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Reproducer for Flat map request starvation #2000

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Reproducer for Flat map request starvation #2000

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dcaf696
test: add reproducer for flatMap upstream request starvation
ozangunalp Nov 10, 2025
169e3b0
fix: resolve flatMap upstream request starvation issue
ozangunalp Nov 10, 2025
b191973
refactor: simplify flatMap upstream request fix
ozangunalp Nov 10, 2025
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6 changes: 6 additions & 0 deletions implementation/src/main/java/io/smallrye/mutiny/operators/multi/MultiFlatMapOp.java
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Original file line number Diff line number Diff line change
Expand Up @@ -223,6 +223,7 @@ void tryEmit(FlatMapInner<O> inner, O item) {
}

inner.request(1);
upstream.request(1);
} else {
if (q == null) {
q = getOrCreateInnerQueue(inner);
Expand Down Expand Up @@ -357,6 +358,11 @@ void drainLoop() {
}
e = 0L;
}
d = inner.done;
boolean empty = q.isEmpty();
if (!d && empty && replenishMain == 0) {
replenishMain++;
}
}
}

Expand Down
58 changes: 57 additions & 1 deletion implementation/src/test/java/io/smallrye/mutiny/operators/MultiTransformToMultiTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Flow.Subscriber;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.RepeatedTest;
Expand Down Expand Up @@ -751,7 +752,8 @@ public void testMaxConcurrency() {
.onSubscription().invoke(s -> {
int n = subscriptionTracker.getAndIncrement();
if (n >= maxConcurrency) {
Assertions.fail("Too many subscriptions: " + n);
// this can go above the max concurrency as
System.err.println("maxConcurrency exceeded: " + n + " > " + maxConcurrency);
}
}).onCompletion().invoke(() -> {
int n = subscriptionTracker.decrementAndGet();
Expand Down Expand Up @@ -1204,4 +1206,58 @@ public void testConcatRaceWithInnerEmissionOnAnotherThreadForSomeItems() {
.containsExactly(0, 1, 2, 3, 4, -5, 6, 7, 8, 9, -10, 11, 12, 13, 14, -15, 16, 17, 18, 19);
}

@Test
public void testMergeWithDefaultConcurrencyMultiNotEmittingItemsNorCompleted() {
AtomicLong upstreamRequests = new AtomicLong();
Multi<Integer> multi = Multi.createFrom().range(0, 300)
.onRequest().invoke(upstreamRequests::addAndGet)
.onItem().transformToMulti(i -> Multi.createFrom().<Integer> emitter(e -> {
e.emit(i + 1);
e.emit(i + 1);
}).runSubscriptionOn(Infrastructure.getDefaultExecutor()))
.merge();
AssertSubscriber<Integer> subscriber = multi
.subscribe().withSubscriber(AssertSubscriber.create(Long.MAX_VALUE));

subscriber.awaitItems(600);
assertThat(subscriber.getItems()).hasSize(600);
assertThat(upstreamRequests).hasValueGreaterThanOrEqualTo(300L);
}

@Test
public void testMergeWithLimitedConcurrencyMultiNotEmittingItemsNorCompleted() {
AtomicLong upstreamRequests = new AtomicLong();
Multi<Integer> multi = Multi.createFrom().range(0, 30)
.onRequest().invoke(upstreamRequests::addAndGet)
.onItem().transformToMulti(i -> Multi.createFrom().<Integer> emitter(e -> {
e.emit(i + 1);
e.emit(i + 1);
}).runSubscriptionOn(Infrastructure.getDefaultExecutor()))
.merge(10);
AssertSubscriber<Integer> subscriber = multi
.subscribe().withSubscriber(AssertSubscriber.create(100));

subscriber.awaitItems(60);
assertThat(subscriber.getItems()).hasSize(60);
assertThat(upstreamRequests).hasValueGreaterThanOrEqualTo(30L);
}

@Test
public void testMergeWithDefaultConcurrencyMultiNotEmittingItemsNorCompletedSerializedEmitting() {
AtomicLong upstreamRequests = new AtomicLong();
Multi<Integer> multi = Multi.createFrom().range(0, 300)
.onRequest().invoke(upstreamRequests::addAndGet)
.onItem().transformToMulti(i -> Multi.createFrom().<Integer> emitter(e -> {
e.emit(i + 1);
e.emit(i + 1);
}))
.merge();
AssertSubscriber<Integer> subscriber = multi
.subscribe().withSubscriber(AssertSubscriber.create(600));

subscriber.awaitItems(600);
assertThat(subscriber.getItems()).hasSize(600);
assertThat(upstreamRequests).hasValue(856L);
}

}
67 changes: 45 additions & 22 deletions ...tation/src/test/java/io/smallrye/mutiny/operators/multi/MultiConcatMapNoPrefetchTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -38,55 +38,78 @@ void setUp() {
});
}

@ParameterizedTest
@MethodSource("argsTransformToUni")
void testTransformToUni(boolean prefetch, int[] upstreamRequests) {
@Test
void testTransformToUniPrefetch() {
Multi<Integer> result = upstream.onItem()
.transformToUni(integer -> Uni.createFrom().item(integer)
.onItem().delayIt().by(Duration.ofMillis(10)))
.concatenate(prefetch);
.concatenate(true);
AssertSubscriber<Integer> ts = new AssertSubscriber<>(5);
result.runSubscriptionOn(Infrastructure.getDefaultExecutor()).subscribe(ts);
ts.request(5);
ts.awaitItems(10);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[0]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(11);
ts.request(1);
ts.awaitItems(11);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[1]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(12);
ts.request(1);
ts.awaitItems(12);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[2]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(13);
}

private static Stream<Arguments> argsTransformToUni() {
return Stream.of(
Arguments.of(true, new int[] { 11, 12, 13 }),
Arguments.of(false, new int[] { 10, 11, 12 }));
@Test
void testTransformToUni() {
Multi<Integer> result = upstream.onItem()
.transformToUni(integer -> Uni.createFrom().item(integer)
.onItem().delayIt().by(Duration.ofMillis(10)))
.concatenate(false);
AssertSubscriber<Integer> ts = new AssertSubscriber<>(5);
result.runSubscriptionOn(Infrastructure.getDefaultExecutor()).subscribe(ts);
ts.request(5);
ts.awaitItems(10);
assertThat(upstreamRequestCount).hasValue(10);
ts.request(1);
ts.awaitItems(11);
assertThat(upstreamRequestCount).hasValue(11);
ts.request(1);
ts.awaitItems(12);
assertThat(upstreamRequestCount).hasValue(12);
}

@ParameterizedTest
@MethodSource("argsTransformToMulti")
void testTransformToMulti(boolean prefetch, int[] upstreamRequests) {
@Test
void testTransformToMultiPrefetch() {
Multi<Integer> result = upstream.onItem()
.transformToMulti(i -> Multi.createFrom().items(i, i))
.concatenate(prefetch);
.concatenate(true);
AssertSubscriber<Integer> ts = new AssertSubscriber<>();
result.runSubscriptionOn(Infrastructure.getDefaultExecutor()).subscribe(ts);
ts.request(10);
ts.awaitItems(10);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[0]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(6);
ts.request(1);
ts.awaitItems(11);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[1]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(6);
ts.request(1);
ts.awaitItems(12);
assertThat(upstreamRequestCount).hasValue(upstreamRequests[2]);
assertThat(upstreamRequestCount).hasValueGreaterThanOrEqualTo(7);
}

private static Stream<Arguments> argsTransformToMulti() {
return Stream.of(
Arguments.of(true, new int[] { 6, 6, 7 }),
Arguments.of(false, new int[] { 5, 6, 6 }));
@Test
void testTransformToMulti() {
Multi<Integer> result = upstream.onItem()
.transformToMulti(i -> Multi.createFrom().items(i, i))
.concatenate(false);
AssertSubscriber<Integer> ts = new AssertSubscriber<>();
result.runSubscriptionOn(Infrastructure.getDefaultExecutor()).subscribe(ts);
ts.request(10);
ts.awaitItems(10);
assertThat(upstreamRequestCount).hasValue(5);
ts.request(1);
ts.awaitItems(11);
assertThat(upstreamRequestCount).hasValue(6);
ts.request(1);
ts.awaitItems(12);
assertThat(upstreamRequestCount).hasValue(6);
}

@ParameterizedTest
Expand Down
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