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Race condition in send()

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#424 1 条评论 0 个 reaction 已指派 0 人 在 GitHub 查看

@JulienPalard 已经在做这个了。

开始于 2026年5月19日。

  • #425 来自 @JulienPalard —— 未关闭

评估

难度
4/5
预计耗时
3-5 天
新手友好度
45/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
冷清
技术栈
python
领域
backend, networking

调研方向

在缓存和适配器流程中追踪 adapter.send()、conditional_headers()、build_response()、update_cached_response() 和 _load_from_cache()。使用并发请求复现所描述的双进程序列，然后验证收到 304 的请求仍然会返回缓存的响应或正常的缓存未命中，而不是将 304 暴露给调用方。

由索引模型根据 Issue 内容生成。

描述

I spotted this while using tox -vvv -p all, so via pip, since pip==26.1.

The issue with tox -p all is that it can run multiple versions of Python (in my config from 3.10 to 3.14), in parallel, I bet some with zstd some without.

TL;DR:

  • pip wants to query /simple/setuptools/
  • Cache is checked, an etag is found in the cache.
  • The HTTP request with an If-None-Match is sent.
  • In the meantime the cache gets modified, from Accept-Encoding: gzip, deflate to Accept-Encoding: gzip, deflate, zstd or the other way around, by another process.
  • The HTTP response arrives, it's a 304 Not Modified, the cache has to be updated, but this time it does no longer loads, and the 304 itself is returned.

In details:

It all starts in those two lines of adapter.send():

        request.headers.update(self.controller.conditional_headers(request))
    resp = super().send(request, stream, timeout, verify, cert, proxies)

Here conditional_headers does a _load_from_cache which can succeed, let say it succeed.

Then super().send() will eventually call adapter.build_response() with a 304 Not Modified so build_response calls self.controller.update_cached_response which runs another _load_from_cache, which can fail if the cache was modified by another process. Let's say it fails it runs this code:

    if not cached_response:
        # we didn't have a cached response
        return response

So the 304 response is returned, up to the user. In case of pip an exception is raised because the answer don't have a Content-Type. But pip or not, the user just did a get with no If-None-Match so he does not expect a 304, either a cache hit (hopefully 200 OK) or a cache miss (hopefully 200 OK) but never a 304.

The chronology with two processes

It think it looks like this, [1] and [2] denoting two distinct processes:

  • [1] runs a requests.get("https://pypi.org/simple/setuptools/")
  • [2] runs a requests.get("https://pypi.org/simple/setuptools/")
  • [1] Read the cache, extract an ETag from the cache
  • [2] Read the cache, extract an ETag from the cache
  • [1] Sends the HTTP request with If-None-Match
  • [2] Sends the HTTP request with If-None-Match
  • [1] Gets a response, adapter.build_response calls self.controller.update_cached_response(), changing the headers on disk. The Accept-Encoding in my case.
  • [1] Returns the cached response.
  • [2] Gets a response, adapter.build_response calls self.controller.update_cached_response() which calls _load_from_cache which refuses to load because the headers has changed (Vary tells the answer varies on Accept-Encoding and the Accept-Encoding changed), update_cached_response returns the 304 response. build_response returns the 304 response. The user gets a 304, which is unexpected.
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环境准备

  • 没有 Dockerfile 或 Docker Compose 文件
  • 没有 Pull Request 模板
  • 阅读贡献指南

从这里开始

  1. 先读完整个 Issue，再读项目的贡献指南。
  2. 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
  3. Fork 仓库，在一个分支上完成修改。
  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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