Add safety comment for get_account_locks_unchecked

Author: ryoqun

runtime/src/bank.rs

@@ -4235,6 +4235,9 @@ impl Bank {
         transaction: &'a SanitizedTransaction,
     ) -> TransactionBatch<'_, '_> {
         let tx_account_lock_limit = self.get_transaction_account_lock_limit();
+        // Note that switching this to .get_account_locks_unchecked() is unacceptable currently.
+        // The unified scheduler relies on the checks enforced here.
+        // See a comment in SchedulingStateMachine::create_task().
         let lock_result = transaction
             .get_account_locks(tx_account_lock_limit)
             .map(|_| ());

unified-scheduler-logic/src/lib.rs

@@ -657,7 +657,20 @@ impl SchedulingStateMachine {
         index: usize,
         page_loader: &mut impl FnMut(Pubkey) -> Page,
     ) -> Task {
-        // this is safe bla bla
+        // Calling the _unchecked() version here is safe for faster operation, because
+        // `get_account_locks()` (the safe variant) is ensured to be called in
+        // DefaultTransactionHandler::handle() via Bank::prepare_unlocked_batch_from_single_tx().
+        //
+        // The safe variant has additional account-locking related verifications, which is crutial.
+        //
+        // Currently the replaying stage is redundantly calling `get_accont_locks()` when unified
+        // scheduler is enabled on the given transaction at the blockstore. This will be relaxed
+        // for optimization in the future. As for banking stage with unified scheduler, it will
+        // need to run .get_account_locks() at least once somewhere in the code path. In the
+        // distant future, this function `create_task()` should be adjusted so that both stages do
+        // the checks before calling this (say, with some ad-hoc type like
+        // `SanitizedTransactionWithCheckedAccountLocks`) or do the chccks here, resulting in
+        // eliminating the redudant one in the replaying stage and in the handler.
         let locks = transaction.get_account_locks_unchecked();
 
         let writable_locks = locks