Sorry for the delay Thomas. This is complex and important to me issue and I want to be attentive to it.

I think we should preserve long existing behavior even if it is not documented (documenting or deprecating it is other issue). Concurrent reading and wring with concurrent reading always (at least since adding ZipFile.open()) worked, but was never documented nor tested. Concurrent writing was added rather as a side effect of issue14099. If there is a benefit from getting rid of it, we can break it.

For preserving current behavior ZipFile.open(mode='w') should acquire the lock and it should be released in _ZipWriteFile.close().

I have added other comments on Rietveld. The patch needs to be updated to resolve conflicts with committed zipinfo-from-file5.patch.