urllib3 test_client_no_intermediate test failure (Fix)

The Fix

pip install urllib3==1.25.9

Based on closed urllib3/urllib3 issue #1766 · PR/commit linked

Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.

Open PR/Commit

@@ -173,29 +173,8 @@ def test_client_no_intermediate(self):
             ca_certs=DEFAULT_CA,
         ) as https_pool:
-            try:
+            with pytest.raises((SSLError, ProtocolError)):
                 https_pool.request("GET", "/certificate", retries=False)

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25.9\nWhen NOT to use: This fix should not be used if the test requires specific exception handling for validation.\n\n

Why This Fix Works in Production

Trigger: ==================================================================== FAILURES =====================================================================
Mechanism: The test_client_no_intermediate test was relying on specific exception values, causing flakiness
Why the fix works: The test_client_no_intermediate test was modified to stop looking at exception values, improving its reliability. (first fixed release: 1.25.9).

Production impact:

If left unfixed, failures can be intermittent under concurrency (hard to reproduce; shows up as sporadic 5xx/timeouts).

Why This Breaks in Prod

Shows up under Python 2.7 in real deployments (not just unit tests).
The test_client_no_intermediate test was relying on specific exception values, causing flakiness
Surfaces as: ==================================================================== FAILURES =====================================================================

Proof / Evidence

GitHub issue: #1766
Fix PR: https://github.com/urllib3/urllib3/pull/1811
First fixed release: 1.25.9
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“@hodbn Thanks for the follow-up! @mgorny Thanks for trying this again! This was fixed in https://github.com/urllib3/urllib3/pull/1811, sorry I had forgot about this issue. We can…”

@pquentin · 2020-05-25 · confirmation · source

“Can you point to a failing CI build or describe your setup?”

@hodbn · 2020-05-24 · source

“@mgorny Thanks for tracking this, let us know if you have more issues with this test. Thanks for maintaining urllib3 dists on Gentoo :)”

@sethmlarson · 2020-05-24 · source

“That would be a good question back in December”

@mgorny · 2020-05-24 · source

Failure Signature (Search String)

==================================================================== FAILURES =====================================================================

Error Message

Stack trace

error.txt

Error Message
-------------
==================================================================== FAILURES =====================================================================
Exception in thread Thread-120:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/tmp/urllib3/dummyserver/server.py", line 145, in run
    self.server = self._start_server()
  File "/tmp/urllib3/dummyserver/server.py", line 141, in _start_server
    self.socket_handler(sock)
  File "/tmp/urllib3/test/with_dummyserver/test_socketlevel.py", line 1331, in socket_handler
    ca_certs=DEFAULT_CA,
  File "/usr/lib64/python2.7/ssl.py", line 931, in wrap_socket
    ciphers=ciphers)
  File "/usr/lib64/python2.7/ssl.py", line 599, in __init__
    self.do_handshake()
  File "/usr/lib64/python2.7/ssl.py", line 828, in do_handshake
    self._sslobj.do_handshake()
error: [Errno 0] Error

Exception in thread Thread-119:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/tmp/urllib3/dummyserver/server.py", line 145, in run
    self.server = self._start_server()
  File "/tmp/urllib3/dummyserver/server.py", line 141, in _start_server
    self.socket_handler(sock)
  File "/tmp/urllib3/test/with_dummyserver/test_socketlevel.py", line 1331, in socket_handler
    ca_certs=DEFAULT_CA,
  File
... (truncated) ...

Environment

Python: 2.7

What Broke

The test intermittently failed, leading to unreliable CI results.

Why It Broke

The test_client_no_intermediate test was relying on specific exception values, causing flakiness

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install urllib3==1.25.9

When NOT to use: This fix should not be used if the test requires specific exception handling for validation.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/urllib3/urllib3/pull/1811

First fixed release: 1.25.9

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the test requires specific exception handling for validation.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
1.25.9	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:race-condition urllib3 hub urllib3 best practices All hubs All clusters

Related clusters: Configuration error Data consistency SSL handshake

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.