Spring Transaction isolation level cannot be passed to R2DBC connection in Spring Boot 3

The isolation level cannot be set/passed to the r2dbc connection after upgrading to Spring Boot 3 and updating r2dbc.

This can be reproduced with the following test (the second exception is thrown):

@Transactional(isolation = Isolation.SERIALIZABLE)
public Mono<Void> runAndEnsureSerializable() {
    return Mono.deferContextual(ctx -> {
        var transactionContext = ctx.get(TransactionContext.class);

        if (transactionContext.getCurrentTransactionIsolationLevel() != Isolation.SERIALIZABLE.value()) {
            throw new RuntimeException("Expected isolation level to be SERIALIZABLE");
        }

        return db.inConnection(connection -> {
            if (connection.getTransactionIsolationLevel() != IsolationLevel.SERIALIZABLE) {
                throw new RuntimeException("Expected isolation level to be SERIALIZABLE");
            }

            return Mono.empty();
        });
    });
}

and the following configuration:

@Bean
public ConnectionFactory connectionFactory(PostgreSQLContainer postgres) {
    return new ConnectionPool(ConnectionPoolConfiguration.builder()
            .initialSize(0)
            .validationDepth(ValidationDepth.REMOTE)
            .connectionFactory(ConnectionFactories.get(ConnectionFactoryOptions.builder()
                    .option(DRIVER, "postgresql")
                    .option(HOST, postgres.getHost())
                    .option(PORT, postgres.getFirstMappedPort())
                    .option(USER, postgres.getUsername())
                    .option(PASSWORD, postgres.getPassword())
                    .option(DATABASE, postgres.getDatabaseName())
                    .build()))
            .customizer(builder -> builder.releaseHandler(Connection::rollbackTransaction))
            .build());
}
````

We are using:

org.springframework.boot spring-boot-starter-parent 3.0.2 ... io.r2dbc r2dbc-spi 1.0.0.RELEASE org.postgresql r2dbc-postgresql 1.0.0.RELEASE io.r2dbc r2dbc-pool

The problem seems to be that autocommit is switched in the `R2dbcTransactionManager` in spring-r2dbc:6.0.4 but not in spring-r2dbc:5.3.24:

spring-r2dbc:6.0.4:
```java
connectionMono.flatMap(con -> switchAutoCommitIfNecessary(con, transaction)
      .then(Mono.from(doBegin(definition, con)))
      .then(prepareTransactionalConnection(con, definition))

spring-r2dbc:5.3.24:

connectionMono.flatMap(con -> prepareTransactionalConnection(con, definition, transaction)
      .then(Mono.from(con.beginTransaction()))
      .then(prepareTransactionalConnection(con, definition))

Any ideas?

Comment From: jhoeller

The isolation level is being passed through R2DBC 1.0's TransactionDefinition in the begin call now. This should apply in any case, even with an explicit auto-commit off switch before. @mp911de any insight into the impact of the call sequence here?

Comment From: jhoeller

This should finally be fixed through #30508 for the upcoming 6.0.10 release. The auto-commit handling is implicit with beginTransaction in R2DBC 1.0 now, so we have removed our custom auto-commit check completely.