openzeppelin-contracts/contracts/proxy/transparent/TransparentUpgradeableProxy.sol

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (proxy/transparent/TransparentUpgradeableProxy.sol)

pragma solidity ^0.8.19;

import {ERC1967Utils} from "../ERC1967/ERC1967Utils.sol";
import {ERC1967Proxy} from "../ERC1967/ERC1967Proxy.sol";
import {IERC1967} from "../../interfaces/IERC1967.sol";

/**
 * @dev Interface for {TransparentUpgradeableProxy}. In order to implement transparency, {TransparentUpgradeableProxy}
 * does not implement this interface directly, and some of its functions are implemented by an internal dispatch
 * mechanism. The compiler is unaware that these functions are implemented by {TransparentUpgradeableProxy} and will not
 * include them in the ABI so this interface must be used to interact with it.
 */
interface ITransparentUpgradeableProxy is IERC1967 {
    function upgradeTo(address) external;

    function upgradeToAndCall(address, bytes memory) external payable;
}

/**
 * @dev This contract implements a proxy that is upgradeable by an immutable admin.
 *
 * To avoid https://medium.com/nomic-labs-blog/malicious-backdoors-in-ethereum-proxies-62629adf3357[proxy selector
 * clashing], which can potentially be used in an attack, this contract uses the
 * https://blog.openzeppelin.com/the-transparent-proxy-pattern/[transparent proxy pattern]. This pattern implies two
 * things that go hand in hand:
 *
 * 1. If any account other than the admin calls the proxy, the call will be forwarded to the implementation, even if
 * that call matches one of the admin functions exposed by the proxy itself.
 * 2. If the admin calls the proxy, it can access the admin functions, but its calls will never be forwarded to the
 * implementation. If the admin tries to call a function on the implementation it will fail with an error indicating the
 * proxy admin cannot fallback to the target implementation.
 *
 * These properties mean that the admin account can only be used for upgrading the proxy, so it's best if it's a dedicated
 * account that is not used for anything else. This will avoid headaches due to sudden errors when trying to call a function
 * from the proxy implementation.
 *
 * Our recommendation is for the dedicated account to be an instance of the {ProxyAdmin} contract. If set up this way,
 * you should think of the `ProxyAdmin` instance as the real administrative interface of your proxy, which extends from the
 * {Ownable} contract to allow for changing the proxy's admin owner.
 *
 * NOTE: The real interface of this proxy is that defined in `ITransparentUpgradeableProxy`. This contract does not
 * inherit from that interface, and instead the admin functions are implicitly implemented using a custom dispatch
 * mechanism in `_fallback`. Consequently, the compiler will not produce an ABI for this contract. This is necessary to
 * fully implement transparency without decoding reverts caused by selector clashes between the proxy and the
 * implementation.
 *
 * IMPORTANT: This contract avoids unnecessary storage reads by setting the admin only during construction as an immutable variable,
 * preventing any changes thereafter. However, the admin slot defined in ERC-1967 can still be overwritten by the implementation
 * logic pointed to by this proxy. In such cases, the contract may end up in an undesirable state where the admin slot is different
 * from the actual admin.
 *
 * WARNING: It is not recommended to extend this contract to add additional external functions. If you do so, the compiler
 * will not check that there are no selector conflicts, due to the note above. A selector clash between any new function
 * and the functions declared in {ITransparentUpgradeableProxy} will be resolved in favor of the new one. This could
 * render the admin operations inaccessible, which could prevent upgradeability. Transparency may also be compromised.
 */
contract TransparentUpgradeableProxy is ERC1967Proxy {
    // An immutable address for the admin avoid unnecessary SLOADs before each call
    // at the expense of removing the ability to change the admin once it's set.
    // This is acceptable if the admin is always a ProxyAdmin instance or similar contract
    // with its own ability to transfer the permissions to another account.
    address private immutable _admin;

    /**
     * @dev The proxy caller is the current admin, and can't fallback to the proxy target.
     */
    error ProxyDeniedAdminAccess();

    /**
     * @dev msg.value is not 0.
     */
    error ProxyNonPayableFunction();

    /**
     * @dev Initializes an upgradeable proxy managed by `_admin`, backed by the implementation at `_logic`, and
     * optionally initialized with `_data` as explained in {ERC1967Proxy-constructor}.
     */
    constructor(address _logic, address admin_, bytes memory _data) payable ERC1967Proxy(_logic, _data) {
        _admin = admin_;
        // Set the storage value and emit an event for ERC-1967 compatibility
        ERC1967Utils.changeAdmin(admin_);
    }

    /**
     * @dev If caller is the admin process the call internally, otherwise transparently fallback to the proxy behavior
     */
    function _fallback() internal virtual override {
        if (msg.sender == _admin) {
            bytes memory ret;
            bytes4 selector = msg.sig;
            if (selector == ITransparentUpgradeableProxy.upgradeTo.selector) {
                ret = _dispatchUpgradeTo();
            } else if (selector == ITransparentUpgradeableProxy.upgradeToAndCall.selector) {
                ret = _dispatchUpgradeToAndCall();
            } else {
                revert ProxyDeniedAdminAccess();
            }
            assembly {
                return(add(ret, 0x20), mload(ret))
            }
        } else {
            super._fallback();
        }
    }

    /**
     * @dev Upgrade the implementation of the proxy.
     */
    function _dispatchUpgradeTo() private returns (bytes memory) {
        _requireZeroValue();

        address newImplementation = abi.decode(msg.data[4:], (address));
        ERC1967Utils.upgradeToAndCall(newImplementation, bytes(""), false);

        return "";
    }

    /**
     * @dev Upgrade the implementation of the proxy, and then call a function from the new implementation as specified
     * by `data`, which should be an encoded function call. This is useful to initialize new storage variables in the
     * proxied contract.
     */
    function _dispatchUpgradeToAndCall() private returns (bytes memory) {
        (address newImplementation, bytes memory data) = abi.decode(msg.data[4:], (address, bytes));
        ERC1967Utils.upgradeToAndCall(newImplementation, data, true);

        return "";
    }

    /**
     * @dev To keep this contract fully transparent, the fallback is payable. This helper is here to enforce
     * non-payability of function implemented through dispatchers while still allowing value to pass through.
     */
    function _requireZeroValue() private {
        if (msg.value != 0) {
            revert ProxyNonPayableFunction();
        }
    }
}