Use the getrandom system call on Linux when it is available in CRYPTO_sysrand

[briansmith]

This should be faster than using /dev/urandom. Also, it would be better for sandboxed applications that want to block open and read syscalls completely.

This may obviate the need for the hwrand+ChaCha20 optimization and/or the /dev/urandom buffering. We should benchmark getrandom vs. the current CRYPTO_sysrand implementation vs. the hwrand+ChaCha20 optimization.

[DemiMarie]

I doubt it will solve the speed issue unless the kernel algorithms have changed. See my comment on #58 (tl;dr the kernel uses very slow algorithms).

[briansmith]

This is now #176. One issue is that Travis CI only natively supports Ubuntu 12.04 and Ubuntu 14.04, and the first Ubuntu with getrandom is Ubuntu 16.04. Apparently we could in theory use the Travis CI Docker support (see https://docs.travis-ci.com/user/docker/), but that seems way too complicated. For now I'm just going to punt on the Travis CI testing and do the testing locally instead.

[briansmith]

This was done a while back. See https://briansmith.org/rustdoc/ring/rand/struct.SystemRandom.html for the documentation.