Use getentropy() libc function on Emscripten

[josephlr]

Release 2.0.5: 09/28/2020 of Emscripten introduced a getentropy() call that can be used instead of opening /dev/urandom. We should use this as the preferred (or sole) method to get random data, as it avoids needing to open a file and hold onto a file descriptor.

NOTE: this probably requires getting the emscipten CI working with a recent-ish version

[briansmith]

Release 2.0.5: 09/28/2020 of Emscripten introduced a getentropy() call that can be used instead of opening /dev/urandom. We should use this as the preferred (or sole) method to get random data, as it avoids needing to open a file and hold onto a file descriptor.

I do agree that the file descriptor mechanism is worse. However, isn't Emscripten's getentropy() reimplementing what the getrandom crate does for the browser wasm32 target? Why not just use our own logic instead?

[josephlr]

However, isn't Emscripten's getentropy() reimplementing what the getrandom crate does for the browser wasm32 target? Why not just use our own logic instead?

IIUC, the point of the Emscripten target is to not use things like wasm-bindgen, but instead link to the EMSDK's libc. Then that libc crate can provide all the necessary functionality to emulate an Unix system while running in a wasm/asm.js environment. More practically, wasm-bindgen doesn't support Emscripten targets, so we couldn't do what we do for wasm32-unknown-unknown on wasm32-unknown-emscripten.