Fix RepositoryConnection.getIsolationLevel() returning null when isolation level is set

[Copilot]

Fixed issue where RepositoryConnection#getIsolationLevel() was always returning null even when an isolation level was set via begin(IsolationLevel).

Root Cause: SailRepositoryConnection overrides begin(IsolationLevel) and begin(TransactionSetting...) but doesn't call setIsolationLevel() to update the AbstractRepositoryConnection's isolationLevel field that is returned by getIsolationLevel().

Solution: Modified both methods in SailRepositoryConnection to call setIsolationLevel() before starting the transaction.

Changes Made

• Modified SailRepositoryConnection.begin(IsolationLevel level) to call setIsolationLevel(level)
• Modified SailRepositoryConnection.begin(TransactionSetting... settings) to call setIsolationLevel() for isolation level settings
• Added comprehensive unit tests that verify the fix works for both method variants:
  • testGetIsolationLevel_shouldReturnSetLevel - Tests explicit isolation level setting via begin(IsolationLevel)
  • testGetIsolationLevel_shouldReturnSetLevelFromTransactionSettings - Tests isolation level via begin(TransactionSetting...)
  • testGetIsolationLevel_shouldReturnDefaultLevelWhenNotExplicitlySet - Tests that default isolation level is returned when begin() is called without parameters (currently failing, demonstrates additional issue)

Testing

• ✅ Created failing test that reproduces the original issue
• ✅ Verified that test fails before fix and passes after fix
• ✅ All existing tests continue to pass
• ✅ Code formatted and follows project conventions
• ✅ Added test for default isolation level behavior (exposes additional issue where begin() without parameters doesn't properly set isolation level)

The fix is minimal and surgical - it only adds the missing setIsolationLevel() call that should have been there. This ensures that getIsolationLevel() properly returns the isolation level that was set when beginning a transaction. The additional test case reveals that there's still an issue with the default isolation level behavior that may need to be addressed separately.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Isolation level is always null, if queried from the RepositoryConnection</issue_title>
<issue_description>### Discussed in #4798

^{Originally posted by domkun October 6, 2023}
Hi there,

I had a problem with concurrent transactions, so I wanted to log the isolation level of the current connection using RepositoryConnection#getIsolationLevel method. However it always returns null. The underlying sailConnection property of the SailRepositoryConnection shows the set isolation level. Is this intended?

I am a bit confused about this. Here is a short reproducer:

import org.eclipse.rdf4j.common.transaction.IsolationLevels;
import org.eclipse.rdf4j.repository.Repository;
import org.eclipse.rdf4j.repository.RepositoryConnection;
import org.eclipse.rdf4j.repository.sail.SailRepository;
import org.eclipse.rdf4j.sail.memory.MemoryStore;

class Scratch {
    public static void main(String[] args) {
        Repository sailRepository = new SailRepository(new MemoryStore());

        try (RepositoryConnection connection = sailRepository.getConnection()) {
            connection.begin(IsolationLevels.SERIALIZABLE);
            System.out.println(connection.getIsolationLevel());
            connection.commit();
        }
    }
}

Edit: Screenshot of the debugger window attached

Comments on the Issue (you are @copilot in this section)

Fixes #4802

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[hmottestad]

@copilot remove initial-evidence.txt

[Copilot]

@copilot remove initial-evidence.txt

Removed initial-evidence.txt file as requested in commit c14c3a9.

[hmottestad]

@copilot add a test where the isolation level is not explicitly specified by the user, which means that the default isolation level is used. Only add the test!

[Copilot]

@copilot add a test where the isolation level is not explicitly specified by the user, which means that the default isolation level is used. Only add the test!

Added test testGetIsolationLevel_shouldReturnDefaultLevelWhenNotExplicitlySet that verifies default isolation level behavior when begin() is called without parameters. The test currently fails as expected, showing that getIsolationLevel() returns null instead of the default READ_COMMITTED level. Commit: 6f7e9d5