The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #626 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -204,9 +204,11 @@ def _send_until_done(self, data):
def sendall(self, data):
- while len(data):
- sent = self._send_until_done(data)
- data = data[sent:]
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: Do not use this fix if the application relies on the previous behavior of sendall() for data handling.\n\n
Why This Fix Works in Production
- Trigger: pyopenssl performance hit from issue#412
- Mechanism: The sendall() method introduced excessive memory allocations during large file uploads due to inefficient data handling
- Why the fix works: Fixes significant performance issues in the sendall() method for large file uploads, addressing the performance hit introduced by a previous fix. (first fixed release: 1.25).
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- The sendall() method introduced excessive memory allocations during large file uploads due to inefficient data handling
- Production symptom (often without a traceback): pyopenssl performance hit from issue#412
Proof / Evidence
- GitHub issue: #626
- Fix PR: https://github.com/urllib3/urllib3/pull/627
- First fixed release: 1.25
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.79
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Erg, thank you for the analysis! @darkrain42 Would you be up for making a PR? /cc @vitek”
“Btw, would it make sense to open an issue on PyOpenSSL regarding this too?”
“Does urllib3/contrib/pyopenssl.py come from PyOpenSSL? I thought that was a urllib3 file. Otherwise, could you clarify what the issue for PyOpenSSL would be?”
“urllib3/contrib/pyopenssl.py is something we maintain, a compatibility layer for PyOpenSSL”
Failure Signature (Search String)
- pyopenssl performance hit from issue#412
- - OpenSSL will only write up to 16384 bytes at a time (the limit derives from the maximum amount of plaintext in a single SSL record)
Copy-friendly signature
Failure Signature
-----------------
pyopenssl performance hit from issue#412
- OpenSSL will only write up to 16384 bytes at a time (the limit derives from the maximum amount of plaintext in a single SSL record)
Error Message
Signature-only (no traceback captured)
Error Message
-------------
pyopenssl performance hit from issue#412
- OpenSSL will only write up to 16384 bytes at a time (the limit derives from the maximum amount of plaintext in a single SSL record)
What Broke
Large file uploads took significantly longer, causing timeouts and degraded performance.
Why It Broke
The sendall() method introduced excessive memory allocations during large file uploads due to inefficient data handling
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==1.25
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/urllib3/urllib3/pull/627
First fixed release: 1.25
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if the application relies on the previous behavior of sendall() for data handling.
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 TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
- Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.
Version Compatibility Table
| Version | Status |
|---|---|
| 1.25 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.