Reject old transactions so that we can boot old signatures

src/accountant.rs

@@ -15,6 +15,8 @@ use std::result;
 use std::sync::RwLock;
 use transaction::Transaction;
 
+const MAX_ENTRY_IDS: usize = 1024 * 4;
+
 #[derive(Debug, PartialEq, Eq)]
 pub enum AccountingError {
     InsufficientFunds,
@@ -62,7 +64,9 @@ impl Accountant {
             to: mint.pubkey(),
             tokens: mint.tokens,
         };
-        Self::new_from_deposit(&deposit)
+        let acc = Self::new_from_deposit(&deposit);
+        acc.register_entry_id(&mint.last_id());
+        acc
     }
 
     fn reserve_signature(signatures: &RwLock<HashSet<Signature>>, sig: &Signature) -> bool {
@@ -83,10 +87,19 @@ impl Accountant {
         {
             return Self::reserve_signature(&entry.1, sig);
         }
-        let sigs = RwLock::new(HashSet::new());
-        Self::reserve_signature(&sigs, sig);
-        self.last_ids.write().unwrap().push_back((*last_id, sigs));
-        true
+        false
+    }
+
+    /// Tell the accountant which Entry IDs exist on the ledger. This function
+    /// assumes subsequent calls correspond to later entries, and will boot
+    /// the oldest ones once its internal cache is full. Once boot, the
+    /// accountant will reject transactions using that `last_id`.
+    pub fn register_entry_id(&self, last_id: &Hash) {
+        let mut last_ids = self.last_ids.write().unwrap();
+        if last_ids.len() >= MAX_ENTRY_IDS {
+            last_ids.pop_front();
+        }
+        last_ids.push_back((*last_id, RwLock::new(HashSet::new())));
     }
 
     /// Process a Transaction that has already been verified.
@@ -214,6 +227,8 @@ impl Accountant {
 mod tests {
     use super::*;
     use signature::KeyPairUtil;
+    use hash::hash;
+    use bincode::serialize;
 
     #[test]
     fn test_accountant() {
@@ -331,9 +346,21 @@ mod tests {
         let alice = Mint::new(1);
         let acc = Accountant::new(&alice);
         let sig = Signature::default();
-        let last_id = Hash::default();
-        assert!(acc.reserve_signature_with_last_id(&sig, &last_id));
-        assert!(!acc.reserve_signature_with_last_id(&sig, &last_id));
+        assert!(acc.reserve_signature_with_last_id(&sig, &alice.last_id()));
+        assert!(!acc.reserve_signature_with_last_id(&sig, &alice.last_id()));
+    }
+
+    #[test]
+    fn test_max_entry_ids() {
+        let alice = Mint::new(1);
+        let acc = Accountant::new(&alice);
+        let sig = Signature::default();
+        for i in 0..MAX_ENTRY_IDS {
+            let last_id = hash(&serialize(&i).unwrap()); // Unique hash
+            acc.register_entry_id(&last_id);
+        }
+        // Assert we're no longer able to use the oldest entry ID.
+        assert!(!acc.reserve_signature_with_last_id(&sig, &alice.last_id()));
     }
 }
 
@@ -362,6 +389,8 @@ mod bench {
 
                 // Seed the 'to' account and a cell for its signature.
                 let last_id = hash(&serialize(&i).unwrap()); // Unique hash
+                acc.register_entry_id(&last_id);
+
                 let rando1 = KeyPair::new();
                 let tr = Transaction::new(&rando0, rando1.pubkey(), 1, last_id);
                 acc.process_verified_transaction(&tr).unwrap();

src/accountant_skel.rs

@@ -77,6 +77,7 @@ impl<W: Write + Send + 'static> AccountantSkel<W> {
     pub fn sync(&mut self) -> Hash {
         while let Ok(entry) = self.historian.receiver.try_recv() {
             self.last_id = entry.id;
+            self.acc.register_entry_id(&self.last_id);
             writeln!(self.writer, "{}", serde_json::to_string(&entry).unwrap()).unwrap();
         }
         self.last_id