Michael.W基于Foundry精读Openzeppelin第59期——Proxy.sol

Proxy库对外只暴露了fallback和receive函数，是代理合约的基础实现。所有对Proxy合约的call都将被delegatecall到implement合约且delegatecall的执行结果会原封不动地返还给Proxy合约的调用方。通常称implement合约为代理合约背后的逻辑合约。

### 0. 版本

[openzeppelin]：v4.8.3，[forge-std]：v1.5.6

#### 0.1 Proxy.sol

Github: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v4.8.3/contracts/proxy/Proxy.sol

Proxy库对外只暴露了fallback和receive函数，是代理合约的基础实现。所有对Proxy合约的call都将被delegatecall到implement合约并且delegatecall的执行结果会原封不动地返还给Proxy合约的调用方。我们通常称implement合约为代理合约背后的逻辑合约。

### 1. 目标合约

继承Proxy合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/src/proxy/MockProxy.sol

```solidity
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

import "openzeppelin-contracts/contracts/proxy/Proxy.sol";

contract MockProxy is Proxy {
    address immutable private _IMPLEMENTATION_ADDR;
    bool immutable private _ENABLE_BEFORE_FALLBACK;

event ProxyBeforeFallback(uint value);

constructor(
        address implementationAddress,
        bool enableBeforeFallback
    ){
        _IMPLEMENTATION_ADDR = implementationAddress;
        _ENABLE_BEFORE_FALLBACK = enableBeforeFallback;
    }

function _implementation() internal view override returns (address){
        return _IMPLEMENTATION_ADDR;
    }

function _beforeFallback() internal override {
        if (_ENABLE_BEFORE_FALLBACK) {
            emit ProxyBeforeFallback(msg.value);
        }
    }
}
```

全部foundry测试合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/test/proxy/Proxy/Proxy.t.sol

测试使用的物料合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/test/proxy/Proxy/Implement.sol

```solidity
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

contract Implement {
    uint public i;
    address public addr;
    uint[3] public fixedArray;
    uint[] public dynamicArray;
    mapping(uint => uint) public map;

event ImplementReceive(uint value);
    event ImplementFallback(uint value);

function setUint(uint target) external {
        i = target;
    }

function setUintPayable(uint target) external payable {
        i = target;
    }

function setAddress(address target) external {
        addr = target;
    }

function setAddressPayable(address target) external payable {
        addr = target;
    }

function setFixedArray(uint[3] memory target) external {
        fixedArray = target;
    }

function setFixedArrayPayable(uint[3] memory target) external payable {
        fixedArray = target;
    }

function setDynamicArray(uint[] memory target) external {
        dynamicArray = target;
    }

function setDynamicArrayPayable(uint[] memory target) external payable {
        dynamicArray = target;
    }

function setMapping(uint key, uint value) external {
        map[key] = value;
    }

function setMappingPayable(uint key, uint value) external payable {
        map[key] = value;
    }

function triggerRevert() external pure {
        revert("Implement: revert");
    }

function triggerRevertPayable() external payable {
        revert("Implement: revert");
    }

function getPure() external pure returns (string memory){
        return "pure return value";
    }

receive() external payable {
        emit ImplementReceive(msg.value);
    }

fallback() external payable {
        emit ImplementFallback(msg.value);
    }
}
```

### 2. 代码精读

#### 2.1 _delegate(address implementation) internal

将当前的call，委托调用到implementation地址。

注：通过内联汇编“黑魔法”，使得没有返回值的_delegate()函数可以动态返回delegatecall的返回值。

```solidity
    function _delegate(address implementation) internal virtual {
        // 内联汇编
        assembly {
            // 从当前calldata的position 0开始将全部calldata都复制到内存中。内存中的数据存储也是从位置0开始。
            // 为何此处使用内存的起始position不是从0x40处取空闲内存指针？原因见后文。
            calldatacopy(0, 0, calldatasize())

// 使用delegatecall去调用逻辑合约。
            // 第一个参数：调用delegatecall的过程允许使用的gas上限。为gas()，即执行到此处剩余可用的全部gas;
            // 第二个参数：逻辑合约的地址;
            // 第三个参数：delegatecall所携带的calldata相关。calldata是从当前内存中获取，第三个参数为开始载入的内存position；
            // 第四个参数：delegatecall所携带的calldata相关。第四个参数为从内存中读取calldata的字节长度；
            // 综上可知，delegatecall所用的calldata就是进入_delegate(address implementation)时的calldata；
            // 第五个参数：delegatecall得到的返回数据存储在内存中，第五个参数为开始存储返回值的内存position；
            // 第六个参数：delegatecall得到的返回数据存储在内存中的字节长度。
            // 注：由于第五和第六个参数都设为0，即用来存储返回数据的内存长度为0。很明显delegatecall的返回数据长度（如有）要大于设定的存储空间，
            // 此时，全部的返回数据都要用returndatacopy()来复制到内存中。具体细则详见：https://learnblockchain.cn/article/6309
            let result := delegatecall(gas(), implementation, 0, calldatasize(), 0, 0)

// 由于delegatecall()时设定的存储返回数据的空间为0，要用returndatacopy()和returndatasize()来获取全部的返回数据。
            // 第一个参数：内存中存储返回数据的起始position，即从position 0处开始存储；
            // 第二个参数：返回数据被复制的起始position，即从头开始复制返回数据；
            // 第三个参数：复制返回数据的字节长度。returndatasize()表示未存储到delegatecall()时设定的存储空间的返回数据字节长度，此时该
            // 值应该为全部返回数据字节长度。
            // 综上可知，delegatecall()得到的全部返回数据都存储到从0开始的内存空间中
            returndatacopy(0, 0, returndatasize())

// 判断delegatecall是否成功调用
            switch result
            case 0 {
                // 如果delegatecall调用失败（例如gas不足），result为0
                // 那么就直接revert，revert携带的数据为内存中存储的delegatecall的全部返回数据
                revert(0, returndatasize())
            }
            default {
                // 如果非0（即1），表示delegatecall调用成功
                // 那么就进行函数返回，返回值为内存中存储的delegatecall的全部返回数据
                return(0, returndatasize())
            }
        }
    }
```

为何`_delegate()`中使用内存的起始position不是从0x40处取空闲内存指针，而是直接从position 0开始？

答：因为在该内联汇编代码块结束时直接进行函数返回，不会再有回到solidity代码逻辑的地方。**全部内存都只供汇编代码块使用**。只要在内联汇编中手动管理好内存指针，内存就是安全的。

#### 2.2 _implementation() internal && _beforeFallback() internal

- `_implementation()`：返回逻辑合约的地址。该函数未带实现体，需要在主合约中进行重写；
- ` _beforeFallback()`：执行delegatecall之前会执行的hook函数，如果有需要可以重写该函数并在其中增添逻辑。

```solidity
    function _implementation() internal view virtual returns (address);

function _beforeFallback() internal virtual {}
```

**foundry代码验证：**

```solidity
contract ProxyTest is Test {
    Implement private _implement = new Implement();
    address payable private _testingAddress = payable(address(new MockProxy(address(_implement), false)));

event ImplementFallback(uint value);
    event ImplementReceive(uint value);
    event ProxyBeforeFallback(uint value);

function test_beforeFallback() external {
        _testingAddress = payable(address(new MockProxy(address(_implement), true)));
        Implement proxy = Implement(_testingAddress);
        uint proxyBalance = _testingAddress.balance;
        assertEq(proxyBalance, 0);
        uint ethValue = 1 wei;

// case 1: test setUint()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setUint(1024);

// case 2:test setUintPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setUintPayable{value: ethValue}(1024);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 3: test setAddress()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setAddress(address(1));

// case 4: test setAddressPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setAddressPayable{value: ethValue}(address(1));
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 5: test setFixedArray()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];
        proxy.setFixedArray(targetFixedArray);

// case 6: test setFixedArrayPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setFixedArrayPayable{value: ethValue}(targetFixedArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 7: test setDynamicArray()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;
        proxy.setDynamicArray(targetDynamicArray);

// case 8: test setDynamicArrayPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setDynamicArrayPayable{value: ethValue}(targetDynamicArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 9: test setMapping()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setMapping(1024, 2048);

// case 10: test setMapping()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setMappingPayable{value: ethValue}(1024, 2048);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 11: revert with any static call because it emits event in _beforeFallback()
        // and causes the evm error: "StateChangeDuringStaticCall"
        vm.expectRevert();
        proxy.i();
        vm.expectRevert();
        proxy.addr();
        vm.expectRevert();
        proxy.fixedArray(0);
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        proxy.map(1024);
        vm.expectRevert();
        proxy.triggerRevert();
        vm.expectRevert();
        proxy.getPure();

// case 12: revert in the function of implement during a call
        vm.expectRevert("Implement: revert");
        proxy.triggerRevertPayable{value: ethValue}();

// case 13: call the function not exists in the implement
        // and delegate call to the fallback function of implement
        // without value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        emit ImplementFallback(0);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call(calldata_);
        assertTrue(ok);
        // with value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        emit ImplementFallback(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}(calldata_);
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 14: call the proxy with empty call data
        // and delegate call to the receive function of implement
        // without value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        emit ImplementReceive(0);
        (ok,) = _testingAddress.call("");
        assertTrue(ok);
        // with value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        emit ImplementReceive(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}("");
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
    }
}
```

#### 2.3 fallback() && receive()

- `fallback()`：当本合约被携带calldata的call调用时，进入该函数。随即将该call的calldata直接delegatecall到逻辑合约；
- `receive()`：当本合约被不携带任何calldata的call调用时，进入该函数。随即直接delegatecall到逻辑合约（不携带任何calldata）。

```solidity
    fallback() external payable virtual {
        // 调用_fallback()
        _fallback();
    }

receive() external payable virtual {
        // 调用_fallback()
        _fallback();
    }
    
    // 携带当前对本合约的call的calldata，delegatecall到逻辑合约
    function _fallback() internal virtual {
        // delegatecall之前运行hook函数
        _beforeFallback();
        // 携带当前对本合约的call的calldata，delegatecall到逻辑合约
        _delegate(_implementation());
    }
```

**foundry代码验证：**

```solidity
contract ProxyTest is Test {
    Implement private _implement = new Implement();
    address payable private _testingAddress = payable(address(new MockProxy(address(_implement), false)));

event ImplementFallback(uint value);
    event ImplementReceive(uint value);

function test_Call() external {
        Implement proxy = Implement(_testingAddress);
        // case 1: set uint256
        assertEq(proxy.i(), 0);
        assertEq(_implement.i(), 0);

proxy.setUint(1024);
        // check storage by static call
        assertEq(proxy.i(), 1024);
        assertEq(_implement.i(), 0);
        // check storage by slot number
        bytes32 slotNumber = bytes32(uint(0));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(1024)));

// case 2: set address
        assertEq(proxy.addr(), address(0));
        assertEq(_implement.addr(), address(0));

proxy.setAddress(address(2048));
        // check storage by static call
        assertEq(proxy.addr(), address(2048));
        assertEq(_implement.addr(), address(0));
        // check storage by slot number
        slotNumber = bytes32(uint(1));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(2048)));

// case 3: set fixed array
        assertEq(proxy.fixedArray(0), 0);
        assertEq(_implement.fixedArray(0), 0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];

proxy.setFixedArray(targetFixedArray);
        for (uint i; i < 3; ++i) {
            // check storage by static call
            assertEq(proxy.fixedArray(i), targetFixedArray[i]);
            assertEq(_implement.fixedArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(2 + i));
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetFixedArray[i]));
        }

// case 4: set dynamic array
        // revert during static call because dynamic array isn't initialized
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        _implement.dynamicArray(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;

proxy.setDynamicArray(targetDynamicArray);
        for (uint i; i < 3; ++i) {
            // check storage by static call
            assertEq(proxy.dynamicArray(i), targetDynamicArray[i]);
            vm.expectRevert();
            assertEq(_implement.dynamicArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(keccak256(abi.encodePacked(uint(5)))) + i);
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetDynamicArray[i]));
        }

// case 5: set mapping
        uint key = 1024;
        uint value = 2048;
        assertEq(proxy.map(key), 0);
        assertEq(_implement.map(key), 0);

proxy.setMapping(key, value);
        // check storage by static call
        assertEq(proxy.map(key), value);
        assertEq(_implement.map(key), 0);
        // check storage by slot number
        slotNumber = bytes32(uint(keccak256(abi.encodePacked(key, uint(6)))));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(value));

// case 6: revert with msg
        vm.expectRevert("Implement: revert");
        proxy.triggerRevert();

// case 7: call pure (staticcall)
        assertEq(proxy.getPure(), "pure return value");

// case 8: call the function not exists in the implement
        // and delegate call to the fallback function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementFallback(0);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call(calldata_);
        assertTrue(ok);

// case 9: call without value and calldata
        // and delegate call to the receive function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementReceive(0);
        (ok,) = _testingAddress.call("");
        assertTrue(ok);
    }

function test_PayableCall() external {
        Implement proxy = Implement(_testingAddress);
        uint proxyBalance = _testingAddress.balance;
        assertEq(proxyBalance, 0);

// case 1: set uint256 payable
        assertEq(proxy.i(), 0);
        assertEq(_implement.i(), 0);

uint ethValue = 1 wei;
        proxy.setUintPayable{value: ethValue}(1024);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// check storage by static call
        assertEq(proxy.i(), 1024);
        assertEq(_implement.i(), 0);
        // check storage by slot number
        bytes32 slotNumber = bytes32(uint(0));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(1024)));

// case 2: set address payble
        assertEq(proxy.addr(), address(0));
        assertEq(_implement.addr(), address(0));

proxy.setAddressPayable{value: ethValue}(address(2048));
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// check storage by static call
        assertEq(proxy.addr(), address(2048));
        assertEq(_implement.addr(), address(0));
        // check storage by slot number
        slotNumber = bytes32(uint(1));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(2048)));

// case 3: set fixed array payable
        assertEq(proxy.fixedArray(0), 0);
        assertEq(_implement.fixedArray(0), 0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];

proxy.setFixedArrayPayable{value: ethValue}(targetFixedArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        for (uint i; i < 3; ++i) {
            // check storage by static call
            assertEq(proxy.fixedArray(i), targetFixedArray[i]);
            assertEq(_implement.fixedArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(2 + i));
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetFixedArray[i]));
        }

// case 4: set dynamic array payable
        // revert during static call because dynamic array isn't initialized
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        _implement.dynamicArray(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;

proxy.setDynamicArrayPayable{value: ethValue}(targetDynamicArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        for (uint i; i < 3; ++i) {
            // check storage by static call
            assertEq(proxy.dynamicArray(i), targetDynamicArray[i]);
            vm.expectRevert();
            assertEq(_implement.dynamicArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(keccak256(abi.encodePacked(uint(5)))) + i);
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetDynamicArray[i]));
        }

// case 5: set mapping payable
        uint key = 1024;
        uint value = 2048;
        assertEq(proxy.map(key), 0);
        assertEq(_implement.map(key), 0);

proxy.setMappingPayable{value: ethValue}(key, value);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        // check storage by static call
        assertEq(proxy.map(key), value);
        assertEq(_implement.map(key), 0);
        // check storage by slot number
        slotNumber = bytes32(uint(keccak256(abi.encodePacked(key, uint(6)))));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(value));

// case 6: revert with msg payable
        vm.expectRevert("Implement: revert");
        proxy.triggerRevertPayable{value: ethValue}();

// case 7: call the function not exists in the implement with value
        // and delegate call to the fallback function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementFallback(ethValue);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call{value: ethValue}(calldata_);
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

// case 8: call with value and empty callata
        // and delegate call to the receive function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementReceive(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}("");
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
    }
}
```

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0. 版本

[openzeppelin]：v4.8.3，[forge-std]：v1.5.6

0.1 Proxy.sol

Github: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v4.8.3/contracts/proxy/Proxy.sol

1. 目标合约

继承Proxy合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/src/proxy/MockProxy.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

import "openzeppelin-contracts/contracts/proxy/Proxy.sol";

contract MockProxy is Proxy {
    address immutable private _IMPLEMENTATION_ADDR;
    bool immutable private _ENABLE_BEFORE_FALLBACK;

    event ProxyBeforeFallback(uint value);

    constructor(
        address implementationAddress,
        bool enableBeforeFallback
    ){
        _IMPLEMENTATION_ADDR = implementationAddress;
        _ENABLE_BEFORE_FALLBACK = enableBeforeFallback;
    }

    function _implementation() internal view override returns (address){
        return _IMPLEMENTATION_ADDR;
    }

    function _beforeFallback() internal override {
        if (_ENABLE_BEFORE_FALLBACK) {
            emit ProxyBeforeFallback(msg.value);
        }
    }
}

全部foundry测试合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/test/proxy/Proxy/Proxy.t.sol

测试使用的物料合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/test/proxy/Proxy/Implement.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

contract Implement {
    uint public i;
    address public addr;
    uint[3] public fixedArray;
    uint[] public dynamicArray;
    mapping(uint => uint) public map;

    event ImplementReceive(uint value);
    event ImplementFallback(uint value);

    function setUint(uint target) external {
        i = target;
    }

    function setUintPayable(uint target) external payable {
        i = target;
    }

    function setAddress(address target) external {
        addr = target;
    }

    function setAddressPayable(address target) external payable {
        addr = target;
    }

    function setFixedArray(uint[3] memory target) external {
        fixedArray = target;
    }

    function setFixedArrayPayable(uint[3] memory target) external payable {
        fixedArray = target;
    }

    function setDynamicArray(uint[] memory target) external {
        dynamicArray = target;
    }

    function setDynamicArrayPayable(uint[] memory target) external payable {
        dynamicArray = target;
    }

    function setMapping(uint key, uint value) external {
        map[key] = value;
    }

    function setMappingPayable(uint key, uint value) external payable {
        map[key] = value;
    }

    function triggerRevert() external pure {
        revert("Implement: revert");
    }

    function triggerRevertPayable() external payable {
        revert("Implement: revert");
    }

    function getPure() external pure returns (string memory){
        return "pure return value";
    }

    receive() external payable {
        emit ImplementReceive(msg.value);
    }

    fallback() external payable {
        emit ImplementFallback(msg.value);
    }
}

2. 代码精读

2.1 _delegate(address implementation) internal

将当前的call，委托调用到implementation地址。

注：通过内联汇编“黑魔法”，使得没有返回值的_delegate()函数可以动态返回delegatecall的返回值。

    function _delegate(address implementation) internal virtual {
        // 内联汇编
        assembly {
            // 从当前calldata的position 0开始将全部calldata都复制到内存中。内存中的数据存储也是从位置0开始。
            // 为何此处使用内存的起始position不是从0x40处取空闲内存指针？原因见后文。
            calldatacopy(0, 0, calldatasize())

            // 使用delegatecall去调用逻辑合约。
            // 第一个参数：调用delegatecall的过程允许使用的gas上限。为gas()，即执行到此处剩余可用的全部gas;
            // 第二个参数：逻辑合约的地址;
            // 第三个参数：delegatecall所携带的calldata相关。calldata是从当前内存中获取，第三个参数为开始载入的内存position；
            // 第四个参数：delegatecall所携带的calldata相关。第四个参数为从内存中读取calldata的字节长度；
            // 综上可知，delegatecall所用的calldata就是进入_delegate(address implementation)时的calldata；
            // 第五个参数：delegatecall得到的返回数据存储在内存中，第五个参数为开始存储返回值的内存position；
            // 第六个参数：delegatecall得到的返回数据存储在内存中的字节长度。
            // 注：由于第五和第六个参数都设为0，即用来存储返回数据的内存长度为0。很明显delegatecall的返回数据长度（如有）要大于设定的存储空间，
            // 此时，全部的返回数据都要用returndatacopy()来复制到内存中。具体细则详见：https://learnblockchain.cn/article/6309
            let result := delegatecall(gas(), implementation, 0, calldatasize(), 0, 0)

            // 由于delegatecall()时设定的存储返回数据的空间为0，要用returndatacopy()和returndatasize()来获取全部的返回数据。
            // 第一个参数：内存中存储返回数据的起始position，即从position 0处开始存储；
            // 第二个参数：返回数据被复制的起始position，即从头开始复制返回数据；
            // 第三个参数：复制返回数据的字节长度。returndatasize()表示未存储到delegatecall()时设定的存储空间的返回数据字节长度，此时该
            // 值应该为全部返回数据字节长度。
            // 综上可知，delegatecall()得到的全部返回数据都存储到从0开始的内存空间中
            returndatacopy(0, 0, returndatasize())

            // 判断delegatecall是否成功调用
            switch result
            case 0 {
                // 如果delegatecall调用失败（例如gas不足），result为0
                // 那么就直接revert，revert携带的数据为内存中存储的delegatecall的全部返回数据
                revert(0, returndatasize())
            }
            default {
                // 如果非0（即1），表示delegatecall调用成功
                // 那么就进行函数返回，返回值为内存中存储的delegatecall的全部返回数据
                return(0, returndatasize())
            }
        }
    }

为何_delegate()中使用内存的起始position不是从0x40处取空闲内存指针，而是直接从position 0开始？

答：因为在该内联汇编代码块结束时直接进行函数返回，不会再有回到solidity代码逻辑的地方。全部内存都只供汇编代码块使用。只要在内联汇编中手动管理好内存指针，内存就是安全的。

2.2 _implementation() internal && _beforeFallback() internal

_implementation()：返回逻辑合约的地址。该函数未带实现体，需要在主合约中进行重写；
_beforeFallback()：执行delegatecall之前会执行的hook函数，如果有需要可以重写该函数并在其中增添逻辑。

    function _implementation() internal view virtual returns (address);

    function _beforeFallback() internal virtual {}

foundry代码验证：

contract ProxyTest is Test {
    Implement private _implement = new Implement();
    address payable private _testingAddress = payable(address(new MockProxy(address(_implement), false)));

    event ImplementFallback(uint value);
    event ImplementReceive(uint value);
    event ProxyBeforeFallback(uint value);

    function test_beforeFallback() external {
        _testingAddress = payable(address(new MockProxy(address(_implement), true)));
        Implement proxy = Implement(_testingAddress);
        uint proxyBalance = _testingAddress.balance;
        assertEq(proxyBalance, 0);
        uint ethValue = 1 wei;

        // case 1: test setUint()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setUint(1024);

        // case 2:test setUintPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setUintPayable{value: ethValue}(1024);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 3: test setAddress()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setAddress(address(1));

        // case 4: test setAddressPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setAddressPayable{value: ethValue}(address(1));
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 5: test setFixedArray()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];
        proxy.setFixedArray(targetFixedArray);

        // case 6: test setFixedArrayPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setFixedArrayPayable{value: ethValue}(targetFixedArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 7: test setDynamicArray()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;
        proxy.setDynamicArray(targetDynamicArray);

        // case 8: test setDynamicArrayPayable()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setDynamicArrayPayable{value: ethValue}(targetDynamicArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 9: test setMapping()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        proxy.setMapping(1024, 2048);

        // case 10: test setMapping()
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        proxy.setMappingPayable{value: ethValue}(1024, 2048);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 11: revert with any static call because it emits event in _beforeFallback()
        // and causes the evm error: "StateChangeDuringStaticCall"
        vm.expectRevert();
        proxy.i();
        vm.expectRevert();
        proxy.addr();
        vm.expectRevert();
        proxy.fixedArray(0);
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        proxy.map(1024);
        vm.expectRevert();
        proxy.triggerRevert();
        vm.expectRevert();
        proxy.getPure();

        // case 12: revert in the function of implement during a call
        vm.expectRevert("Implement: revert");
        proxy.triggerRevertPayable{value: ethValue}();

        // case 13: call the function not exists in the implement
        // and delegate call to the fallback function of implement
        // without value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        emit ImplementFallback(0);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call(calldata_);
        assertTrue(ok);
        // with value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        emit ImplementFallback(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}(calldata_);
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 14: call the proxy with empty call data
        // and delegate call to the receive function of implement
        // without value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(0);
        emit ImplementReceive(0);
        (ok,) = _testingAddress.call("");
        assertTrue(ok);
        // with value
        vm.expectEmit(_testingAddress);
        emit ProxyBeforeFallback(ethValue);
        emit ImplementReceive(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}("");
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
    }
}

2.3 fallback() && receive()

fallback()：当本合约被携带calldata的call调用时，进入该函数。随即将该call的calldata直接delegatecall到逻辑合约；
receive()：当本合约被不携带任何calldata的call调用时，进入该函数。随即直接delegatecall到逻辑合约（不携带任何calldata）。

    fallback() external payable virtual {
        // 调用_fallback()
        _fallback();
    }

    receive() external payable virtual {
        // 调用_fallback()
        _fallback();
    }

    // 携带当前对本合约的call的calldata，delegatecall到逻辑合约
    function _fallback() internal virtual {
        // delegatecall之前运行hook函数
        _beforeFallback();
        // 携带当前对本合约的call的calldata，delegatecall到逻辑合约
        _delegate(_implementation());
    }

foundry代码验证：

contract ProxyTest is Test {
    Implement private _implement = new Implement();
    address payable private _testingAddress = payable(address(new MockProxy(address(_implement), false)));

    event ImplementFallback(uint value);
    event ImplementReceive(uint value);

    function test_Call() external {
        Implement proxy = Implement(_testingAddress);
        // case 1: set uint256
        assertEq(proxy.i(), 0);
        assertEq(_implement.i(), 0);

        proxy.setUint(1024);
        // check storage by static call
        assertEq(proxy.i(), 1024);
        assertEq(_implement.i(), 0);
        // check storage by slot number
        bytes32 slotNumber = bytes32(uint(0));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(1024)));

        // case 2: set address
        assertEq(proxy.addr(), address(0));
        assertEq(_implement.addr(), address(0));

        proxy.setAddress(address(2048));
        // check storage by static call
        assertEq(proxy.addr(), address(2048));
        assertEq(_implement.addr(), address(0));
        // check storage by slot number
        slotNumber = bytes32(uint(1));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(2048)));

        // case 3: set fixed array
        assertEq(proxy.fixedArray(0), 0);
        assertEq(_implement.fixedArray(0), 0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];

        proxy.setFixedArray(targetFixedArray);
        for (uint i; i &lt; 3; ++i) {
            // check storage by static call
            assertEq(proxy.fixedArray(i), targetFixedArray[i]);
            assertEq(_implement.fixedArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(2 + i));
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetFixedArray[i]));
        }

        // case 4: set dynamic array
        // revert during static call because dynamic array isn't initialized
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        _implement.dynamicArray(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;

        proxy.setDynamicArray(targetDynamicArray);
        for (uint i; i &lt; 3; ++i) {
            // check storage by static call
            assertEq(proxy.dynamicArray(i), targetDynamicArray[i]);
            vm.expectRevert();
            assertEq(_implement.dynamicArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(keccak256(abi.encodePacked(uint(5)))) + i);
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetDynamicArray[i]));
        }

        // case 5: set mapping
        uint key = 1024;
        uint value = 2048;
        assertEq(proxy.map(key), 0);
        assertEq(_implement.map(key), 0);

        proxy.setMapping(key, value);
        // check storage by static call
        assertEq(proxy.map(key), value);
        assertEq(_implement.map(key), 0);
        // check storage by slot number
        slotNumber = bytes32(uint(keccak256(abi.encodePacked(key, uint(6)))));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(value));

        // case 6: revert with msg
        vm.expectRevert("Implement: revert");
        proxy.triggerRevert();

        // case 7: call pure (staticcall)
        assertEq(proxy.getPure(), "pure return value");

        // case 8: call the function not exists in the implement
        // and delegate call to the fallback function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementFallback(0);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call(calldata_);
        assertTrue(ok);

        // case 9: call without value and calldata
        // and delegate call to the receive function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementReceive(0);
        (ok,) = _testingAddress.call("");
        assertTrue(ok);
    }

    function test_PayableCall() external {
        Implement proxy = Implement(_testingAddress);
        uint proxyBalance = _testingAddress.balance;
        assertEq(proxyBalance, 0);

        // case 1: set uint256 payable
        assertEq(proxy.i(), 0);
        assertEq(_implement.i(), 0);

        uint ethValue = 1 wei;
        proxy.setUintPayable{value: ethValue}(1024);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // check storage by static call
        assertEq(proxy.i(), 1024);
        assertEq(_implement.i(), 0);
        // check storage by slot number
        bytes32 slotNumber = bytes32(uint(0));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(1024)));

        // case 2: set address payble
        assertEq(proxy.addr(), address(0));
        assertEq(_implement.addr(), address(0));

        proxy.setAddressPayable{value: ethValue}(address(2048));
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // check storage by static call
        assertEq(proxy.addr(), address(2048));
        assertEq(_implement.addr(), address(0));
        // check storage by slot number
        slotNumber = bytes32(uint(1));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(uint(2048)));

        // case 3: set fixed array payable
        assertEq(proxy.fixedArray(0), 0);
        assertEq(_implement.fixedArray(0), 0);
        uint[3] memory targetFixedArray = [uint(1024), 2048, 4096];

        proxy.setFixedArrayPayable{value: ethValue}(targetFixedArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        for (uint i; i &lt; 3; ++i) {
            // check storage by static call
            assertEq(proxy.fixedArray(i), targetFixedArray[i]);
            assertEq(_implement.fixedArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(2 + i));
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetFixedArray[i]));
        }

        // case 4: set dynamic array payable
        // revert during static call because dynamic array isn't initialized
        vm.expectRevert();
        proxy.dynamicArray(0);
        vm.expectRevert();
        _implement.dynamicArray(0);
        // build dynamic array as input
        uint[] memory targetDynamicArray = new uint[](3);
        targetDynamicArray[0] = 1024;
        targetDynamicArray[1] = 2048;
        targetDynamicArray[2] = 4096;

        proxy.setDynamicArrayPayable{value: ethValue}(targetDynamicArray);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        for (uint i; i &lt; 3; ++i) {
            // check storage by static call
            assertEq(proxy.dynamicArray(i), targetDynamicArray[i]);
            vm.expectRevert();
            assertEq(_implement.dynamicArray(i), 0);
            // check storage by slot number
            slotNumber = bytes32(uint(keccak256(abi.encodePacked(uint(5)))) + i);
            assertEq(vm.load(_testingAddress, slotNumber), bytes32(targetDynamicArray[i]));
        }

        // case 5: set mapping payable
        uint key = 1024;
        uint value = 2048;
        assertEq(proxy.map(key), 0);
        assertEq(_implement.map(key), 0);

        proxy.setMappingPayable{value: ethValue}(key, value);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;
        // check storage by static call
        assertEq(proxy.map(key), value);
        assertEq(_implement.map(key), 0);
        // check storage by slot number
        slotNumber = bytes32(uint(keccak256(abi.encodePacked(key, uint(6)))));
        assertEq(vm.load(_testingAddress, slotNumber), bytes32(value));

        // case 6: revert with msg payable
        vm.expectRevert("Implement: revert");
        proxy.triggerRevertPayable{value: ethValue}();

        // case 7: call the function not exists in the implement with value
        // and delegate call to the fallback function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementFallback(ethValue);
        bytes memory calldata_ = abi.encodeWithSignature("unknown()");
        (bool ok,) = _testingAddress.call{value: ethValue}(calldata_);
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
        proxyBalance += ethValue;

        // case 8: call with value and empty callata
        // and delegate call to the receive function of implement
        vm.expectEmit(_testingAddress);
        emit ImplementReceive(ethValue);
        (ok,) = _testingAddress.call{value: ethValue}("");
        assertTrue(ok);
        assertEq(_testingAddress.balance, proxyBalance + ethValue);
    }
}

ps: 本人热爱图灵，热爱中本聪，热爱V神。以下是我个人的公众号，如果有技术问题可以关注我的公众号来跟我交流。同时我也会在这个公众号上每周更新我的原创文章，喜欢的小伙伴或者老伙计可以支持一下！如果需要转发，麻烦注明作者。十分感谢！