Re-entrancy

Re-entrancy occurs when a contract makes a call back into the contract that called it, e.g. Contract A calls Contract B but then Contract B makes a call back into Contract A.

To mitigate security concerns there are two approaches that are commonly used:

• Implement a guard: detect when a re-entrancy occurs
• Defensive programming: perform calls after all state changes have been made

Re-entrancy Guard

Sway provides a stateless re-entrancy guard, which reverts at run-time when re-entrancy is detected.

To use the guard we must import it.

use reentrancy::reentrancy_guard;

Then call it in a contract function.

    fn deposit() {
        reentrancy_guard();

        // code
    }

Checks-Effects-Interactions Pattern

The pattern states that all state (storage) changes should be made before a call is made.

    fn withdraw() {
        // Step 1. Perform any state changes to update balance
        // Step 2. After all state changes make a call
    }