作者： Davy
2020年 Defi 大火，動盪的年份裡也有個最靚的崽，難得。 當然，這個崽一樣譭譽參半，愛的人吹捧，厭的人大罵。本文只談技術，不論這些江湖上的是非。
Defi 大火，帶頭大哥是 Uniswap 無疑。於是，跟風者無數。Uniswap 是開源的，趕緊抄一個吧。
然而，Uniswap 程式碼中有一個坑，讓很多程式設計師在 “複製貼上” 工程中陷入困境。Uniswap 合約一共那麼幾個，都能部署上，但交易執行卻總是失敗，訊息是這樣子的：
Gas estimation errored with the following message (see below). The transaction execution will likely fail. Do you want to force sending?gas required exceeds allowance (8000029) or always failing transaction
uniswap 的教程有很多，網上一搜就有，然而，都跳不過這個坑。很多程式設計師在教程評論中留下了迷茫的跟帖：
有的脾氣不好，甚至大發雷霆失去控制，罵起人來。
好，下面就幫助大家，輕鬆跳過這個大坑。
Uniswap 的開原始碼在這裡：https://github.com/Uniswap
其中用到兩個庫，一個是 Uniswap V2 core：https://github.com/Uniswap/uniswap-v2-core。一個是 Uniswap V2 periphery：https://github.com/Uniswap/uniswap-v2-periphery
主要的合約是這麼幾個： 工廠合約 (UniswapV2Factory.sol)，負責生成“交易對合約”，並管理所有的“交易對合約”；路由合約 (UniswapV2Router02.sol)，處理交易和流動性；流動性代幣合約，與本文無關，不去管他。
基於工廠合約，可以生成很多的“交易對合約”，比如 ETH/ABC（ABC是一個 erc20 代幣）。注意，每一個“交易對合約” 的位元組碼都是一樣的，但地址不一樣。 “交易對合約”與 erc20 代幣的關係，在工廠合約中維護，工廠合約中用 maping 對映和陣列，在 “交易對合約”地址與 “erc20 合約”地址之間建立關聯。
工廠合約生成 “交易對合約” 的方法是 create2，併產生 “交易對合約” 的地址。
CREATE2 是以太坊在 2019 年 2 月 28 日的君士坦丁堡(Constantinople)硬分叉中引入 的一個新操作碼。CREATE2 的目的是：在部署智慧合約前就可以預先計算出合約的部署地址。
部署地址的計算公式如下:
keccak256 (0xff ++ address ++ salt ++ keccak256 (init_code)) [12:]
其中:
- address:要呼叫CREATE2的智慧合約地址，在 uniswap 裡就是工廠地址
- salt: 隨機值
- init_code:要部署合約的位元組碼
而路由合約是管交易和存入流動性的（就是存幣），在路由合約中要處理兩個幣種的交易時，就要呼叫這兩個幣種的 “交易對合約”。怎麼呼叫呢？路由合約並不儲存所有的“交易對合約”地址，那是工廠合約的任務。 路由合約透過計算 “交易對合約” 地址的方式呼叫“交易對合約”，計算方法封裝到了一個類庫裡 (UniswapV2Library)，程式碼如下:
// calculates the CREATE2 address for a pair without making any external calls
function pairFor(address factory, address tokenA, address tokenB) internal pure
returns (address pair) {
(address token0, address token1) = sortTokens(tokenA, tokenB);pair = address(uint(keccak256(abi.encodePacked(
hex,
factory,
keccak256(abi.encodePacked(token0, token1)),
hex //initcode hash
}
以上程式碼中 hex 對應的就是 “交易對合約” 位元組碼的keccak256 雜湊值。
天坑就在這裡，看明白了，也就一米寬，一躍而過，看不明白，就掉下去了。
uniswap 在這段程式碼裡留的雜湊值，很可能是 uniswap 自己所用的“交易對合約”的位元組碼，而開源出來的 “交易對合約”的位元組碼卻不一樣。程式設計師們在“複製貼上”工程中，就原樣抄上去了。
大學抄作業，把人家的名字也抄上去。這個坑，就是這麼一回事。
至於 uniswap 到底是不是故意的，大家猜猜吧。
下面來驗證一下。
1. 動手部署工廠合約。本文用 remix 部署，合約原始碼地址:https://cn.etherscan.com/address/0x5C69bEe701ef814a2B6a3EDD4B1652CB9cc5aA6f#code
在remix中編譯之後獲得 “交易對合約” 的位元組碼如下:
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這是一個16進位制的字串，使用 web3j 計算 keccak256 雜湊，輸出的結果是:
0a12837db10ca6a14dc85c90628faab669b8d5ed48f336b4f45100c2e9160239 。顯然路由合約
中的 96e8ac4277198ff8b6f785478aa9a39f403cb768dd02cbee326c3e7da348845f 是不對的。
當然不對，那是人家 uniswap 的“交易對合約”，不是你的。
這個錯誤不會導致合約部署出問題，但在合約呼叫時會報錯，就是這個：
Gas estimation errored with the following message (see below). The transaction execution will likely fail. Do you want to force sending?gas required exceeds allowance (8000029) or always failing transaction
好了，程式設計師們彆著急上火了，抄要抄得高高興興，抄要抄個明明白白。祝大家搞 Defi 順順利利。
