SVM - requirement of load_and_execute_sanitized_transaction

[apfitzge]

Problem

• load_and_execute_sanitized_transactions has a undocumented requirement that sanitized_txs has no conflicting transactions.
• If conflicting transactions are passed in, it will not crash, but may give incorrect results
  • The validation and leader code currently check before calling, so this is not happening in our production environment
  • However, this function requirement is not checked nor documented in this new publicly exposed interface
  • If other groups begin using this interface, it is a non-obvious requirement

Proposed Solution

• Short term:
  • Add function documentation for this requirement
• Longer term:
  • Make load_and_execute_sanitized_transactions allow for conflicting transactions to be passed in
    • This relates to pending SIMD-0083: Relax Entry Constraints solana-foundation/solana-improvement-documents#83

[apfitzge]

Example test which gives incorrect results but does not fail:

#[test]
fn test_conflicting_simple_transfers() {
    let kp1 = Keypair::new();

    let mut account_data = AccountSharedData::default();
    account_data.set_lamports(1_000_000_000_000);

    let mut mock_bank = mock_bank::MockBankCallback::default();

    mock_bank
        .account_shared_data
        .insert(kp1.pubkey(), account_data.clone());

    let tx1 = SanitizedTransaction::from_transaction_for_tests(system_transaction::transfer(
        &kp1,
        &Pubkey::new_unique(),
        1_000_000,
        Hash::default(),
    ));
    let tx2 = SanitizedTransaction::from_transaction_for_tests(system_transaction::transfer(
        &kp1,
        &Pubkey::new_unique(),
        1_000_000,
        Hash::default(),
    ));

    let batch_processor = TransactionBatchProcessor::<MockForkGraph>::default();
    batch_processor
        .loaded_programs_cache
        .write()
        .unwrap()
        .fork_graph = Some(Arc::new(RwLock::new(MockForkGraph {})));

    let mut check_results = [(Ok(()), None, Some(5000)), (Ok(()), None, Some(5000))];
    let result = batch_processor.load_and_execute_sanitized_transactions(
        &mock_bank,
        &[tx1, tx2],
        &mut check_results,
        &mut TransactionErrorMetrics::default(),
        ExecutionRecordingConfig::new_single_setting(false),
        &mut ExecuteTimings::default(),
        None,
        [solana_system_program::id()].iter(),
        None,
        false,
    );

    let (ltx1, _nonce) = &result.loaded_transactions[0];
    let (ltx2, _nonce) = &result.loaded_transactions[1];
    let ltx1 = ltx1.as_ref().unwrap();
    let ltx2 = ltx2.as_ref().unwrap();

    assert!(result.execution_results[0].was_executed());
    assert!(result.execution_results[1].was_executed());

    assert!(!result.execution_results[0].was_executed_successfully());
    assert!(!result.execution_results[1].was_executed_successfully());

    // Fees drained from the account. This should be 999_999_990_000 since 2 fees
    // were charged for the same account.
    assert_eq!(ltx1.accounts[0].0, kp1.pubkey());
    assert_eq!(ltx2.accounts[0].0, kp1.pubkey());
    assert_eq!(ltx1.accounts[0].1.lamports(), 999_999_995_000);
    assert_eq!(ltx2.accounts[0].1.lamports(), 999_999_995_000); // This is a bug
}