Publisher#flatMapConcatIterable may skip emitting items

Motivation:
Publisher#flatMapConcatIterable may not emit some items due to race
conditions and visibility issues. The iterator state is written to
outside the scope of holding the lock. After a drain loop completes
we may request 1 more iterator. However it is possible the thread
emitting holds the lock while another thread invokes onNext(t).
The emitting thread may not see the iterator, and instead see
`EmptyIterator.instance()` and cause it to request 1 more item, but
then the not-visible iterator contents won't be emitted.

Modifications:
- Make FlatMapIterableSubscriber iterator state volatile and atomically
update it. There is only ever 1 valid iterator because only 1 outstanding
demand is issued only after the current iterator `!hasNext()`. The iterator
state is re-read on each drain loop, and the terminal condition must atomically
set to EmptyIterator.

servicetalk-concurrent-api/src/main/java/io/servicetalk/concurrent/api/PublisherConcatMapIterable.java

@@ -23,6 +23,7 @@
 import java.util.NoSuchElementException;
 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 import java.util.concurrent.atomic.AtomicLongFieldUpdater;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 import java.util.function.Function;
 import javax.annotation.Nullable;
 
@@ -57,6 +58,9 @@ private static final class FlatMapIterableSubscriber<T, U> implements Subscriber
         @SuppressWarnings("rawtypes")
         private static final AtomicIntegerFieldUpdater<FlatMapIterableSubscriber> emittingUpdater =
                 AtomicIntegerFieldUpdater.newUpdater(FlatMapIterableSubscriber.class, "emitting");
+        @SuppressWarnings("rawtypes")
+        private static final AtomicReferenceFieldUpdater<FlatMapIterableSubscriber, Iterator> iterUpdater =
+                AtomicReferenceFieldUpdater.newUpdater(FlatMapIterableSubscriber.class, Iterator.class, "iterator");
         private final Function<? super T, ? extends Iterable<? extends U>> mapper;
         private final Subscriber<? super U> target;
         @Nullable
@@ -74,7 +78,7 @@ private static final class FlatMapIterableSubscriber<T, U> implements Subscriber
          * <p>
          * Visibility and thread safety provided by {@link #emitting}.
          */
-        private Iterator<? extends U> currentIterator = emptyIterator();
+        private volatile Iterator<? extends U> iterator = emptyIterator();
         @SuppressWarnings("unused")
         private volatile long requestN;
         @SuppressWarnings("unused")
@@ -98,8 +102,9 @@ public void onSubscribe(Subscription s) {
         public void onNext(T u) {
             // If Function.apply(...) throws we just propagate it to the caller which is responsible to terminate
             // its subscriber and cancel the subscription.
-            currentIterator = requireNonNull(mapper.apply(u).iterator(),
-                    () -> "Iterator from mapper " + mapper + " is null");
+            // Safe to assign because we only ever have demand outstanding of 1, so we never
+            // should concurrently access nextIterator or have multiple iterators being valid at any given time.
+            iterator = requireNonNull(mapper.apply(u).iterator(), () -> "Iterator from mapper " + mapper + " is null");
             tryDrainIterator(ErrorHandlingStrategyInDrain.Throw);
         }
 
@@ -151,8 +156,9 @@ public void cancel() {
 
         private void doCancel() {
             assert sourceSubscription != null;
-            final Iterator<? extends U> currentIterator = this.currentIterator;
-            this.currentIterator = EmptyIterator.instance();
+            @SuppressWarnings("unchecked")
+            final Iterator<? extends U> currentIterator =
+                    (Iterator<? extends U>) iterUpdater.getAndSet(this, EmptyIterator.instance());
             try {
                 tryClose(currentIterator);
             } finally {
@@ -181,13 +187,14 @@ private void tryDrainIterator(ErrorHandlingStrategyInDrain errorHandlingStrategy
                 if (!tryAcquireLock(emittingUpdater, this)) {
                     break;
                 }
+                Iterator<? extends U> currIter = iterator;
                 long currRequestN = this.requestN;
                 final long initialRequestN = currRequestN;
                 try {
                     try {
-                        while ((hasNext = currentIterator.hasNext()) && currRequestN > 0) {
+                        while ((hasNext = currIter.hasNext()) && currRequestN > 0) {
                             --currRequestN;
-                            target.onNext(currentIterator.next());
+                            target.onNext(currIter.next());
                         }
                     } catch (Throwable cause) {
                         switch (errorHandlingStrategyInDrain) {
@@ -199,16 +206,16 @@ private void tryDrainIterator(ErrorHandlingStrategyInDrain errorHandlingStrategy
                             case Propagate:
                                 terminated = true;
                                 safeOnError(target, cause);
-                                tryClose(currentIterator);
+                                tryClose(currIter);
                                 return; // hard return to avoid potential for duplicate terminal events
                             case Throw:
                                 // since we only request 1 at a time we maybe holding requestN demand, in this case we
                                 // discard the current iterator and request 1 more from upstream (if there is demand).
                                 hasNext = false;
                                 thrown = true;
-                                final Iterator<? extends U> currentIterator = this.currentIterator;
-                                this.currentIterator = EmptyIterator.instance();
-                                tryClose(currentIterator);
+                                iterUpdater.compareAndSet(this, currIter, EmptyIterator.instance());
+                                tryClose(currIter);
+                                currIter = EmptyIterator.instance();
                                 // let the exception propagate so the upstream source can do the cleanup.
                                 throw cause;
                             default:
@@ -235,15 +242,16 @@ private void tryDrainIterator(ErrorHandlingStrategyInDrain errorHandlingStrategy
                                     doCancel();
                                 }
                             } else if (terminalNotification == null && !hasNext && currRequestN > 0 &&
-                                    (currentIterator != EmptyIterator.instance() || thrown)) {
-                                // We only request 1 at a time, and therefore we don't have any outstanding demand, so
-                                // we will not be getting an onNext call, so we write to the currentIterator variable
-                                // here before we unlock emitting so visibility to other threads should be taken care of
-                                // by the write to emitting below (and later read).
-                                currentIterator = EmptyIterator.instance();
-                                if (sourceSubscription != null) {
-                                    sourceSubscription.request(1);
-                                }
+                                    (currIter != EmptyIterator.instance() || thrown) &&
+                                    // We only request 1 at a time, and therefore we don't have outstanding demand.
+                                    // We will not be getting an onNext call concurrently, but the onNext(..) call may
+                                    // be on a different thread outside the emitting lock. For this reason we do a CAS
+                                    // to ensure the currIter read at the beginning of the outer loop is still the
+                                    // current iterator. If the CAS fails the outer loop will re-read iterator and try
+                                    // to emit if items are present and demand allows it.
+                                    iterUpdater.compareAndSet(this, currIter, EmptyIterator.instance())) {
+                                assert sourceSubscription != null;
+                                sourceSubscription.request(1);
                             }
                         } finally {
                             // The lock must be released after we interact with the subscription for thread safety

servicetalk-concurrent-api/src/test/java/io/servicetalk/concurrent/api/PublisherConcatMapIterableTest.java

@@ -16,6 +16,7 @@
 package io.servicetalk.concurrent.api;
 
 import io.servicetalk.concurrent.BlockingIterable;
+import io.servicetalk.concurrent.BlockingIterator;
 import io.servicetalk.concurrent.PublisherSource.Processor;
 import io.servicetalk.concurrent.PublisherSource.Subscriber;
 import io.servicetalk.concurrent.PublisherSource.Subscription;
@@ -26,6 +27,7 @@
 import org.junit.jupiter.api.extension.RegisterExtension;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.CyclicBarrier;
@@ -38,6 +40,7 @@
 import static io.servicetalk.concurrent.api.Processors.newPublisherProcessor;
 import static io.servicetalk.concurrent.api.Publisher.failed;
 import static io.servicetalk.concurrent.api.Publisher.from;
+import static io.servicetalk.concurrent.api.Publisher.fromIterable;
 import static io.servicetalk.concurrent.api.SourceAdapters.fromSource;
 import static io.servicetalk.concurrent.api.SourceAdapters.toSource;
 import static io.servicetalk.concurrent.internal.DeliberateException.DELIBERATE_EXCEPTION;
@@ -46,8 +49,10 @@
 import static java.util.Collections.singletonList;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.function.Function.identity;
+import static java.util.stream.IntStream.range;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.contains;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.Matchers.sameInstance;
@@ -422,6 +427,20 @@ void exceptionFromSubscriptionRequestNIsPropagated() {
         assertThat(subscriber.awaitOnError(), is(DELIBERATE_EXCEPTION));
     }
 
+    @Test
+    void concurrencyEmitsInOrder() throws Exception {
+        try (BlockingIterator<Integer> iterable = fromIterable(() -> range(0, 10_000).iterator())
+                .publishOn(Executors.global())
+                .flatMapConcatIterable(Collections::singletonList)
+                .toIterable()
+                .iterator()) {
+            int expected = 0;
+            while (iterable.hasNext()) {
+                assertThat(iterable.next(), equalTo(expected++));
+            }
+        }
+    }
+
     private void verifyTermination(boolean success) {
         if (success) {
             publisher.onComplete();