#48 - Atomically close connections.

We now make sure to close connections only once by tracking the cleanup state. Flux.usingWhen/Mono.usingWhen do not ensure atomic cleanup in situations where the subscription completes and then the subscription is terminated.

This behavior has lead to closing a connection multiple times.

Related ticket: reactor/reactor-core#1486

src/main/java/org/springframework/data/r2dbc/function/DefaultDatabaseClient.java

@@ -38,6 +38,7 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Supplier;
@@ -122,15 +123,16 @@ class DefaultDatabaseClient implements DatabaseClient, ConnectionAccessor {
 
 		Assert.notNull(action, "Callback object must not be null");
 
-		Mono<Connection> connectionMono = getConnection();
-		// Create close-suppressing Connection proxy, also preparing returned Statements.
+		Mono<ConnectionCloseHolder> connectionMono = getConnection()
+				.map(it -> new ConnectionCloseHolder(it, this::closeConnection));
 
 		return Mono.usingWhen(connectionMono, it -> {
 
-			Connection connectionToUse = createConnectionProxy(it);
+			// Create close-suppressing Connection proxy
+			Connection connectionToUse = createConnectionProxy(it.connection);
 
 			return doInConnection(connectionToUse, action);
-		}, this::closeConnection, this::closeConnection, this::closeConnection) //
+		}, ConnectionCloseHolder::close, ConnectionCloseHolder::close, ConnectionCloseHolder::close) //
 				.onErrorMap(R2dbcException.class, ex -> translateException("execute", getSql(action), ex));
 	}
 
@@ -149,15 +151,16 @@ class DefaultDatabaseClient implements DatabaseClient, ConnectionAccessor {
 
 		Assert.notNull(action, "Callback object must not be null");
 
-		Mono<Connection> connectionMono = getConnection();
-		// Create close-suppressing Connection proxy, also preparing returned Statements.
+		Mono<ConnectionCloseHolder> connectionMono = getConnection()
+				.map(it -> new ConnectionCloseHolder(it, this::closeConnection));
 
 		return Flux.usingWhen(connectionMono, it -> {
 
-			Connection connectionToUse = createConnectionProxy(it);
+			// Create close-suppressing Connection proxy, also preparing returned Statements.
+			Connection connectionToUse = createConnectionProxy(it.connection);
 
 			return doInConnectionMany(connectionToUse, action);
-		}, this::closeConnection, this::closeConnection, this::closeConnection) //
+		}, ConnectionCloseHolder::close, ConnectionCloseHolder::close, ConnectionCloseHolder::close) //
 				.onErrorMap(R2dbcException.class, ex -> translateException("executeMany", getSql(action), ex));
 	}
 
@@ -1104,4 +1107,26 @@ class DefaultDatabaseClient implements DatabaseClient, ConnectionAccessor {
 			}
 		}
 	}
+
+	/**
+	 * Holder for a connection that makes sure the close action is invoked atomically only once.
+	 */
+	@RequiredArgsConstructor
+	static class ConnectionCloseHolder extends AtomicBoolean {
+
+		final Connection connection;
+		final Function<Connection, Publisher<Void>> closeFunction;
+
+		Mono<Void> close() {
+
+			return Mono.defer(() -> {
+
+				if (compareAndSet(false, true)) {
+					return Mono.from(closeFunction.apply(connection));
+				}
+
+				return Mono.empty();
+			});
+		}
+	}
 }

src/test/java/org/springframework/data/r2dbc/function/DefaultDatabaseClientUnitTests.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2019 the original author or 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 org.springframework.data.r2dbc.function;
+
+import static org.mockito.Mockito.*;
+
+import io.r2dbc.spi.Connection;
+import io.r2dbc.spi.ConnectionFactory;
+import reactor.core.CoreSubscriber;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscription;
+import org.springframework.data.r2dbc.support.R2dbcExceptionTranslator;
+
+/**
+ * Unit tests for {@link DefaultDatabaseClient}.
+ *
+ * @author Mark Paluch
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class DefaultDatabaseClientUnitTests {
+
+	@Mock ConnectionFactory connectionFactory;
+	@Mock Connection connection;
+	@Mock ReactiveDataAccessStrategy strategy;
+	@Mock R2dbcExceptionTranslator translator;
+
+	@Before
+	public void before() {
+		when(connectionFactory.create()).thenReturn((Publisher) Mono.just(connection));
+		when(connection.close()).thenReturn(Mono.empty());
+	}
+
+	@Test // gh-48
+	public void shouldCloseConnectionOnlyOnce() {
+
+		DefaultDatabaseClient databaseClient = (DefaultDatabaseClient) DatabaseClient.builder()
+				.connectionFactory(connectionFactory).dataAccessStrategy(strategy).exceptionTranslator(translator).build();
+
+		Flux<Object> flux = databaseClient.inConnectionMany(it -> {
+			return Flux.empty();
+		});
+
+		flux.subscribe(new CoreSubscriber<Object>() {
+			Subscription subscription;
+
+			@Override
+			public void onSubscribe(Subscription s) {
+				s.request(1);
+				subscription = s;
+			}
+
+			@Override
+			public void onNext(Object o) {}
+
+			@Override
+			public void onError(Throwable t) {}
+
+			@Override
+			public void onComplete() {
+				subscription.cancel();
+			}
+		});
+
+		verify(connection, times(1)).close();
+	}
+}