On the pit of delegatecall of solidity

There is a problem today , Record this pit

pragma solidity ^0.4.26;

contract Proxy {

    address internal masterCopy;
    constructor(address _masterCopy)
        public
    {
        require(_masterCopy != address(0), "Invalid master copy address provided");
        masterCopy = _masterCopy;
    }

    /// @dev Fallback function forwards all transactions and returns all received return data.
    function ()
        external
        payable
    {
        // solium-disable-next-line security/no-inline-assembly
        assembly {
            let masterCopy := and(sload(0), 0xffffffffffffffffffffffffffffffffffffffff)
            // 0xa619486e == keccak("masterCopy()"). The value is right padded to 32-bytes with 0s
            if eq(calldataload(0), 0xa619486e00000000000000000000000000000000000000000000000000000000) {
                mstore(0, masterCopy)
                return(0, 0x20)
            }
            calldatacopy(0, 0, calldatasize())
            let success := delegatecall(gas, masterCopy, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            if eq(success, 0) { revert(0, returndatasize()) }
            return(0, returndatasize())
        }
    }
}


contract Erc20 {
    address public sender;
    event DoSomething(address); 
    function dosomthing() public returns(bool){
        sender = msg.sender;
        emit DoSomething(msg.sender);
        return true;
    }
}


contract Hacker {
    event Ok(address,bytes,uint256);
    event Failed(bool);

    function setErc20Addr(address addr) public returns(bool) {
        erc20 = Erc20(addr);
    }

    Erc20  public erc20 ;
    function exec(address addr, bytes data, uint256 amount)  public payable returns(bool){
        bool success = erc20.dosomthing();
        if(success) {
            emit Ok(addr, data, amount);
            return true;
        } else {
            emit Failed(false);
            return false;
        }
    }   
}

call setErc20Addr after , Call directly Hacker The successful , But by calling proxy Of fallback Functions always fail .

remix The errors reported in the report are as follows

What is the reason ？

as a result of delegatecall The mechanism of : https://solidity-cn.readthedocs.io/zh/develop/introduction-to-smart-contracts.html?highlight=delegatecall#index-13

in other words , bool success = erc20.dosomthing(); This sentence ,erc20 yes Hacker Members of , When in Proxy Of fallback Call up Hacker Of exec when , Just put exec Function code is used to , Not yet Hacker Of erc20 Copy the past together , therefore ,erc20 It's empty. ！

After the modification ：

contract Hacker {
    event Ok(address,bytes,uint256);
    event Failed(bool);

    function exec(address addr, bytes data, uint256 amount)  public payable returns(bool){
        Erc20 erc20 = Erc20(addr);
        bool success = erc20.dosomthing();
        if(success) {
            emit Ok(addr, data, amount);
            return true;
        } else {
            emit Failed(false);
            return false;
        }
    }   
}

in other words ,erc20 Contract address by Proxy Of fallback Pass in when calling , Reinitialize erc20. Now you can call it normally .