Strawman for flashloans for flashbots

Loans.sol

pragma solidity ^0.7.0;

// Each mining pool that intends to provide flash loans deploys a Loaner contract and transfers ETH to it
// When testing each bundle, the diff in balance in this contract is taking into account for calculating effective gas price
// The contract loans funds only on blocks mined by the miner and on zero-gasprice txs
contract Loaner {
    address immutable owner;
    
    constructor(address _owner) {
        owner = _owner;
    }
    
    function loan(address payable to, uint256 amount) public returns (uint256) {
        require(tx.gasprice == 0 && block.coinbase == owner);
        to.transfer(amount);
        return amount;
    }
    
    function withdraw(address payable to) public {
      require(msg.sender == owner);
      to.transfer(address(this).balance);
    }
}

// Users interact with the router, which redirects to the corresponding loaner based on the miner that picked up the bundle
contract Router {
    mapping(address => Loaner) registry;
    
    function getBalance() public view returns(uint256) {
        Loaner loaner = registry[block.coinbase];
        if (address(loaner) == address(0)) return 0;
        return address(loaner).balance;
    }
    
    function loan(address payable to, uint256 amount) public returns(uint256) {
        Loaner loaner = registry[block.coinbase];
        if (address(loaner) == address(0)) return 0;
        return loaner.loan(to, amount);
    }
    
    function register(Loaner loaner) public {
        registry[msg.sender] = loaner;
    }
}

the builder has full control of the coinbase

I'm not sure that's the case, but even if it were, there's still risk: the validator that's proposing the current block could set the block.coinbase of the block they're producing to be the address of a loaner, and include a zero-gas-price tx that siphons out all those funds, since it passes the tx.gasprice == 0 && block.coinbase == owner check.

a zero-gas-price tx

This bit probably needs to be adjusted to post-1559