Add notes when various functions were added (#257)

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/bsd_arandom.rs

@@ -31,6 +31,7 @@ fn kern_arnd(buf: &mut [u8]) -> libc::ssize_t {
 }
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getrandom(2) was introduces in FreeBSD 12.0 and NetBsd 10.0
     #[cfg(target_os = "freebsd")]
     {
         use crate::util_libc::Weak;

src/dragonfly.rs

@@ -17,6 +17,7 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
     type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
 
+    // getrandom(2) was introduced in DragonflyBSD 5.7
     if let Some(fptr) = GETRANDOM.ptr() {
         let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
         return sys_fill_exact(dest, |buf| unsafe { func(buf.as_mut_ptr(), buf.len(), 0) });

src/linux_android.rs

@@ -14,6 +14,7 @@ use crate::{
 };
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getrandom(2) was introduced in Linux 3.17
     static HAS_GETRANDOM: LazyBool = LazyBool::new();
     if HAS_GETRANDOM.unsync_init(is_getrandom_available) {
         sys_fill_exact(dest, |buf| unsafe {

src/macos.rs

@@ -17,6 +17,7 @@ use core::mem;
 type GetEntropyFn = unsafe extern "C" fn(*mut u8, libc::size_t) -> libc::c_int;
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getentropy(2) was added in 10.12, Rust supports 10.7+
     static GETENTROPY: Weak = unsafe { Weak::new("getentropy\0") };
     if let Some(fptr) = GETENTROPY.ptr() {
         let func: GetEntropyFn = unsafe { mem::transmute(fptr) };

src/openbsd.rs

@@ -10,6 +10,7 @@
 use crate::{util_libc::last_os_error, Error};
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getentropy(2) was added in OpenBSD 5.6, so we can use it unconditionally.
     for chunk in dest.chunks_mut(256) {
         let ret = unsafe { libc::getentropy(chunk.as_mut_ptr() as *mut libc::c_void, chunk.len()) };
         if ret == -1 {

src/solaris_illumos.rs

@@ -30,6 +30,7 @@ type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) ->
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::c_int;
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getrandom(2) was introduced in Solaris 11.3 for Illumos in 2015.
     static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
     if let Some(fptr) = GETRANDOM.ptr() {
         let func: GetRandomFn = unsafe { mem::transmute(fptr) };

src/windows.rs

@@ -24,6 +24,7 @@ extern "system" {
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     // Prevent overflow of u32
     for chunk in dest.chunks_mut(u32::max_value() as usize) {
+        // BCryptGenRandom was introduced in Windows Vista
         let ret = unsafe {
             BCryptGenRandom(
                 ptr::null_mut(),