Use esp_fill_random instead of libc::getrandom

Import esp_fill_random by using extern "C" and skip depending of
esp-idf-sys.

Opted for esp_fill_random instead of esp_random since esp_fill_random
implements logic for filling the u8 array efficiently from a u32 source.
Figured it was more efficient than what I would implement.

Author: thorhs

src/espidf.rs

@@ -8,17 +8,22 @@
 
 //! Implementation for ESP-IDF
 use crate::Error;
+use core::ffi::c_void;
+
+extern "C" {
+    fn esp_fill_random(buf: *mut c_void, len: usize) -> u32;
+}
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    // ESP-IDF fails and returns -1 only when the passed buffer is NULL, which cannot happen in our case:
-    // https://github.com/espressif/esp-idf/blob/master/components/newlib/random.c#L33
-    //
     // Not that NOT enabling WiFi, BT, or the voltage noise entropy source (via `bootloader_random_enable`)
     // will cause ESP-IDF to return pseudo-random numbers based on the voltage noise entropy, after the initial boot process:
     // https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/random.html
     //
     // However tracking if some of these entropy sources is enabled is way too difficult to implement here
-    unsafe { libc::getrandom(dest.as_mut_ptr().cast(), dest.len(), 0) };
+    //
+    // Using esp_fill_random since it has some optimizations regarding filling a byte array from an
+    // u32 source.  See https://github.com/espressif/esp-idf/blob/master/components/esp_hw_support/hw_random.c
+    unsafe { esp_fill_random(dest.as_mut_ptr().cast(), dest.len()) };
 
     Ok(())
 }

src/lib.rs

@@ -27,7 +27,7 @@
 //! | Hermit            | `x86_64-*-hermit`  | [`RDRAND`][18]
 //! | SGX               | `x86_64‑*‑sgx`     | [RDRAND][18]
 //! | VxWorks           | `*‑wrs‑vxworks‑*`  | `randABytes` after checking entropy pool initialization with `randSecure`
-//! | ESP-IDF           | `*‑espidf`         | [`getrandom()`][23]
+//! | ESP-IDF           | `*‑espidf`         | [`esp_fill_random()`][23]
 //! | Emscripten        | `*‑emscripten`     | `/dev/random` (identical to `/dev/urandom`)
 //! | WASI              | `wasm32‑wasi`      | [`random_get`][17]
 //! | Web Browser       | `wasm32‑*‑unknown` | [`Crypto.getRandomValues()`][14], see [WebAssembly support][16]
@@ -143,7 +143,7 @@
 //! [20]: https://www.unix.com/man-page/mojave/4/random/
 //! [21]: https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable
 //! [22]: https://leaf.dragonflybsd.org/cgi/web-man?command=getrandom
-//! [23]: https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/include/sys/random.h;hb=HEAD#l22
+//! [23]: https://github.com/espressif/esp-idf/blob/master/components/esp_hw_support/hw_random.c
 
 #![doc(
     html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",