Slimmer implementation of credit-only accounts (#4592)

* Add credit-only debit/data check to verify_instruction

* Store credits and pass to accounts_db

* Add InstructionErrors and tests

* Relax account locks for credit-only accounts

* Collect credit-only account credits before passing to accounts_db to store properly

* Convert System Transfer  accounts to credit-only, and fixup test

* Functionalize collect_accounts to unit test

* Review comments

* Rebase

sdk/src/account.rs

@@ -55,6 +55,8 @@ impl Account {
     }
 }
 
+pub type LamportCredit = u64;
+
 #[repr(C)]
 #[derive(Debug)]
 pub struct KeyedAccount<'a> {

sdk/src/instruction.rs

@@ -52,6 +52,12 @@ pub enum InstructionError {
     /// Program modified the data of an account that doesn't belong to it
     ExternalAccountDataModified,
 
+    /// Credit-only account spent lamports
+    CreditOnlyLamportSpend,
+
+    /// Credit-only account modified data
+    CreditOnlyDataModified,
+
     /// An account was referenced more than once in a single instruction
     DuplicateAccountIndex,
 

sdk/src/message.rs

@@ -216,7 +216,7 @@ impl Message {
             .position(|&&pubkey| pubkey == self.account_keys[index])
     }
 
-    fn is_credit_debit(&self, i: usize) -> bool {
+    pub fn is_debitable(&self, i: usize) -> bool {
         i < (self.header.num_required_signatures - self.header.num_credit_only_signed_accounts)
             as usize
             || (i >= self.header.num_required_signatures as usize
@@ -228,7 +228,7 @@ impl Message {
         let mut credit_debit_keys = vec![];
         let mut credit_only_keys = vec![];
         for (i, key) in self.account_keys.iter().enumerate() {
-            if self.is_credit_debit(i) {
+            if self.is_debitable(i) {
                 credit_debit_keys.push(key);
             } else {
                 credit_only_keys.push(key);
@@ -516,7 +516,7 @@ mod tests {
     }
 
     #[test]
-    fn test_is_credit_debit() {
+    fn test_is_debitable() {
         let key0 = Pubkey::new_rand();
         let key1 = Pubkey::new_rand();
         let key2 = Pubkey::new_rand();
@@ -534,12 +534,12 @@ mod tests {
             recent_blockhash: Hash::default(),
             instructions: vec![],
         };
-        assert_eq!(message.is_credit_debit(0), true);
-        assert_eq!(message.is_credit_debit(1), false);
-        assert_eq!(message.is_credit_debit(2), false);
-        assert_eq!(message.is_credit_debit(3), true);
-        assert_eq!(message.is_credit_debit(4), true);
-        assert_eq!(message.is_credit_debit(5), false);
+        assert_eq!(message.is_debitable(0), true);
+        assert_eq!(message.is_debitable(1), false);
+        assert_eq!(message.is_debitable(2), false);
+        assert_eq!(message.is_debitable(3), true);
+        assert_eq!(message.is_debitable(4), true);
+        assert_eq!(message.is_debitable(5), false);
     }
 
     #[test]

sdk/src/system_instruction.rs

@@ -90,7 +90,7 @@ pub fn assign(from_pubkey: &Pubkey, program_id: &Pubkey) -> Instruction {
 pub fn transfer(from_pubkey: &Pubkey, to_pubkey: &Pubkey, lamports: u64) -> Instruction {
     let account_metas = vec![
         AccountMeta::new(*from_pubkey, true),
-        AccountMeta::new(*to_pubkey, false),
+        AccountMeta::new_credit_only(*to_pubkey, false),
     ];
     Instruction::new(
         system_program::id(),