[PR #38] p3: MockERC20 missing transferFrom — mock is incomplete for executeOperation unit tests

[obchain]

Summary

MockERC20 in CharonLiquidator.t.sol implements balanceOf, mint, transfer, approve, and allowance, but not transferFrom. The current test usage is limited to rescue tests where only transfer is needed.

However, any future mocked unit test for executeOperation will require transferFrom because:

• PancakeSwap's exactInputSingle pulls tokens from the caller via transferFrom
• Some ERC-20 implementations of liquidateBorrow on the debt side use transferFrom

Without transferFrom, any attempt to wire MockERC20 into an executeOperation unit test will silently fail (fallback to no-op or revert) and mask the real behavior.

Location

contracts/test/CharonLiquidator.t.sol — MockERC20 contract definition

Fix

Add transferFrom to MockERC20:

function transferFrom(address from, address to, uint256 amount) external returns (bool) {
    require(balanceOf[from] >= amount, "insufficient");
    balanceOf[from] -= amount;
    balanceOf[to] += amount;
    return true;
}

Refs #38

[obchain]

Fixed via commit 6447016. The in-file MockERC20 now implements the full ERC-20 surface required by the callback tests:

• allowance is tracked as a mapping(address => mapping(address => uint256)) (previously returned a hardcoded zero).
• approve writes to that mapping instead of no-opping.
• transferFrom(from, to, amount) enforces both balance and allowance, debits the allowance (unless type(uint256).max for infinite approvals), and moves balances.
  This is what lets the in-file MockSwapRouter.exactInputSingle actually pull tokenIn from the liquidator via transferFrom — closing the hole that previously made a full end-to-end callback test impossible.