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Race condition in send()

Aperta
  • Qualcosa non va?
#424 1 commento 0 reazioni 0 assegnatari Vedi su GitHub

@JulienPalard ci sta già lavorando.

Dal 19/5/2026.

  • #425 di @JulienPalard — aperta

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
45/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Stack tecnologico
python
Ambito
backend, networking

Direzione di ricerca

Traccia adapter.send(), conditional_headers(), build_response(), update_cached_response() e _load_from_cache() nel flusso della cache e dell’adapter. Riproduci la sequenza a due processi descritta con richieste concorrenti, quindi verifica che una richiesta che ha ricevuto un 304 restituisca comunque la risposta memorizzata nella cache o un normale cache miss, invece di esporre 304 al chiamante.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

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.
Lingua principale
Python
Stelle
505
Fork
153
Merge medio
5g 6h
PR unite (30g)
5

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