Fix "Operator has been terminated"

src/main/java/oracle/r2dbc/impl/OracleResultImpl.java

@@ -925,14 +925,11 @@ void addDependent() {
     protected final <T extends Segment, U> Publisher<U> mapSegments(
       Class<T> segmentType, Function<? super T, U> segmentMapper) {
 
-      @SuppressWarnings("unchecked")
-      Publisher<U> removeDependent = (Publisher<U>) dependentCounter.decrement();
+      Publisher<Void> removeDependent = dependentCounter.decrement();
 
-      return Flux.concatDelayError(
+      return Publishers.concatTerminal(
         mapDependentSegments(segmentType, segmentMapper),
-        removeDependent)
-        .doOnCancel(() ->
-          Mono.from(removeDependent).subscribe());
+        removeDependent);
     }
 
     /**

src/main/java/oracle/r2dbc/impl/OracleStatementImpl.java

@@ -1026,18 +1026,16 @@ private JdbcStatement(PreparedStatement preparedStatement, Object[] binds) {
      */
     final Publisher<OracleResultImpl> execute() {
 
-      Mono<OracleResultImpl> deallocate =
-        Mono.from(deallocate()).cast(OracleResultImpl.class);
+      Publisher<Void> deallocate = deallocate();
 
-      return Flux.concatDelayError(
+      return Publishers.concatTerminal(
         Mono.from(bind())
           .thenMany(executeJdbc())
           .map(this::getWarnings)
           .onErrorResume(R2dbcException.class, r2dbcException ->
             Mono.just(createErrorResult(r2dbcException)))
           .doOnNext(OracleResultImpl::addDependent),
-          deallocate)
-        .doOnCancel(deallocate::subscribe);
+        deallocate);
     }
 
     /**

src/main/java/oracle/r2dbc/impl/Publishers.java

@@ -0,0 +1,78 @@
+/*
+  Copyright (c) 2020, 2022, Oracle and/or its affiliates.
+
+  This software is dual-licensed to you under the Universal Permissive License
+  (UPL) 1.0 as shown at https://oss.oracle.com/licenses/upl or Apache License
+  2.0 as shown at http://www.apache.org/licenses/LICENSE-2.0. You may choose
+  either license.
+
+  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
+
+     https://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 oracle.r2dbc.impl;
+
+import org.reactivestreams.Publisher;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+/**
+ * Factory methods that create a {@code Publisher}. These methods cover special
+ * cases which are not already supported by Project Reactor.
+ */
+class Publishers {
+
+  private Publishers() {}
+
+  /**
+   * A publisher that immediately emits onNext and onComplete to subscribers
+   */
+  private static final Publisher<Object> COMPLETED_PUBLISHER =
+    Mono.just(new Object());
+
+  /**
+   * <p>
+   * Returns a publisher that emits the concatenated signals of a
+   * {@code publisher} and {@code onTerminationPublisher}. If the
+   * {@code onTerminationPublisher} emits an error, it will suppress any error
+   * emitted by the first {@code publisher}. If a subscription to the returned
+   * publisher is cancelled, the {@code onTerminationPublisher} is subscribed to
+   * but it can not emit any error through the cancelled subscription.
+   * </p><p>
+   * The returned publisher behaves similarly to: <pre>{@code
+   * Flux.concatDelayError(
+   *   publisher,
+   *   onTerminationPublisher)
+   *   .doOnCancel(onTerminationPublisher::subscribe)
+   * }</pre>
+   * However, the code above can result in:
+   * <pre>
+   *   reactor.core.Exceptions$StaticThrowable: Operator has been terminated
+   * </pre>
+   * This seems to happen when the concatDelayError publisher receives a cancel
+   * from a downstream subscriber after it has already received onComplete from
+   * a upstream publisher.
+   * </p>
+   * @param publisher First publisher which is subscribed to.
+   * @param onTerminationPublisher Publisher which is subscribed to when the
+   * first publisher terminates, or a subcription is cancelled.
+   * @return The concatenated publisher.
+   * @param <T> Type of objects emitted to onNext
+   */
+  static <T> Publisher<T> concatTerminal(
+    Publisher<T> publisher, Publisher<Void> onTerminationPublisher) {
+    return Flux.usingWhen(
+      COMPLETED_PUBLISHER,
+      ignored -> publisher,
+      ignored -> onTerminationPublisher);
+  }
+}