On macOS, try `getentropy()` before opening `/dev/urandom`?

[hatoo]

Background

Hi, I'm developing a HTTP load generator called oha. The application often exceeds the number of open file limit and then, tokio runtime crashes because it tries to open /dev/urandom to generate random on macOS.

hatoo/oha#23

issue

On macOS, according to https://github.com/rust-lang/rust/blob/master/src/libstd/sys/unix/rand.rs, fill_bytes always read /dev/urandom but opening /dev/urandom may fail because of the limit of the number of open files.
So it would be better to try getentropy like getrandom before reading /dev/urandom.

Thanks.

[RalfJung]

Miri would also like that, because it is easier to emulate. :)