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The attached patch turns running kill_python(_d).exe into a PreBuildEvent in pythoncore.vcxproj. This should guarantee that kill_python(_d).exe is always run to guarantee no zombie pythons in the background can prevent overwriting the Python dll. The pythoncore project already depends on the kill_python project, so kill_python should always be built and ready before this PreBuildEvent fires. The patch also removes the separate building and running of kill_python_d.exe from the buildbot build scripts, since it should be unnecessary with the patch to pythoncore.vcxproj.
Tim Peters, this patch was partly inspired by your issues in #19779; I suspect that perhaps your rebuild didn't take due to the segfaulted interpreter still hanging around in the background since the same thing happened to me doing a rebuild from the command line. Would you mind testing this patch to see if it would have helped in your case? Steps to test should be something like:
- update to e39db21df580
- build, run test_concurrent_futures (expect segfault)
- update to default
- rebuild, run test_concurrent_futures (expect build error and segfault again)
- apply patch
- rebuild, run test_concurrent_futures (expect success (or usual result))

Zach, I'll try the patch soon. Will this kill _all_ Python processes? I often have some unrelated Python program(s) running when waiting for a development build to finish, so that could create real havoc.
BTW, I'm sure you're right that zombie Pythons were the cause of my earlier woes.

OK, looked at the code and sees that it tries to kill only pythons created by the build process - I feel batter now :-)

Done. I wasn't able to provoke the failing test into leaving behind rogue processes, so I eventually just ran some python_d processes of my own. Of course that stopped the build from replacing the executables, so test_concurrent_futures kept failing.
But after applying your patch, my python_d processes were indeed killed off, as expected, and test_concurrent_futures started working again.
So: ship it! :-) Thanks!

New changeset ca600150205a by Zachary Ware in branch '3.3':
Issue #19788: kill_python(_d).exe is now run as a PreBuildEvent on the
http://hg.python.org/cpython/rev/ca600150205a
New changeset 2c1e041cb504 by Zachary Ware in branch 'default':
Issue #19788: kill_python(_d).exe is now run as a PreBuildEvent on the
http://hg.python.org/cpython/rev/2c1e041cb504 

Thanks for the test, Tim :)