feat(guards): implement flash-loan reentrancy guard using block number verification

contracts/flash_loan_guard/src/lib.rs

@@ -0,0 +1,36 @@
+use soroban_sdk::{contract, contractimpl, contracttype, Address, Env};
+
+#[derive(Clone)]
+#[contracttype]
+pub enum DataKey {
+    DepositBlock(Address),
+}
+
+@contract
+pub struct FlashLoanGuardContract;
+
+@contractimpl
+impl FlashLoanGuardContract {
+    pub fn deposit(env: Env, user: Address) {
+        user.require_auth();
+        let current_block = env.ledger().sequence();
+        env.storage().persistent().set(&DataKey::DepositBlock(user.clone()), &current_block);
+    }
+
+    pub fn withdraw(env: Env, user: Address) {
+        user.require_auth();
+        let current_block = env.ledger().sequence();
+
+        let deposit_block: u32 = env.storage()
+            .persistent()
+            .get(&DataKey::DepositBlock(user.clone()))
+            .unwrap_or(0);
+
+        if current_block <= deposit_block {
+            panic!("FlashLoanReentrancy: withdrawal prohibited in the same ledger block as deposit");
+        }
+
+        // Proceed with withdrawal logic...
+        env.storage().persistent().remove(&DataKey::DepositBlock(user));
+    }
+}

contracts/flash_loan_guard/src/test.rs

@@ -0,0 +1,37 @@
+#[cfg(test)]
+mod test {
+    use super::*;
+    use soroban_sdk::{Env, Address};
+
+    #[test]
+    #[should_panic(expected = "FlashLoanReentrancy")]
+    fn test_prevents_same_block_withdrawal() {
+        let env = Env::default();
+        env.mock_all_auths();
+        let contract_id = env.register(FlashLoanGuardContract, ());
+        let client = FlashLoanGuardContractClient::new(&env, &contract_id);
+        let user = Address::generate(&env);
+
+        env.ledger().set_sequence_number(100);
+        client.deposit(&user);
+
+        // Attempt withdrawal in the same block (100) -> should panic
+        client.withdraw(&user);
+    }
+
+    #[test]
+    fn test_allows_subsequent_block_withdrawal() {
+        let env = Env::default();
+        env.mock_all_auths();
+        let contract_id = env.register(FlashLoanGuardContract, ());
+        let client = FlashLoanGuardContractClient::new(&env, &contract_id);
+        let user = Address::generate(&env);
+
+        env.ledger().set_sequence_number(100);
+        client.deposit(&user);
+
+        // Advance ledger block
+        env.ledger().set_sequence_number(101);
+        client.withdraw(&user); // Should succeed without panicking
+    }
+}