Fix double compression issue when enable_compression() is called on pre-encoded responses

[bdraco]

Summary

This PR fixes a flaky test failure caused by double compression when enable_compression() is called on responses that already have a Content-Encoding header (e.g., FileResponse serving pre-compressed files).

What's Changed

• Modified StreamResponse.enable_compression() to check for existing Content-Encoding header before enabling compression
• Added explicit test coverage for this behavior
• Added detailed comment explaining the rationale

Problem

The test test_static_file_with_encoding_and_enable_compression was failing intermittently with:

aiohttp.client_exceptions.ClientResponseError: 400, message="Expected HTTP/:
  bytearray(b'\\x0b\\t\\x80Hello aiohttp! :-)\\n\\x03')
               ^"

This occurred when:

1. FileResponse detected a pre-compressed file (e.g., file.txt.gz) and set _compression = False with appropriate Content-Encoding header
2. Test code called resp.enable_compression(forced_compression) which unconditionally set _compression = True
3. The already-compressed content was compressed again, resulting in corrupted data

Solution

The fix is minimal and backwards-compatible:

• enable_compression() now returns early if Content-Encoding header is already present
• This prevents double compression while maintaining existing API behavior
• No exceptions are raised to ensure backwards compatibility

Why This Approach?

1. Prevents the actual problem: Double compression is avoided
2. Doesn't break existing code: Silent ignore maintains backwards compatibility
3. "Do the right thing" principle: The framework handles the edge case gracefully
4. Predictable and safe: Behavior is consistent and prevents data corruption

Testing

• All existing tests pass
• Added new test test_enable_compression_with_existing_encoding to verify the fix
• The previously flaky test now passes consistently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.76%. Comparing base (1878509) to head (38fb249).
Report is 3 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10974      +/-   ##
==========================================
+ Coverage   98.10%   98.76%   +0.66%     
==========================================
  Files         129      129              
  Lines       39541    39555      +14     
  Branches     2185     2186       +1     
==========================================
+ Hits        38792    39068     +276     
+ Misses        591      339     -252     
+ Partials      158      148      -10     

Flag	Coverage Δ
CI-GHA	98.64% <100.00%> (+0.65%)	⬆️
OS-Linux	98.35% <100.00%> (+0.66%)	⬆️
OS-Windows	96.53% <100.00%> (+0.52%)	⬆️
OS-macOS	97.51% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.40% <100.00%> (+0.29%)	⬆️
Py-3.10.17	97.93% <100.00%> (+0.63%)	⬆️
Py-3.11.12	98.01% <100.00%> (?)
Py-3.11.9	97.48% <100.00%> (+0.29%)	⬆️
Py-3.12.10	98.41% <100.00%> (+0.93%)	⬆️
Py-3.13.3	98.41% <100.00%> (+1.11%)	⬆️
Py-3.9.13	97.27% <100.00%> (+<0.01%)	⬆️
Py-3.9.22	97.80% <100.00%> (+0.66%)	⬆️
Py-pypy7.3.16	82.18% <100.00%> (?)
VM-macos	97.51% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.35% <100.00%> (+0.66%)	⬆️
VM-windows	96.53% <100.00%> (+0.52%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #10974 will not alter performance

_{Comparing double_compress (38fb249) with master (a023a24)}

Summary

✅ 59 untouched benchmarks

[patchback]

Backport to 3.12: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 9c6da05 on top of patchback/backports/3.12/9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b/pr-10974

Backporting merged PR #10974 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/3.12/9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b/pr-10974 upstream/3.12

4. Now, cherry-pick PR Fix double compression issue when enable_compression() is called on pre-encoded responses #10974 contents into that branch:

   $ git cherry-pick -x 9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b

   If it'll yell at you with something like fatal: Commit 9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix double compression issue when enable_compression() is called on pre-encoded responses #10974 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.12/9c6da05b4233f07d41dd60bf68b8f7a4fe425d9b/pr-10974

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!

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