阿里云函数计算的 bug 与 workaround

去年写的阿里云函数计算是直接用 WSGI 接口撸的，最近引入 flask 框架重构了。但是重构并在本地测试通过后，放到云上遇到了问题。函数计算会对 url 中存在非 ascii 字符的情况报错。

错误 demo

对于这样一个简单的代码：

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/<string:name>')
def echo(name):
    return jsonify({'name': name})

def handler(environ, start_response):
    return app(environ, start_response)

在 python2 下，

GET /test
GET /测试

都可以通过，但是对于 python3 环境，

GET /test
GET /测试

第二个却会抛出 UnicodeDecodeError 异常。

错误原因

通过翻阅 WSGI 接口规范 pep-3333，可以发现阿里云函数计算实现的 WSGI 接口不规范。先看看 pep-3333 怎么定义的字符串和编码问题：

But in many Python versions and implementations, strings are Unicode, rather than bytes. This requires a careful balance between a usable API and correct translations between bytes and text in the context of HTTP... especially to support porting code between Python implementations with different str types.

WSGI therefore defines two kinds of "string":

• "Native" strings (which are always implemented using the type named str) that are used for request/response headers and metadata
• "Bytestrings" (which are implemented using the bytes type in Python 3, and str elsewhere), that are used for the bodies of requests and responses (e.g. POST/PUT input data and HTML page outputs).

Do not be confused however: even if Python's str type is actually Unicode "under the hood", the content of native strings must still be translatable to bytes via the Latin-1 encoding! (See the section on Unicode Issues later in this document for more details.)

可以看到，pep-3333 定义的 string 都是 latin-1 编码的 str，为了将其转换为 python3 中的字符串，werkzeug 库是这样实现的：

return s.encode('latin-1').decode('utf8)

而阿里云函数计算传入的是一个 unicode string，所以没法正确被 werkzeug 处理。

解决方法

知道了错误原理，我们就可以进行暂时的 workaround：

# aliyun FC workaround
def handler(environ, start_response):
    environ['PATH_INFO'] = environ['PATH_INFO'].encode('utf8').decode('latin1')
    return app(environ, start_response)

我已经将这个 bug 通过工单系统提交给了阿里云，不知道他们愿不愿意修复。