Chunk scan mode does not isolate each scanned item in its own transaction

[fxpaquette]

Hello Spring Batch team,

Bug description
In Spring Batch 6.0.3, chunk scanning after rollback in writer appears to run in a single transaction instead of running each scanned item in its own transaction.
This appears to be a regression from Spring Batch 5.

Environment

• Spring Batch: 6.0.3
• Java: 17+
• Database: H2
• Build tool: Maven

Steps to reproduce

The example uses the following scenario:
Fours items (1,2,3,4) are written in the same chunk, a skip policy skips every exception, with a configurable skip limit.
Item 2 and 3 throw exceptions, after performing an insert in the database.

With skipLimit=1, the step fails and no item is committed. Expected: item 1 should be committed before items 2 and 3 make the step fail.
With skipLimit=2, items 2 and 3 are committed even though they threw exceptions, database contains all items 1,2,3,4. Expected: only items 1 and 4 should be commited.

@Bean
public ListItemReader<String> reader() {
    return new ListItemReader<>(Arrays.asList("1", "2", "3", "4"));
}

@Bean
public ItemWriter<String> writer(JdbcTemplate jdbcTemplate) {
    return items -> {
        System.out.println(); System.out.println("Writing chunk of items..." + items); System.out.println();
        for (String number : items) {
            System.out.println("Writing item: " + number);
            writeItem(jdbcTemplate, number);
            System.out.println();
        }
    };
}

private void writeItem(JdbcTemplate jdbcTemplate, String number) {
    // Write item to database
    jdbcTemplate.execute("insert into delivery (item_number) values ('" + number + "')");

    // Throw error if item 2 or 3
    if (number.equals("2") || number.equals("3")) {
        System.out.println("Simulating error on item: " + number);
        throw new RuntimeException("Simulated write error for item: " + number);
    }
}

@Bean
public Step step(
        JobRepository jobRepository, PlatformTransactionManager tm,
        ListItemReader<String> reader, ItemWriter<String> writer) {
  return new StepBuilder("step", jobRepository)
          .<String, String>chunk(4)
          .transactionManager(tm)
          .reader(reader)
          .writer(writer)
          .faultTolerant()
          .skip(Exception.class)
          .skipLimit(skipLimit)
          .build();
}

1. Build and run the Spring Batch 6 project.
2. Run once with --skipLimit=1 and once with --skipLimit=2.
3. Command line to run the example:

mvn package exec:java -Dexec.mainClass=org.springframework.batch.MyBatchJobConfiguration -Dexec.args="--skipLimit=1"
mvn package exec:java -Dexec.mainClass=org.springframework.batch.MyBatchJobConfiguration -Dexec.args="--skipLimit=2"

4. Observe the final database content printed at the end of the execution.

Output below refers to what is printed by:

System.out.println("*** Deliveries in database after job execution - Start ***");
jdbcTemplate.queryForStream("select * from delivery", new DataClassRowMapper<>(Delivery.class))
        .forEach(delivery -> System.out.println("Delivery item number: " + delivery.itemNumber()));
System.out.println("*** Deliveries in database after job execution - End ***");

Expected behavior
During scan mode, each scanned item should be processed in its own transaction and committed/rolled back independently.

• skipLimit=1: item 1 should remain committed before the step fails.
• skipLimit=2: items 2 and 3 should be skipped (not committed), so only 1 and 4 should remain.

Minimal Complete Reproducible example
Two minimal Maven projects are provided:

• Spring Batch 5 project (reference)
• Spring Batch 6 project (issue)

Observed vs expected output:

Scenario A: skipLimit=1

Obtained:

*** Deliveries in database after job execution - Start ***
*** Deliveries in database after job execution - End ***

Expected:

*** Deliveries in database after job execution - Start ***
Delivery item number: 1
*** Deliveries in database after job execution - End ***

Scenario B: skipLimit=2

Obtained:

*** Deliveries in database after job execution - Start ***
Delivery item number: 1
Delivery item number: 2
Delivery item number: 3
Delivery item number: 4
*** Deliveries in database after job execution - End ***

Expected:

*** Deliveries in database after job execution - Start ***
Delivery item number: 1
Delivery item number: 4
*** Deliveries in database after job execution - End ***

spring-batch-5.zip
spring-batch-6.zip