rent collector improvments (#6888)

* avoid account copying + pre-empt rent

* adding support for base rent

runtime/src/accounts.rs

@@ -114,7 +114,10 @@ impl Accounts {
                 .filter(|key| !message.program_ids().contains(key))
             {
                 let (account, rent) = AccountsDB::load(storage, ancestors, accounts_index, key)
-                    .and_then(|(account, _)| rent_collector.update(account))
+                    .and_then(|(mut account, _)| {
+                        let rent_due = rent_collector.update(&mut account);
+                        Some((account, rent_due))
+                    })
                     .unwrap_or_default();
 
                 accounts.push(account);

runtime/src/rent_collector.rs

@@ -33,9 +33,9 @@ impl RentCollector {
     // updates this account's lamports and status and returns
     //  the account rent collected, if any
     //
-    pub fn update(&self, mut account: Account) -> Option<(Account, u64)> {
-        if account.data.is_empty() || account.rent_epoch > self.epoch {
-            Some((account, 0))
+    pub fn update(&self, account: &mut Account) -> u64 {
+        if account.rent_epoch > self.epoch {
+            0
         } else {
             let slots_elapsed: u64 = (account.rent_epoch..=self.epoch)
                 .map(|epoch| self.epoch_schedule.get_slots_in_epoch(epoch + 1))
@@ -51,13 +51,15 @@ impl RentCollector {
                 if account.lamports > rent_due {
                     account.rent_epoch = self.epoch + 1;
                     account.lamports -= rent_due;
-                    Some((account, rent_due))
+                    rent_due
                 } else {
-                    None
+                    let rent_charged = account.lamports;
+                    *account = Account::default();
+                    rent_charged
                 }
             } else {
                 // maybe collect rent later, leave account alone
-                Some((account, 0))
+                0
             }
         }
     }