httpx LineDecoder is accidentally quadratic: iter

The Fix

Upgrade to version 0.24.0 or later.

Based on closed encode/httpx issue #2422 · 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%.

Jump to Verify Open PR/Commit

@@ -259,66 +259,56 @@ class LineDecoder:
     Handles incrementally reading lines from text.
 
-    Uses universal line decoding, supporting any of `\n`, `\r`, or `\r\n`
-    as line endings, normalizing to `\n`.
+    Has the same behaviour as the stdllib splitlines, but handling the input iteratively.

repro.py

def decode(self, text: str) -> typing.List[str]:
        if self.buffer:
            text = self.buffer + text

        if text.endswith('\n'):
            lines = text.splitlines()
            self.buffer = ""
        else:
            lines = text.splitlines()
            self.buffer = lines.pop()

        return lines

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.24.0 or later.\nWhen NOT to use: This fix is not suitable if maintaining the original string slicing behavior is required.\n\n

Why This Fix Works in Production

Trigger: LineDecoder is accidentally quadratic: iter_lines() seems to hang forever
Mechanism: LineDecoder's string copying behavior is quadratic, causing significant delays in processing large chunks
Why the fix works: upstream changes in 0.24.0 address the mechanism above.

Production impact:

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

LineDecoder's string copying behavior is quadratic, causing significant delays in processing large chunks
Surfaces as: LineDecoder is accidentally quadratic: iter_lines() seems to hang forever

Proof / Evidence

GitHub issue: #2422
Fix PR: https://github.com/encode/httpx/pull/2423
First fixed release: 0.24.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.70
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.54

Discussion

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

Jump to Sources Open on GitHub

“Just a quick follow-on, even if the chunks are already line-decoded, processing the 31MB file takes 6 seconds!! It would probably be a lot easier…”

@gtedesco-r7 · 2022-10-26 · source

“Actually, the unit tests clued me in that text.splitlines() is no good because you want to keep the delimiters in. Switched to re, and that…”

@giannitedesco · 2022-10-26 · source

“Ah but okay, there's some whacky behaviours to do with CRLF and CR line-feeds, I will work on the PR as I get some time…”

@giannitedesco · 2022-10-26 · source

“Actually, it's easier than that since splitlines() already returns a list”

@gtedesco-r7 · 2022-10-26 · source

Failure Signature (Search String)

LineDecoder is accidentally quadratic: iter_lines() seems to hang forever

Error Message

Stack trace

error.txt

Error Message
-------------
LineDecoder is accidentally quadratic: iter_lines() seems to hang forever

Minimal Reproduction

repro.py

def decode(self, text: str) -> typing.List[str]:
        if self.buffer:
            text = self.buffer + text

        if text.endswith('\n'):
            lines = text.splitlines()
            self.buffer = ""
        else:
            lines = text.splitlines()
            self.buffer = lines.pop()

        return lines

Why It Broke

LineDecoder's string copying behavior is quadratic, causing significant delays in processing large chunks

Fix Options (Details)

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

Upgrade to version 0.24.0 or later.

When NOT to use: This fix is not suitable if maintaining the original string slicing behavior is required.

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

Fix reference: https://github.com/encode/httpx/pull/2423

First fixed release: 0.24.0

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

When NOT to Use This Fix

This fix is not suitable if maintaining the original string slicing behavior is required.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

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.

Version Compatibility Table

Version	Status
0.24.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:configuration-error All hubs All clusters

Related clusters: Data consistency Other Timeout

Sources

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

httpx LineDecoder is accidentally quadratic: iter_lines() seems to hang forever (Fix)