible head value is released
+         * in a bounded buffer.
+         */
+        void trimHead();
     }
 
     static final class ReplayDisposable<T> extends AtomicInteger implements Disposable {
@@ -619,10 +643,16 @@ public void add(T value) {
         @Override
         public void addFinal(Object notificationLite) {
             buffer.add(notificationLite);
+            trimHead();
             size++;
             done = true;
         }
 
+        @Override
+        public void trimHead() {
+            // no-op in this type of buffer
+        }
+
         @Override
         @Nullable
         @SuppressWarnings("unchecked")
@@ -839,9 +869,24 @@ public void addFinal(Object notificationLite) {
             size++;
             t.lazySet(n); // releases both the tail and size
 
+            trimHead();
             done = true;
         }
 
+        /**
+         * Replace a non-empty head node with an empty one to
+         * allow the GC of the inaccessible old value.
+         */
+        @Override
+        public void trimHead() {
+            Node<Object> h = head;
+            if (h.value != null) {
+                Node<Object> n = new Node<Object>(null);
+                n.lazySet(h.get());
+                head = n;
+            }
+        }
+
         @Override
         @Nullable
         @SuppressWarnings("unchecked")
@@ -1047,12 +1092,24 @@ void trimFinal() {
             for (;;) {
                 TimedNode<Object> next = h.get();
                 if (next.get() == null) {
-                    head = h;
+                    if (h.value != null) {
+                        TimedNode<Object> lasth = new TimedNode<Object>(null, 0L);
+                        lasth.lazySet(h.get());
+                        head = lasth;
+                    } else {
+                        head = h;
+                    }
                     break;
                 }
 
                 if (next.time > limit) {
-                    head = h;
+                    if (h.value != null) {
+                        TimedNode<Object> lasth = new TimedNode<Object>(null, 0L);
+                        lasth.lazySet(h.get());
+                        head = lasth;
+                    } else {
+                        head = h;
+                    }
                     break;
                 }
 
@@ -1085,6 +1142,20 @@ public void addFinal(Object notificationLite) {
             done = true;
         }
 
+        /**
+         * Replace a non-empty head node with an empty one to
+         * allow the GC of the inaccessible old value.
+         */
+        @Override
+        public void trimHead() {
+            TimedNode<Object> h = head;
+            if (h.value != null) {
+                TimedNode<Object> n = new TimedNode<Object>(null, 0);
+                n.lazySet(h.get());
+                head = n;
+            }
+        }
+
         @Override
         @Nullable
         @SuppressWarnings("unchecked")

src/test/java/io/reactivex/subjects/ReplaySubjectTest.java

@@ -30,6 +30,7 @@
 import io.reactivex.functions.Function;
 import io.reactivex.observers.*;
 import io.reactivex.schedulers.*;
+import io.reactivex.subjects.ReplaySubject.*;
 
 public class ReplaySubjectTest extends SubjectTest<Integer> {
 
@@ -1184,4 +1185,92 @@ public void timedNoOutdatedData() {
 
         source.test().assertResult();
     }
+
+    @Test
+    public void noHeadRetentionCompleteSize() {
+        ReplaySubject<Integer> source = ReplaySubject.createWithSize(1);
+
+        source.onNext(1);
+        source.onNext(2);
+        source.onComplete();
+
+        SizeBoundReplayBuffer<Integer> buf = (SizeBoundReplayBuffer<Integer>)source.buffer;
+
+        assertNull(buf.head.value);
+
+        Object o = buf.head;
+
+        source.cleanupBuffer();
+
+        assertSame(o, buf.head);
+    }
+
+
+    @Test
+    public void noHeadRetentionSize() {
+        ReplaySubject<Integer> source = ReplaySubject.createWithSize(1);
+
+        source.onNext(1);
+        source.onNext(2);
+
+        SizeBoundReplayBuffer<Integer> buf = (SizeBoundReplayBuffer<Integer>)source.buffer;
+
+        assertNotNull(buf.head.value);
+
+        source.cleanupBuffer();
+
+        assertNull(buf.head.value);
+
+        Object o = buf.head;
+
+        source.cleanupBuffer();
+
+        assertSame(o, buf.head);
+    }
+
+    @Test
+    public void noHeadRetentionCompleteTime() {
+        ReplaySubject<Integer> source = ReplaySubject.createWithTime(1, TimeUnit.MINUTES, Schedulers.computation());
+
+        source.onNext(1);
+        source.onNext(2);
+        source.onComplete();
+
+        SizeAndTimeBoundReplayBuffer<Integer> buf = (SizeAndTimeBoundReplayBuffer<Integer>)source.buffer;
+
+        assertNull(buf.head.value);
+
+        Object o = buf.head;
+
+        source.cleanupBuffer();
+
+        assertSame(o, buf.head);
+    }
+
+    @Test
+    public void noHeadRetentionTime() {
+        TestScheduler sch = new TestScheduler();
+
+        ReplaySubject<Integer> source = ReplaySubject.createWithTime(1, TimeUnit.MILLISECONDS, sch);
+
+        source.onNext(1);
+
+        sch.advanceTimeBy(2, TimeUnit.MILLISECONDS);
+
+        source.onNext(2);
+
+        SizeAndTimeBoundReplayBuffer<Integer> buf = (SizeAndTimeBoundReplayBuffer<Integer>)source.buffer;
+
+        assertNotNull(buf.head.value);
+
+        source.cleanupBuffer();
+
+        assertNull(buf.head.value);
+
+        Object o = buf.head;
+
+        source.cleanupBuffer();
+
+        assertSame(o, buf.head);
+    }
 }