Add Publisher.onCompleteError

Motivation:
There are some scenarios where a stream terminating with
onComplete is not expected, and translating to an error
simplifies recovery.

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

@@ -0,0 +1,77 @@
+/*
+ * Copyright © 2023 Apple Inc. and the ServiceTalk project authors
+ *
+ * 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.
+ */
+package io.servicetalk.concurrent.api;
+
+import java.util.function.Supplier;
+import javax.annotation.Nullable;
+
+import static java.util.Objects.requireNonNull;
+
+final class OnCompleteErrorPublisher<T> extends AbstractSynchronousPublisherOperator<T, T> {
+    private final Supplier<? extends Throwable> errorSupplier;
+
+    OnCompleteErrorPublisher(final Publisher<T> original, final Supplier<? extends Throwable> errorSupplier) {
+        super(original);
+        this.errorSupplier = requireNonNull(errorSupplier);
+    }
+
+    @Override
+    public Subscriber<? super T> apply(final Subscriber<? super T> subscriber) {
+        return new OnCompleteErrorSubscriber<>(subscriber, errorSupplier);
+    }
+
+    private static final class OnCompleteErrorSubscriber<T> implements Subscriber<T> {
+        private final Subscriber<? super T> subscriber;
+        private final Supplier<? extends Throwable> errorSupplier;
+
+        private OnCompleteErrorSubscriber(final Subscriber<? super T> subscriber,
+                                          final Supplier<? extends Throwable> errorSupplier) {
+            this.subscriber = subscriber;
+            this.errorSupplier = errorSupplier;
+        }
+
+        @Override
+        public void onSubscribe(final Subscription subscription) {
+            subscriber.onSubscribe(subscription);
+        }
+
+        @Override
+        public void onNext(@Nullable final T t) {
+            subscriber.onNext(t);
+        }
+
+        @Override
+        public void onError(final Throwable t) {
+            subscriber.onError(t);
+        }
+
+        @Override
+        public void onComplete() {
+            final Throwable cause;
+            try {
+                cause = errorSupplier.get();
+            } catch (Throwable cause2) {
+                subscriber.onError(cause2);
+                return;
+            }
+            if (cause == null) {
+                subscriber.onComplete();
+            } else {
+                subscriber.onError(cause);
+            }
+        }
+    }
+}

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

@@ -561,6 +561,28 @@ public final <E extends Throwable> Publisher<T> onErrorReturn(
         return onErrorReturn(type::isInstance, rawSupplier);
     }
 
+    /**
+     * Transform this {@link Publisher}s {@link Subscriber#onComplete()} signal into
+     * {@link Subscriber#onError(Throwable)} signal (unless {@code null} error returned from {@code errorSupplier}).
+     * <p>
+     * This method provides a data transformation in sequential programming similar to:
+     * <pre>{@code
+     *     List<T> results = resultOfThisPublisher();
+     *     terminalOfThisPublisher();
+     *     Throwable cause = errorSupplier.get()
+     *     if (cause != null) {
+     *       throw cause;
+     *     }
+     * }</pre>
+     * @param errorSupplier returns the error to emit to {@link Subscriber#onError(Throwable)}. if the return value
+     * is {@code null} then complete with {@link Subscriber#onComplete()}.
+     * @return A {@link Publisher} which transform this {@link Publisher}s {@link Subscriber#onComplete()} signal into
+     * {@link Subscriber#onError(Throwable)} signal (unless {@code null} error returned from {@code errorSupplier}).
+     */
+    public final Publisher<T> onCompleteError(final Supplier<? extends Throwable> errorSupplier) {
+        return new OnCompleteErrorPublisher<>(this, errorSupplier);
+    }
+
     /**
      * Transform errors emitted on this {@link Publisher} which match {@code predicate} into
      * {@link Subscriber#onNext(Object)} then {@link Subscriber#onComplete()} signals (e.g. swallows the error).

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

@@ -0,0 +1,65 @@
+/*
+ * Copyright © 2023 Apple Inc. and the ServiceTalk project authors
+ *
+ * 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.
+ */
+package io.servicetalk.concurrent.api;
+
+import io.servicetalk.concurrent.test.internal.TestPublisherSubscriber;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import static io.servicetalk.concurrent.api.Publisher.from;
+import static io.servicetalk.concurrent.api.SourceAdapters.toSource;
+import static io.servicetalk.concurrent.internal.DeliberateException.DELIBERATE_EXCEPTION;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.sameInstance;
+
+final class OnCompleteErrorPublisherTest {
+    private final TestPublisherSubscriber<Integer> subscriber = new TestPublisherSubscriber<>();
+
+    @Test
+    void errorPassThrough() {
+        toSource(Publisher.<Integer>failed(DELIBERATE_EXCEPTION)
+                .onCompleteError(() -> new IllegalStateException("shouldn't get here"))
+        ).subscribe(subscriber);
+        assertThat(subscriber.awaitOnError(), sameInstance(DELIBERATE_EXCEPTION));
+    }
+
+    @Test
+    void nullCompletes() {
+        toSource(Publisher.<Integer>empty()
+                .onCompleteError(() -> null)
+        ).subscribe(subscriber);
+        subscriber.awaitOnComplete();
+    }
+
+    @ParameterizedTest(name = "{displayName} [{index}] shouldThrow={0}")
+    @ValueSource(booleans = {false, true})
+    void completeToError(boolean shouldThrow) {
+        toSource(from(1)
+                .onCompleteError(() -> {
+                    if (shouldThrow) {
+                        throw DELIBERATE_EXCEPTION;
+                    }
+                    return DELIBERATE_EXCEPTION;
+                })
+        ).subscribe(subscriber);
+        subscriber.awaitSubscription().request(1);
+        assertThat(subscriber.takeOnNext(), equalTo(1));
+        assertThat(subscriber.awaitOnError(), sameInstance(DELIBERATE_EXCEPTION));
+    }
+}

servicetalk-concurrent-reactivestreams/src/test/java/io/servicetalk/concurrent/reactivestreams/tck/PublisherOnCompleteErrorTckTest.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright © 2023 Apple Inc. and the ServiceTalk project authors
+ *
+ * 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.
+ */
+package io.servicetalk.concurrent.reactivestreams.tck;
+
+import io.servicetalk.concurrent.api.Publisher;
+
+import org.testng.annotations.Test;
+
+@Test
+public class PublisherOnCompleteErrorTckTest extends AbstractPublisherOperatorTckTest<Integer> {
+    @Override
+    protected Publisher<Integer> composePublisher(final Publisher<Integer> publisher, final int elements) {
+        return publisher.onCompleteError(() -> null);
+    }
+}