Paused contract: burn() behaviour before/after 0.6.0

jmahhh · February 26, 2020, 2:00pm

Am I correct in seeing that pre-0.6.0, a Paused contract would still allow burning, but post-0.6.0 it would not because of the way _beforeTokenTransfer is implemented?

Environment
Currently on 0.5.16

abcoathup · February 27, 2020, 8:01am

Hi @jmahhh,

Welcome to the community

Yes, that is how it appears to me.
In OpenZeppelin Contracts 2.5 burning wasn’t impacted by Pausable, whilst in OpenZeppelin Contracts 3.0 burning is impacted by Pausable.

Is this a problem?

OpenZeppelin Contracts v2.5 (Solidity 0.5)

github.com

OpenZeppelin/openzeppelin-contracts/blob/release-v2.5.0/contracts/token/ERC20/ERC20Pausable.sol

pragma solidity ^0.5.0;

import "./ERC20.sol";
import "../../lifecycle/Pausable.sol";

/**
 * @title Pausable token
 * @dev ERC20 with pausable transfers and allowances.
 *
 * Useful if you want to stop trades until the end of a crowdsale, or have
 * an emergency switch for freezing all token transfers in the event of a large
 * bug.
 */
contract ERC20Pausable is ERC20, Pausable {
    function transfer(address to, uint256 value) public whenNotPaused returns (bool) {
        return super.transfer(to, value);
    }

    function transferFrom(address from, address to, uint256 value) public whenNotPaused returns (bool) {
        return super.transferFrom(from, to, value);

This file has been truncated. show original

OpenZeppelin Contracts v3.0 beta release (Solidity 0.6)

github.com

OpenZeppelin/openzeppelin-contracts/blob/v3.0.0-beta.0/contracts/token/ERC20/ERC20Pausable.sol#L22


* bug.
*/
contract ERC20Pausable is ERC20, Pausable {
   /**
    * @dev See {ERC20-_beforeTokenTransfer}.
    *
    * Requirements:
    *
    * - the contract must not be paused.
    */
   function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual override {
       super._beforeTokenTransfer(from, to, amount);


       require(!paused(), "ERC20Pausable: token transfer while paused");
   }
}

github.com

OpenZeppelin/openzeppelin-contracts/blob/v3.0.0-beta.0/contracts/token/ERC20/ERC20.sol#L195


 * @dev Destroys `amount` tokens from `account`, reducing the
 * total supply.
 *
 * Emits a {Transfer} event with `to` set to the zero address.
 *
 * Requirements
 *
 * - `account` cannot be the zero address.
 * - `account` must have at least `amount` tokens.
 */
function _burn(address account, uint256 amount) internal virtual {
    require(account != address(0), "ERC20: burn from the zero address");


    _beforeTokenTransfer(account, address(0), amount);


    _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
    _totalSupply = _totalSupply.sub(amount);
    emit Transfer(account, address(0), amount);
}


/**

jmahhh · February 27, 2020, 10:46am

Hi @abcoathup! IMO this is a change significant enough to be considered a “breaking change”? People combining these two contracts in 2.5 should be (made) aware of the new behaviour?

abcoathup · February 28, 2020, 3:50am

Hi @jmahhh,

This would be a breaking change going from OpenZeppelin Contracts 2.5 to 3.0.
OpenZeppelin Contracts 2.5 Pausable didn’t pause ERC20 burn.
OpenZeppelin Contracts 3.0 Pausable does pause ERC20 burn.

See OpenZeppelin Contracts API Stability for details on when breaking changes may occur.

Would be great to get your feedback on this change.

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Paused contract: burn() behaviour before/after 0.6.0

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