Apply read-only enforcement after R2DBC transaction begin

Includes prepareTransactionalConnection variant aligned with JDBC DataSourceTransactionManager.

Closes gh-28610

spring-r2dbc/src/main/java/org/springframework/r2dbc/connection/R2dbcTransactionManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -47,7 +47,7 @@ import org.springframework.util.Assert;
  * <p><b>Note: The {@code ConnectionFactory} that this transaction manager
  * operates on needs to return independent {@code Connection}s.</b>
  * The {@code Connection}s may come from a pool (the typical case), but the
- * {@code ConnectionFactory} must not return scoped scoped {@code Connection}s
+ * {@code ConnectionFactory} must not return scoped {@code Connection}s
  * or the like. This transaction manager will associate {@code Connection}
  * with context-bound transactions itself, according to the specified propagation
  * behavior. It assumes that a separate, independent {@code Connection} can
@@ -142,8 +142,8 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 	 * transactional connection: "SET TRANSACTION READ ONLY" as understood by Oracle,
 	 * MySQL and Postgres.
 	 * <p>The exact treatment, including any SQL statement executed on the connection,
-	 * can be customized through through {@link #prepareTransactionalConnection}.
+	 * can be customized through {@link #prepareTransactionalConnection(Connection, TransactionDefinition)}.
-	 * @see #prepareTransactionalConnection
+	 * @see #prepareTransactionalConnection(Connection, TransactionDefinition)
 	 */
 	public void setEnforceReadOnly(boolean enforceReadOnly) {
 		this.enforceReadOnly = enforceReadOnly;
@@ -179,6 +179,7 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 		return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
 	}
 
+	@SuppressWarnings("deprecation")
 	@Override
 	protected Mono<Void> doBegin(TransactionSynchronizationManager synchronizationManager, Object transaction,
 			TransactionDefinition definition) throws TransactionException {
@@ -202,8 +203,9 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 				connectionMono = Mono.just(txObject.getConnectionHolder().getConnection());
 			}
 
-			return connectionMono.flatMap(con -> {
+			return connectionMono.flatMap(con -> prepareTransactionalConnection(con, definition, transaction)
-				return prepareTransactionalConnection(con, definition, transaction).then(Mono.from(con.beginTransaction()))
+					.then(Mono.from(con.beginTransaction()))
+					.then(prepareTransactionalConnection(con, definition))
 					.doOnSuccess(v -> {
 						txObject.getConnectionHolder().setTransactionActive(true);
 						Duration timeout = determineTimeout(definition);
@@ -221,8 +223,7 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 									.then(Mono.error(e));
 						}
 						return Mono.error(e);
-						});
+			})).onErrorResume(e -> {
-			}).onErrorResume(e -> {
 				CannotCreateTransactionException ex = new CannotCreateTransactionException(
 						"Could not open R2DBC Connection for transaction", e);
 				return Mono.error(ex);
@@ -350,31 +351,17 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 	}
 
 	/**
-	 * Prepare the transactional {@link Connection} right after transaction begin.
+	 * Prepare the transactional {@link Connection} right before transaction begin.
-	 * <p>The default implementation executes a "SET TRANSACTION READ ONLY" statement if the
+	 * @deprecated in favor of {@link #prepareTransactionalConnection(Connection, TransactionDefinition)}
-	 * {@link #setEnforceReadOnly "enforceReadOnly"} flag is set to {@code true} and the
+	 * since this variant gets called too early (before transaction begin) for read-only customization
-	 * transaction definition indicates a read-only transaction.
-	 * <p>The "SET TRANSACTION READ ONLY" is understood by Oracle, MySQL and Postgres
-	 * and may work with other databases as well. If you'd like to adapt this treatment,
-	 * override this method accordingly.
-	 * @param con the transactional R2DBC Connection
-	 * @param definition the current transaction definition
-	 * @param transaction the transaction object
-	 * @see #setEnforceReadOnly
 	 */
+	@Deprecated
 	protected Mono<Void> prepareTransactionalConnection(
 			Connection con, TransactionDefinition definition, Object transaction) {
 
 		ConnectionFactoryTransactionObject txObject = (ConnectionFactoryTransactionObject) transaction;
-
 		Mono<Void> prepare = Mono.empty();
 
-		if (isEnforceReadOnly() && definition.isReadOnly()) {
-			prepare = Mono.from(con.createStatement("SET TRANSACTION READ ONLY").execute())
-					.flatMapMany(Result::getRowsUpdated)
-					.then();
-		}
-
 		// Apply specific isolation level, if any.
 		IsolationLevel isolationLevelToUse = resolveIsolationLevel(definition.getIsolationLevel());
 		if (isolationLevelToUse != null && definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
@@ -404,6 +391,29 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 		return prepare;
 	}
 
+	/**
+	 * Prepare the transactional {@link Connection} right after transaction begin.
+	 * <p>The default implementation executes a "SET TRANSACTION READ ONLY" statement if the
+	 * {@link #setEnforceReadOnly "enforceReadOnly"} flag is set to {@code true} and the
+	 * transaction definition indicates a read-only transaction.
+	 * <p>The "SET TRANSACTION READ ONLY" is understood by Oracle, MySQL and Postgres
+	 * and may work with other databases as well. If you'd like to adapt this treatment,
+	 * override this method accordingly.
+	 * @param con the transactional R2DBC Connection
+	 * @param definition the current transaction definition
+	 * @since 5.3.22
+	 * @see #setEnforceReadOnly
+	 */
+	protected Mono<Void> prepareTransactionalConnection(Connection con, TransactionDefinition definition) {
+		Mono<Void> prepare = Mono.empty();
+		if (isEnforceReadOnly() && definition.isReadOnly()) {
+			prepare = Mono.from(con.createStatement("SET TRANSACTION READ ONLY").execute())
+					.flatMapMany(Result::getRowsUpdated)
+					.then();
+		}
+		return prepare;
+	}
+
 	/**
 	 * Resolve the {@linkplain TransactionDefinition#getIsolationLevel() isolation level constant} to a R2DBC
 	 * {@link IsolationLevel}. If you'd like to extend isolation level translation for vendor-specific