don't return a state in state_at if the db prunes and the block is before guaranteed history

ethcore/src/client/client.rs

@@ -371,6 +371,14 @@ impl<V> Client<V> where V: Verifier {
 			return Some(self.state())
 		}
 
+		let block_number = self.block_number(id.clone());
+
+		// check that the block is not too old -- blocks within `HISTORY` blocks of the best will
+		// always be available.
+		if self.state_db.does_pruning() && self.best_block_number() >= block_number + HISTORY {
+			return None;
+		}
+
 		self.block_header(id).map(|header| {
 			let db = self.state_db.lock().unwrap().boxed_clone();
 			State::from_existing(db, HeaderView::new(&header).state_root(), self.engine.account_start_nonce())

util/src/journaldb/archivedb.rs

@@ -175,6 +175,8 @@ impl JournalDB for ArchiveDB {
 	fn state(&self, id: &H256) -> Option<Bytes> {
 		self.backing.get_by_prefix(&id.bytes()[0..12]).and_then(|b| Some(b.to_vec()))
 	}
+
+	fn does_pruning(&self) -> bool { false }
 }
 
 #[cfg(test)]

util/src/journaldb/traits.rs

@@ -42,4 +42,7 @@ pub trait JournalDB : HashDB + Send + Sync {
 	fn state(&self, _id: &H256) -> Option<Bytes> {
 		None
 	}
+
+	/// Whether this database does pruning.
+	fn does_pruning(&self) -> bool { true }
 }