Reentrancy Guard Pattern

A reentrancy guard is a security mechanism implemented in smart contracts to prevent recursive calls from compromising the contract state. It typically involves a state variable that acts as a lock, ensuring that a function cannot be entered again until the previous execution has completed.

When a function is called, the guard checks the status of this lock; if it is active, the transaction reverts, preventing the exploit. This is a standard defensive programming practice in Solidity development, particularly for functions that interact with external addresses or transfer tokens.

By enforcing sequential execution, it protects protocols from sophisticated reentrancy attacks that attempt to drain assets. It is a fundamental component of secure vault and lending protocol design.