[jf-paquin-web]

Summary

xdist workers start with a copy of the master's args, so -n is still set when CovPlugin.__init__ runs in a worker. As a result, each worker started a DistMaster, then started a DistWorker on top of it without stopping the first one. Two collectors ended up on coverage's stack. Pausing the worker's coverage for no_cover brought the stray tracer back, so no_cover tests were still traced under xdist.

 worker: _prepareconfig(master args, -n still set)
   CovPlugin.__init__
     start(DistMaster)              # stray: erase() + cov.start()
 worker: pytest_sessionstart
+  cov_controller.cov.stop()        # pop the stray collector
   start(DistWorker)

Evidence

New test test_no_cover_marker_stops_tracing (nodist / 1xdist / 2xdist / 3xdist). Each inner test records how many collectors are active and whether a tracer is set:

• Before: 3 failed, 1 passed. In workers, the normal test saw collectors=2 and the no_cover test still had a tracer.
  After: 4 passed. Normal test: collectors=1. no_cover test: collectors=0, sys.gettrace() is None.

Full tox run (pytest 9.0.2, xdist 3.8.0, coverage 7.13.5):

Env	Result
py310, py311, py312, py313, py314, py315	218 passed, 4 skipped
pypy310, pypy311	216 passed, 6 skipped
check (build, twine, prek)	passed
docs	HTML build passed

Skips are pre-existing and platform-only: redis on macOS, SIGBREAK on Windows, and two PyPy-specific skips.