FastAPI自定义中间件调用await request.form()后无响应（连接超时）

问题根源 #

在中间件中调用await request.form()会直接消耗请求体的数据流。FastAPI底层的Starlette框架中，请求体是流式且不可重复读取的。当后续call_next(request)把请求传递给业务路由时，路由尝试读取请求体时会发现没有数据，一直处于等待状态，最终导致连接超时。

解决办法 #

方法1：缓存请求体，允许重复读取 #

先读取请求体并缓存，再重新构建请求对象，确保下游路由能正常读取请求体：

from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware

class AuthMiddleWare(BaseHTTPMiddleware):
    def __init__(self, app, some_attribute: str):
        super().__init__(app)
        self.some_attribute = some_attribute

    async def dispatch(self, request: Request, call_next):
        # 缓存原始请求体
        body = await request.body()
        # 替换请求内部的body缓存，让后续读取有效
        request._body = body
        
        await self.before_request(request)
        response = await call_next(request)
        self.after_request(request)
        print('call next executes')
        return response

    async def before_request(self, request: Request):
        try:
            print('------------------ before request starts ----------------')
            print(request.url)
            print(request.method)
            print(request.client.host)
            print(request.client.port)
            print(request.base_url)
            # 此时读取form不会消耗请求体
            form = await request.form()
            firstname = form.get('firstname')
            print(firstname)
            print('------------------ before request ends ----------------')
        except Exception as e:
            print(f'发生异常: {str(e)}')

    def after_request(self, request: Request):
        pass

方法2：将form数据存入request.state，下游直接复用 #

如果中间件读取form后，下游路由不需要再处理原始请求体，可将解析后的form数据存入request.state，下游直接取用即可：

from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware

class AuthMiddleWare(BaseHTTPMiddleware):
    def __init__(self, app, some_attribute: str):
        super().__init__(app)
        self.some_attribute = some_attribute

    async def dispatch(self, request: Request, call_next):
        await self.before_request(request)
        response = await call_next(request)
        self.after_request(request)
        print('call next executes')
        return response

    async def before_request(self, request: Request):
        try:
            print('------------------ before request starts ----------------')
            print(request.url)
            print(request.method)
            print(request.client.host)
            print(request.client.port)
            print(request.base_url)
            form = await request.form()
            firstname = form.get('firstname')
            print(firstname)
            # 把form数据存入request.state，下游路由直接用
            request.state.form_data = form
            print('------------------ before request ends ----------------')
        except Exception as e:
            print(f'发生异常: {str(e)}')
            request.state.form_data = None

    def after_request(self, request: Request):
        pass

下游路由示例：

from fastapi import FastAPI, Request

app = FastAPI()
app.add_middleware(AuthMiddleWare, some_attribute="test")

@app.post("/test")
async def test(request: Request):
    return {"firstname": request.state.form_data.get("firstname") if request.state.form_data else None}

内容的提问来源于stack exchange，提问作者M. Mudassar