Intermittent parallel test crash on macOS

I see an intermittent parallel test crash on macOS 10.14.

pytest==6.2.2
pytest-cov==2.10.1
pytest-forked==1.3.0
pytest-xdist==2.2.1

Crashed Thread:        2
--
 
Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY
 
Application Specific Information:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '+[NSUndoManager(NSInternal) _endTopLevelGroupings] is only safe to invoke on the main thread.'
terminating with uncaught exception of type NSException
abort() called

It's very infrequent (<1/100) so it's been difficult to debug, but it's a problem for our CI.

This occurs because execnet does not guarantee that workers are started in the main thread (see pytest-dev/pytest-xdist/issues/469)

We believe it can be fixed by

1. detecting when a worker is started in a not-main-thread
2. restarting the worker until the worker is in the main thread

We have code to do 1, but need help doing 2.

Start by reading the existing worker-start detection code and the related pytest-dev/pytest-xdist issue 469, then examine how execnet starts workers during parallel tests. Reproduce the macOS failure in CI if possible; done means workers started off the main thread are restarted until they run on the main thread, with coverage for the retry behavior.