feat(gateway): add instance ttl extension

contracts/axelar-gateway/src/contract.rs

@@ -9,6 +9,15 @@ use crate::{auth, event};
 
 const CONTRACT_VERSION: &str = env!("CARGO_PKG_VERSION");
 
+/// Parameters for extending the contract instance and its instance storage.
+/// 
+/// If the instance's time to live falls below 7 days, it will be extended by 60 days.
+/// 
+/// If at least one message is approved per week, the instance should never be archived.
+const LEDGERS_PER_DAY: u32 = (24 * 3600) / 5;
+const INSTANCE_TTL_THRESHOLD: u32 = 7 * LEDGERS_PER_DAY;
+const INSTANCE_TTL_EXTEND_TO: u32 = 60 * LEDGERS_PER_DAY;
+
 #[contract]
 pub struct AxelarGateway;
 
@@ -188,6 +197,8 @@ impl AxelarGateway {
             event::approve_message(&env, message);
         }
 
+        Self::extend_instance_ttl(&env);
+
         Ok(())
     }
 
@@ -307,4 +318,10 @@ impl AxelarGateway {
     fn message_approval_hash(env: &Env, message: Message) -> MessageApprovalValue {
         MessageApprovalValue::Approved(env.crypto().keccak256(&message.to_xdr(env)).into())
     }
+
+    pub fn extend_instance_ttl(env: &Env) {
+        env.storage()
+            .instance()
+            .extend_ttl(INSTANCE_TTL_THRESHOLD, INSTANCE_TTL_EXTEND_TO);
+    }
 }