Expire all transactions after some amount of time

Reject old transactions so that we can calculate an upper bound
for memory usage, and therefore ensure the server won't slow
down over time to crash due to memory exhaustion.

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,
@@ -93,8 +95,11 @@ impl Accountant {
     /// 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 sigs = RwLock::new(HashSet::new());
-        self.last_ids.write().unwrap().push_back((*last_id, sigs));
+        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.
@@ -222,6 +227,8 @@ impl Accountant {
 mod tests {
     use super::*;
     use signature::KeyPairUtil;
+    use hash::hash;
+    use bincode::serialize;
 
     #[test]
     fn test_accountant() {
@@ -342,6 +349,19 @@ mod tests {
         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()));
+    }
 }
 
 #[cfg(all(feature = "unstable", test))]