Fix WebSocket compressed sends to be cancellation safe

[bdraco]

What do these changes do?

Fixes data corruption in WebSocket compressed messages when send operations are cancelled. WebSocket uses a stateful compressor that persists across the connection lifetime. If a send_frame() call is cancelled during compression, the compressor state becomes corrupted, causing subsequent messages to contain mixed/corrupted data.

The fix wraps compressed sends in asyncio.shield() + lock pattern to ensure atomicity:

• Shield ensures compression completes even if task is cancelled
• Lock serializes sends to prevent interleaving
• Python 3.12+ optimization uses eager task creation

HTTP doesn't need this protection because connections close on error, getting a fresh compressor for the next request.

Are there changes in behavior for the user?

No breaking changes. This is a bug fix that prevents data corruption. Users may notice:

• Cancelled WebSocket sends now complete in the background (shielded)
• Slightly different cancellation timing for compressed messages only

Is it a substantial burden for the maintainers to support this?

No. The implementation is straightforward

The fix follows established patterns and includes cancellation tests for both single and multiple concurrent sends across all zlib backends.

Related issue number

Fixes #11725

[bdraco]

real world performance is awful for small compressed payloads. codspeed will likely show this as well. Our current tests will actually undershow it

[bdraco]

I need to optimize it for the small payload case which is the majority of cases

[codspeed-hq]

CodSpeed Performance Report

Merging #11726 will not alter performance

_{Comparing websocket_send_cancel_safe_large_payloads (7d6154e) with master (0d77d0d)}

Summary

✅ 59 untouched

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.73%. Comparing base (1fbb1bb) to head (7d6154e).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #11726    +/-   ##
========================================
  Coverage   98.73%   98.73%            
========================================
  Files         127      127            
  Lines       43546    43663   +117     
  Branches     2320     2325     +5     
========================================
+ Hits        42996    43112   +116     
  Misses        390      390            
- Partials      160      161     +1     

Flag	Coverage Δ
CI-GHA	98.60% <99.12%> (-0.01%)	⬇️
OS-Linux	98.34% <99.12%> (+<0.01%)	⬆️
OS-Windows	96.67% <99.12%> (-0.01%)	⬇️
OS-macOS	97.56% <99.12%> (+<0.01%)	⬆️
Py-3.10.11	97.11% <98.24%> (+<0.01%)	⬆️
Py-3.10.18	97.60% <98.24%> (-0.01%)	⬇️
Py-3.11.13	97.81% <98.24%> (+<0.01%)	⬆️
Py-3.11.9	97.32% <98.24%> (-0.01%)	⬇️
Py-3.12.10	97.42% <98.24%> (+<0.01%)	⬆️
Py-3.12.11	97.91% <98.24%> (+<0.01%)	⬆️
Py-3.13.7	97.91% <98.24%> (-0.01%)	⬇️
Py-3.13.9	97.39% <98.24%> (-0.01%)	⬇️
Py-3.14.0	98.12% <98.24%> (+<0.01%)	⬆️
Py-3.14.0t	97.18% <98.24%> (+<0.01%)	⬆️
Py-pypy3.10.16-7.3.19	91.31% <77.19%> (-4.07%)	⬇️
VM-macos	97.56% <99.12%> (+<0.01%)	⬆️
VM-ubuntu	98.34% <99.12%> (+<0.01%)	⬆️
VM-windows	96.67% <99.12%> (-0.01%)	⬇️

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[Copilot]

Pull Request Overview

This PR fixes a data corruption bug in WebSocket compressed message sending when operations are cancelled. The WebSocket connection maintains a stateful compressor that can become corrupted if send_frame() is cancelled during compression, causing subsequent messages to contain mixed data.

Key changes:

• Implements shield + lock pattern for large compressed sends to ensure atomicity
• Synchronous compression path for small payloads (≤16KiB) with lock protection
• Removed lock from ZLibCompressor.compress() and added cancellation safety warnings

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
aiohttp/_websocket/writer.py	Refactored send_frame() to use shield+lock pattern for large compressed frames; added sync compression path for small frames
aiohttp/compression_utils.py	Removed internal lock from ZLibCompressor.compress(); added documentation warnings about cancellation safety
tests/test_websocket_writer.py	Added regression tests for single and multiple cancelled compression operations
tests/conftest.py	Added slow_executor fixture to simulate slow operations for testing race conditions
CHANGES/11725.bugfix.rst	Added changelog entry for the bug fix

---

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[patchback]

Backport to 3.13: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 6cffcfd on top of patchback/backports/3.13/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726

Backporting merged PR #11726 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.13/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726 upstream/3.13

4. Now, cherry-pick PR Fix WebSocket compressed sends to be cancellation safe #11726 contents into that branch:

   $ git cherry-pick -x 6cffcfd749734551c0c824658f3a5c9250e9a165

   If it'll yell at you with something like fatal: Commit 6cffcfd749734551c0c824658f3a5c9250e9a165 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 6cffcfd749734551c0c824658f3a5c9250e9a165

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix WebSocket compressed sends to be cancellation safe #11726 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.13/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.14: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 6cffcfd on top of patchback/backports/3.14/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726

Backporting merged PR #11726 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.14/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726 upstream/3.14

4. Now, cherry-pick PR Fix WebSocket compressed sends to be cancellation safe #11726 contents into that branch:

   $ git cherry-pick -x 6cffcfd749734551c0c824658f3a5c9250e9a165

   If it'll yell at you with something like fatal: Commit 6cffcfd749734551c0c824658f3a5c9250e9a165 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 6cffcfd749734551c0c824658f3a5c9250e9a165

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix WebSocket compressed sends to be cancellation safe #11726 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.14/6cffcfd749734551c0c824658f3a5c9250e9a165/pr-11726

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.