These are very nice statements. What is you actually want?
Good question. Vinay provide a link to the issue he opened. In his opening message he wrote:
This behaviour works well when the file descriptors of these semaphores can be shared with children processes.
But, it doesn't work when an unrelated process which needs access to shared semaphore tries to open it.
Seems clear enough to me, but I'm not an expert. Here's the URL: https://bugs.python.org/issue38035
I think this answers Rhodri's question. I'm sure Vinay will correct me if I've mispoken, and add further comments appropriate.
By the way, it was core developer Tal Einat who suggested that Vinay post to this list.