fix: remove discarded transactions (paradigmxyz#4164)

crates/transaction-pool/src/pool/txpool.rs

@@ -415,7 +415,10 @@ impl<T: TransactionOrdering> TxPool<T> {
             let PoolUpdate { id, hash, current, destination } = update;
             match destination {
                 Destination::Discard => {
+                    // remove the transaction from the pool and subpool
+                    self.prune_transaction_by_hash(&hash);
                     outcome.discarded.push(hash);
+                    self.metrics.removed_transactions.increment(1);
                 }
                 Destination::Pool(move_to) => {
                     debug_assert!(!move_to.eq(&current), "destination must be different");
@@ -1683,4 +1686,30 @@ mod tests {
 
         assert_eq!(pool.all_transactions.txs.get(&id).unwrap().subpool, SubPool::BaseFee)
     }
+
+    #[test]
+    fn discard_nonce_too_low() {
+        let mut f = MockTransactionFactory::default();
+        let mut pool = TxPool::new(MockOrdering::default(), Default::default());
+
+        let tx = MockTransaction::eip1559().inc_price_by(10);
+        let validated = f.validated(tx.clone());
+        let id = *validated.id();
+        pool.add_transaction(validated, U256::from(1_000), 0).unwrap();
+
+        let next = tx.next();
+        let validated = f.validated(next.clone());
+        pool.add_transaction(validated, U256::from(1_000), 0).unwrap();
+
+        assert_eq!(pool.pending_pool.len(), 2);
+
+        let mut changed_senders = HashMap::new();
+        changed_senders.insert(
+            id.sender,
+            SenderInfo { state_nonce: next.get_nonce(), balance: U256::from(1_000) },
+        );
+        let outcome = pool.update_accounts(changed_senders);
+        assert_eq!(outcome.discarded.len(), 1);
+        assert_eq!(pool.pending_pool.len(), 1);
+    }
 }