Use worksteal distribution strategy with pytest-xdist

[mhucka]

Pytest-xdist has options to control distribution of jobs across workers. Experimenting with different choices led to the discovery that the worksteal strategy speeds up execution of the Qualtran test suite. It appears to have no negative consequences. Using --dist worksteal affords a roughly 20–40% speedup for the quick test and 6% for the full tests.

Rough average timing results:

pytest-quick:

• Before: ~100 seconds
• After: ~80 seconds

pytest-full:

• Before: ~630 seconds
• After: ~380 seconds

[gemini-code-assist]

Code Review

The pull request refactors pytest_configure in qualtran/conftest.py by extracting thread-limiting logic into _config_set_thread_limits and introducing _config_set_xdist_worksteal to set --dist worksteal as the default distribution mode when using pytest-xdist. It also adds corresponding unit tests in qualtran/conftest_test.py. Feedback highlights two issues in _config_set_xdist_worksteal: it incorrectly overrides the distribution mode during sequential runs (when parallel execution is disabled) and fails to respect --dist configurations defined in addopts within configuration files. A code suggestion is provided to address these issues and safely handle cases where pytest-xdist is not installed.

[mhucka]

/gemini review

[gemini-code-assist]

This is a great optimization! Using worksteal is a well-known strategy in pytest-xdist for improving load balancing, especially when test execution times vary significantly. Your implementation correctly handles the configuration logic and preserves user-provided overrides, which is crucial for maintainability. The accompanying tests also look solid and cover the necessary edge cases. One minor suggestion: ensure that the pytest_configure function remains compatible with other plugins that might also be modifying config.option.dist by keeping the logic focused on the default case as you have done.