Do not cancel subscription on BlockingIterable#hasNext(long, TimeUnit)

This makes sure that if this method, or in extend next(long, TimeUnit) is
called and times out, the caller is able to retry the operation and also
the upstream source will not be cancelled.

In the context of a Blocking Streaming server, this means that if a timeout
is thrown on the incoming request, the outgoing response can still be modified
since the underlying socket will not be immediately closed.

It also aligns the semantics with Single#toFuture where a blocking get with
a timeout on the future also does not cancel the upstream Single.

A (temporary) system property is introduced which allows to fall back to the
old behavior should incompatibilities be discovered in the wild.

A note for the reader who wonders how to close the subscription now: the close()
method always did cancel the subscription and continues to do so.

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

@@ -72,6 +72,12 @@ public BlockingIterator<T> iterator() {
     }
 
     private static final class SubscriberAndIterator<T> implements Subscriber<T>, BlockingIterator<T> {
+        /**
+         * Allows to re-enable cancelling the subscription on {@link #hasNext(long, TimeUnit)} timeout. This flag
+         * will be removed after a couple releases and no issues identified with the new behavior.
+         */
+        private static final boolean CANCEL_SUBSCRIPTION_ON_HAS_NEXT_TIMEOUT = Boolean.parseBoolean(
+                System.getProperty("io.servicetalk.concurrent.api.cancelSubscriptionOnHasNextTimeout", "false"));
         private static final Logger LOGGER = LoggerFactory.getLogger(SubscriberAndIterator.class);
         private static final Object CANCELLED_SIGNAL = new Object();
         private static final TerminalNotification COMPLETE_NOTIFICATION = complete();
@@ -172,7 +178,9 @@ public boolean hasNext(final long timeout, final TimeUnit unit) throws TimeoutEx
                 next = data.poll(timeout, unit);
                 if (next == null) {
                     terminated = true;
-                    subscription.cancel();
+                    if (CANCEL_SUBSCRIPTION_ON_HAS_NEXT_TIMEOUT) {
+                        subscription.cancel();
+                    }
                     throw new TimeoutException("timed out after: " + timeout + " units: " + unit);
                 }
                 requestMoreIfRequired();

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

@@ -71,7 +71,7 @@ void errorEmittedIsThrown() {
         DeliberateException de = new DeliberateException();
         Iterator<Integer> iterator = Publisher.<Integer>failed(de).toIterable().iterator();
         assertThat("Item expected but not found.", iterator.hasNext(), is(true));
-        assertSame(de, assertThrows(DeliberateException.class, () -> iterator.next()));
+        assertSame(de, assertThrows(DeliberateException.class, iterator::next));
     }
 
     @Test
@@ -80,7 +80,7 @@ void doubleHashNextWithError() {
         Iterator<Integer> iterator = Publisher.<Integer>failed(de).toIterable().iterator();
         assertThat("Item expected but not found.", iterator.hasNext(), is(true));
         assertThat("Second hasNext inconsistent with first.", iterator.hasNext(), is(true));
-        assertSame(de, assertThrows(DeliberateException.class, () -> iterator.next()));
+        assertSame(de, assertThrows(DeliberateException.class, iterator::next));
     }
 
     @Test
@@ -93,7 +93,7 @@ void hasNextWithEmpty() {
     void nextWithEmpty() {
         Iterator<Integer> iterator = Publisher.<Integer>empty().toIterable().iterator();
         assertThat("Item not expected but found.", iterator.hasNext(), is(false));
-        assertThrows(NoSuchElementException.class, () -> iterator.next());
+        assertThrows(NoSuchElementException.class, iterator::next);
     }
 
     @Test
@@ -109,7 +109,10 @@ void hasNextWithTimeout() throws Exception {
 
         assertThrows(TimeoutException.class, () -> iterator.hasNext(10, MILLISECONDS));
         assertThat("Unexpected item found.", iterator.hasNext(-1, MILLISECONDS), is(false));
-        assertTrue(subscription.isCancelled());
+
+        assertThat(subscription.isCancelled(), is(false));
+        iterator.close();
+        assertThat(subscription.isCancelled(), is(true));
     }
 
     @Test
@@ -124,9 +127,11 @@ void nextWithTimeout() throws Exception {
         assertThat("Unexpected item found.", iterator.next(-1, MILLISECONDS), is(2));
 
         assertThrows(TimeoutException.class, () -> iterator.next(10, MILLISECONDS));
-
         assertThat("Unexpected item found.", iterator.hasNext(-1, MILLISECONDS), is(false));
-        assertTrue(subscription.isCancelled());
+
+        assertThat(subscription.isCancelled(), is(false));
+        iterator.close();
+        assertThat(subscription.isCancelled(), is(true));
     }
 
     @Test
@@ -173,7 +178,7 @@ void nextWithoutHasNextAndTerminal() {
         source.onNext(2);
         assertThat("Unexpected item found.", iterator.next(), is(2));
         source.onComplete();
-        assertThrows(NoSuchElementException.class, () -> iterator.next());
+        assertThrows(NoSuchElementException.class, iterator::next);
     }
 
     @Test
@@ -234,7 +239,7 @@ void delayOnNextThenError() {
         DeliberateException de = new DeliberateException();
         source.onError(de);
         assertThat("Item not expected but found.", iterator.hasNext(), is(true));
-        Exception e = assertThrows(DeliberateException.class, () -> iterator.next());
+        Exception e = assertThrows(DeliberateException.class, iterator::next);
         assertThat(e, is(de));
     }
 
@@ -310,7 +315,7 @@ void queueFullButAccommodatesOnError() {
         source.onError(de);
         verifyNextIs(iterator, 1);
         assertThat("Item expected but not found.", iterator.hasNext(), is(true));
-        Exception e = assertThrows(DeliberateException.class, () -> iterator.next());
+        Exception e = assertThrows(DeliberateException.class, iterator::next);
         assertThat(e, sameInstance(de));
     }
 

servicetalk-concurrent/src/main/java/io/servicetalk/concurrent/BlockingIterable.java

@@ -64,6 +64,7 @@ public interface BlockingIterable<T> extends CloseableIterable<T> {
     default void forEach(Consumer<? super T> action, LongSupplier timeoutSupplier, TimeUnit unit)
             throws TimeoutException {
         requireNonNull(action);
+        // TODO: should we here try/catch with close explicitly now?
         BlockingIterator<T> iterator = iterator();
         while (iterator.hasNext(timeoutSupplier.getAsLong(), unit)) {
             action.accept(iterator.next(timeoutSupplier.getAsLong(), unit));
@@ -96,6 +97,8 @@ default void forEach(Consumer<? super T> action, LongSupplier timeoutSupplier, T
      */
     default void forEach(Consumer<? super T> action, long timeout, TimeUnit unit) throws TimeoutException {
         requireNonNull(action);
+        // TODO: should we here try/catch with close explicitly now?
+
         BlockingIterator<T> iterator = iterator();
         long remainingTimeoutNanos = unit.toNanos(timeout);
         long timeStampANanos = nanoTime();