lightspeed survey: copy sys.modules before the extension scan

[ArjunS07]

Purpose

Benchmark discovery stops with RuntimeError: dictionary changed size during iteration for some tasks. The image build then finds 0 benchmarks and the task fails. In the current pandas batch, 5 of 7 failed builds have this error. 2 earlier pandas builds have it too.

How it works

_add_extension_deps loops over sys.modules.values() and calls getattr(mod, "__file__") on each module. A module with a lazy __getattr__ can import another module during that call, so sys.modules changes while the loop reads it. The fix loops over a copy, list(sys.modules.values()).

Changes

• asv/contrib/lightspeed/survey.py: loop over a copy of sys.modules.
  • Effect: the scan finishes when a module import happens during the scan. Modules added during the scan are not scanned.
  • Before: the scan stopped with RuntimeError and the survey recorded no benchmarks.
• test/test_lightspeed_survey.py: a test with a module whose __getattr__ adds a module to sys.modules.

Verification

$ pytest -q --noconftest test/test_lightspeed_survey.py
3 passed

Without the fix, the new test fails with the same error as the builds:

E       RuntimeError: dictionary changed size during iteration
asv/contrib/lightspeed/survey.py:77: RuntimeError

Notes

Build logs with the error: pandas-dev/pandas #23019, #23591, #23981, #24204, #24428, #24792, #25665. The task template pins LSV_REF, so the failed tasks need the new commit in the template before a rebuild.