[tlambert03]

Two problems found while working on funkey/waterz#25.

The lock did not lock anything

The lock file was created in the temporary directory that holds the source, which is a different one for every call. No two processes ever locked the same file, so processes compiling the same module for the first time all compiled it, into the same destination.

With four processes doing that (the new test_concurrent_compilation), on main:

ld: open() failed, errno=17 (File exists) for '.../_witty_module_4a6f29fae0e2e2174acec66e8dc17f6f.cpython-314-darwin.so'
clang: error: linker command failed with exit code 1

Now:

• the lock file is next to the module (<module>.lock in the output directory), so that one process compiles and the others wait and load the result
• the module is built in a temporary directory next to its destination and moved there with os.replace, so that it is never there in parts (for example if the build is interrupted)

On Windows, the lock was on os.path.getsize(lock_file) bytes, which is 0 for a file that was just opened for writing. It is one byte now, and it retries, since LK_LOCK gives up after ten seconds, which is less than a compilation can take. I could not run this on Windows locally; the new test runs there in CI.

Not addressed: nb_combined.o for nanobind modules is built into the cache directory under the lock of the module that needs it, so two different nanobind modules compiled concurrently for the first time can still both build it.

typing_extensions is imported, and not a dependency

$ uv venv --python 3.14 && uv pip install witty && python -c "import witty"
ModuleNotFoundError: No module named 'typing_extensions'

It is only needed for deprecated, which is warnings.deprecated as of python 3.13. It is a dependency for older versions now, and not imported on newer ones.

Testing

• test_concurrent_compilation fails on main and passes here (macOS, python 3.9, 3.12, 3.13, 3.14)
• clean environments with nothing but witty: python 3.12 (installs typing_extensions) and 3.14 (does not)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.69%. Comparing base (d30f274) to head (bf769f0).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/witty/compile_module.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   92.94%   92.69%   -0.25%     
==========================================
  Files           2        2              
  Lines         170      178       +8     
==========================================
+ Hits          158      165       +7     
- Misses         12       13       +1     

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