Purge accounts with lamports=0 on rooted forks (#6315)

runtime/src/accounts_db.rs

@@ -559,6 +559,26 @@ impl AccountsDB {
         false
     }
 
+    pub fn purge_zero_lamport_accounts(&self, ancestors: &HashMap<u64, usize>) {
+        let accounts_index = self.accounts_index.read().unwrap();
+        let mut purges = Vec::new();
+        accounts_index.scan_accounts(ancestors, |pubkey, (account_info, fork)| {
+            if account_info.lamports == 0 && accounts_index.is_root(fork) {
+                purges.push(*pubkey);
+            }
+        });
+        drop(accounts_index);
+        let mut reclaims = Vec::new();
+        let mut accounts_index = self.accounts_index.write().unwrap();
+        for purge in &purges {
+            reclaims.extend(accounts_index.purge(purge));
+        }
+        let last_root = accounts_index.last_root;
+        drop(accounts_index);
+        let mut dead_forks = self.remove_dead_accounts(reclaims);
+        self.cleanup_dead_forks(&mut dead_forks, last_root);
+    }
+
     pub fn scan_accounts<F, A>(&self, ancestors: &HashMap<Fork, usize>, scan_func: F) -> A
     where
         F: Fn(&mut A, Option<(&Pubkey, Account, Fork)>) -> (),
@@ -746,6 +766,10 @@ impl AccountsDB {
     }
 
     pub fn hash_account_data(fork: Fork, lamports: u64, data: &[u8], pubkey: &Pubkey) -> Hash {
+        if lamports == 0 {
+            return Hash::default();
+        }
+
         let mut hasher = Hasher::default();
         let mut buf = [0u8; 8];
 

runtime/src/accounts_index.rs

@@ -29,6 +29,17 @@ impl<T: Clone> AccountsIndex<T> {
         }
     }
 
+    pub fn purge(&mut self, pubkey: &Pubkey) -> Vec<(Fork, T)> {
+        let mut list = self.account_maps.get(&pubkey).unwrap().write().unwrap();
+        let reclaims = list
+            .iter()
+            .filter(|(fork, _)| self.is_root(*fork))
+            .cloned()
+            .collect();
+        list.retain(|(fork, _)| !self.is_root(*fork));
+        reclaims
+    }
+
     // find the latest fork and T in a list for a given ancestor
     // returns index into 'list' if found, None if not.
     fn latest_fork(&self, ancestors: &HashMap<Fork, usize>, list: &[(Fork, T)]) -> Option<usize> {

runtime/src/bank.rs

@@ -1403,6 +1403,28 @@ impl Bank {
             .verify_hash_internal_state(self.slot(), &self.ancestors)
     }
 
+    /// A snapshot bank should be purged of 0 lamport accounts which are not part of the hash
+    /// calculation and could shield other real accounts.
+    pub fn verify_snapshot_bank(&self) -> bool {
+        self.rc
+            .accounts
+            .verify_hash_internal_state(self.slot(), &self.ancestors)
+            && !self.has_accounts_with_zero_lamports()
+    }
+
+    fn has_accounts_with_zero_lamports(&self) -> bool {
+        self.rc.accounts.accounts_db.scan_accounts(
+            &self.ancestors,
+            |collector: &mut bool, option| {
+                if let Some((_, account, _)) = option {
+                    if account.lamports == 0 {
+                        *collector = true;
+                    }
+                }
+            },
+        )
+    }
+
     /// Return the number of ticks per slot
     pub fn ticks_per_slot(&self) -> u64 {
         self.ticks_per_slot
@@ -1584,6 +1606,13 @@ impl Bank {
             .accounts
             .commit_credits(&self.ancestors, self.slot());
     }
+
+    pub fn purge_zero_lamport_accounts(&self) {
+        self.rc
+            .accounts
+            .accounts_db
+            .purge_zero_lamport_accounts(&self.ancestors);
+    }
 }
 
 impl Drop for Bank {
@@ -1753,6 +1782,71 @@ mod tests {
         );
     }
 
+    fn assert_no_zero_balance_accounts(bank: &Arc<Bank>) {
+        assert!(!bank.has_accounts_with_zero_lamports());
+    }
+
+    // Test that purging 0 lamports accounts works.
+    #[test]
+    fn test_purge_empty_accounts() {
+        solana_logger::setup();
+        let (genesis_block, mint_keypair) = create_genesis_block(500_000);
+        let parent = Arc::new(Bank::new(&genesis_block));
+        let mut bank = parent;
+        for _ in 0..10 {
+            let blockhash = bank.last_blockhash();
+            let pubkey = Pubkey::new_rand();
+            let tx = system_transaction::transfer_now(&mint_keypair, &pubkey, 0, blockhash);
+            bank.process_transaction(&tx).unwrap();
+            bank.squash();
+            bank = Arc::new(new_from_parent(&bank));
+        }
+
+        bank.purge_zero_lamport_accounts();
+
+        assert_no_zero_balance_accounts(&bank);
+
+        let bank0 = Arc::new(new_from_parent(&bank));
+        let blockhash = bank.last_blockhash();
+        let keypair = Keypair::new();
+        let tx = system_transaction::transfer_now(&mint_keypair, &keypair.pubkey(), 10, blockhash);
+        bank0.process_transaction(&tx).unwrap();
+
+        let bank1 = Arc::new(new_from_parent(&bank0));
+        let pubkey = Pubkey::new_rand();
+        let blockhash = bank.last_blockhash();
+        let tx = system_transaction::transfer_now(&keypair, &pubkey, 10, blockhash);
+        bank1.process_transaction(&tx).unwrap();
+
+        assert_eq!(bank0.get_account(&keypair.pubkey()).unwrap().lamports, 10);
+        assert_eq!(bank1.get_account(&keypair.pubkey()), None);
+        bank0.purge_zero_lamport_accounts();
+
+        assert_eq!(bank0.get_account(&keypair.pubkey()).unwrap().lamports, 10);
+        assert_eq!(bank1.get_account(&keypair.pubkey()), None);
+        bank1.purge_zero_lamport_accounts();
+
+        assert_eq!(bank0.get_account(&keypair.pubkey()).unwrap().lamports, 10);
+        assert_eq!(bank1.get_account(&keypair.pubkey()), None);
+
+        assert!(bank0.verify_hash_internal_state());
+
+        // Squash and then verify hash_internal value
+        bank0.squash();
+        assert!(bank0.verify_hash_internal_state());
+
+        bank1.squash();
+        assert!(bank1.verify_hash_internal_state());
+
+        // keypair should have 0 tokens on both forks
+        assert_eq!(bank0.get_account(&keypair.pubkey()), None);
+        assert_eq!(bank1.get_account(&keypair.pubkey()), None);
+        bank1.purge_zero_lamport_accounts();
+
+        assert!(bank1.verify_hash_internal_state());
+        assert_no_zero_balance_accounts(&bank1);
+    }
+
     #[test]
     fn test_two_payments_to_one_party() {
         let (genesis_block, mint_keypair) = create_genesis_block(10_000);