[issue21305] PEP 466: update os.urandom

[Charles-François Natali]

Charles-François Natali added the comment:

> Using os.urandom is the *right* thing to do for getting random in an application, but the current implementation effectively punishes people who use it if their application is highly concurrent.

And I argue that this scenario is almost as likely as the one you
depict above: we never had a bug report before, and if you have a look
at the the bug report which led to the change in question, it's not
clear at all that all threads were indeed reading from /dev/urandom
when EMFILE was raised. Since reading from /dev/urandom shouldn't
block, it's not clear at all how a realistic workload would actually
hit the file descriptor limit because RLIMIT_NOFILE threads are
reading from /dev/urandom.

But don't get me wrong, I'm not saying this change is useless, it
actually makes sense to use a persistent FD. But backporting always
has a risk, which has to be balanced.

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