Refactor accounts.rs with Justin's comments to improve lock accounts (#21406)

with results code path.
- fix a bug that could unlock accounts that weren't locked
- add test to the refactored function
- skip enumerating transaction accounts if qos results is an error
- add #[must_use] annotation
- avoid clone error in results
- add qos error code to unlock_accounts match statement
- remove unnecessary AbiExample

core/src/banking_stage.rs

@@ -940,7 +940,8 @@ impl BankingStage {
         // Once accounts are locked, other threads cannot encode transactions that will modify the
         // same account state
         let mut lock_time = Measure::start("lock_time");
-        let batch = bank.prepare_sanitized_batch_with_results(txs, transactions_qos_results.iter());
+        let batch =
+            bank.prepare_sanitized_batch_with_results(txs, transactions_qos_results.into_iter());
         lock_time.stop();
 
         // retryable_txs includes AccountInUse, WouldExceedMaxBlockCostLimit and

core/src/qos_service.rs

@@ -24,6 +24,10 @@ use {
 };
 
 pub struct QosService {
+    // cost_model instance is owned by validator, shared between replay_stage and
+    // banking_stage. replay_stage writes the latest on-chain program timings to
+    // it; banking_stage's qos_service reads that information to calculate
+    // transaction cost, hence RwLock wrapped.
     cost_model: Arc<RwLock<CostModel>>,
     metrics: Arc<QosServiceMetrics>,
     reporting_thread: Option<JoinHandle<()>>,