Add more test cases

runtime/src/bank.rs

@@ -16523,6 +16523,19 @@ pub(crate) mod tests {
         )
         .unwrap();
 
+        // Dummy message to determine fee amount
+        let dummy_message = SanitizedMessage::try_from(Message::new_with_blockhash(
+            &[system_instruction::transfer(
+                &rent_exempt_fee_payer.pubkey(),
+                &recipient,
+                sol_to_lamports(1.),
+            )],
+            Some(&rent_exempt_fee_payer.pubkey()),
+            &recent_blockhash,
+        ))
+        .unwrap();
+        let fee = bank.get_fee_for_message(&dummy_message).unwrap();
+
         // RentPaying fee-payer can remain RentPaying
         let tx = Transaction::new(
             &[&rent_paying_fee_payer, &mint_keypair],
@@ -16570,6 +16583,23 @@ pub(crate) mod tests {
             &rent_paying_fee_payer.pubkey()
         ));
 
+        // RentPaying fee-payer can be emptied with fee and transaction
+        let tx = Transaction::new(
+            &[&rent_paying_fee_payer],
+            Message::new(
+                &[system_instruction::transfer(
+                    &rent_paying_fee_payer.pubkey(),
+                    &recipient,
+                    bank.get_balance(&rent_paying_fee_payer.pubkey()) - fee,
+                )],
+                Some(&rent_paying_fee_payer.pubkey()),
+            ),
+            recent_blockhash,
+        );
+        let result = bank.process_transaction(&tx);
+        assert!(result.is_ok());
+        assert_eq!(0, bank.get_balance(&rent_paying_fee_payer.pubkey()));
+
         // RentExempt fee-payer cannot become RentPaying via rent
         let tx = Transaction::new(
             &[&rent_exempt_fee_payer, &mint_keypair],
@@ -16613,6 +16643,86 @@ pub(crate) mod tests {
         assert!(check_account_is_rent_exempt(
             &rent_exempt_fee_payer.pubkey()
         ));
+
+        // For good measure, show that a RentExempt fee-payer that is also debited by a transaction
+        // cannot become RentPaying by that debit, but can still be charged for the fee
+        bank.transfer(fee, &mint_keypair, &rent_exempt_fee_payer.pubkey())
+            .unwrap();
+        let fee_payer_balance = bank.get_balance(&rent_exempt_fee_payer.pubkey());
+        assert_eq!(fee_payer_balance, rent_exempt_minimum + fee);
+        let tx = Transaction::new(
+            &[&rent_exempt_fee_payer],
+            Message::new(
+                &[system_instruction::transfer(
+                    &rent_exempt_fee_payer.pubkey(),
+                    &recipient,
+                    fee,
+                )],
+                Some(&rent_exempt_fee_payer.pubkey()),
+            ),
+            recent_blockhash,
+        );
+        let result = bank.process_transaction(&tx);
+        assert_eq!(
+            result.unwrap_err(),
+            TransactionError::InvalidRentPayingAccount
+        );
+        assert_eq!(
+            fee_payer_balance - fee,
+            bank.get_balance(&rent_exempt_fee_payer.pubkey())
+        );
+        assert!(check_account_is_rent_exempt(
+            &rent_exempt_fee_payer.pubkey()
+        ));
+
+        // Also show that a RentExempt fee-payer can be completely emptied via fee and transaction
+        bank.transfer(fee + 1, &mint_keypair, &rent_exempt_fee_payer.pubkey())
+            .unwrap();
+        assert!(bank.get_balance(&rent_exempt_fee_payer.pubkey()) > rent_exempt_minimum + fee);
+        let tx = Transaction::new(
+            &[&rent_exempt_fee_payer],
+            Message::new(
+                &[system_instruction::transfer(
+                    &rent_exempt_fee_payer.pubkey(),
+                    &recipient,
+                    bank.get_balance(&rent_exempt_fee_payer.pubkey()) - fee,
+                )],
+                Some(&rent_exempt_fee_payer.pubkey()),
+            ),
+            recent_blockhash,
+        );
+        let result = bank.process_transaction(&tx);
+        assert!(result.is_ok());
+        assert_eq!(0, bank.get_balance(&rent_exempt_fee_payer.pubkey()));
+
+        // ... but not if the fee alone would make it RentPaying
+        bank.transfer(
+            rent_exempt_minimum + 1,
+            &mint_keypair,
+            &rent_exempt_fee_payer.pubkey(),
+        )
+        .unwrap();
+        assert!(bank.get_balance(&rent_exempt_fee_payer.pubkey()) < rent_exempt_minimum + fee);
+        let tx = Transaction::new(
+            &[&rent_exempt_fee_payer],
+            Message::new(
+                &[system_instruction::transfer(
+                    &rent_exempt_fee_payer.pubkey(),
+                    &recipient,
+                    bank.get_balance(&rent_exempt_fee_payer.pubkey()) - fee,
+                )],
+                Some(&rent_exempt_fee_payer.pubkey()),
+            ),
+            recent_blockhash,
+        );
+        let result = bank.process_transaction(&tx);
+        assert_eq!(
+            result.unwrap_err(),
+            TransactionError::InvalidRentPayingAccount
+        );
+        assert!(check_account_is_rent_exempt(
+            &rent_exempt_fee_payer.pubkey()
+        ));
     }
 
     #[test]