Add wasi select, FD_SET, FD_ZERO, FD_ISSET

(backport <rust-lang#3681>)
(cherry picked from commit 1edaad1)

src/wasi.rs

@@ -175,6 +175,11 @@ s! {
         pub st_ctim: timespec,
         __reserved: [c_longlong; 3],
     }
+
+    pub struct fd_set {
+        __nfds: usize,
+        __fds: [c_int; FD_SETSIZE as usize],
+    }
 }
 
 // Declare dirent outside of s! so that it doesn't implement Copy, Eq, Hash,
@@ -442,6 +447,28 @@ pub const NOEXPR: ::nl_item = 0x50001;
 pub const YESSTR: ::nl_item = 0x50002;
 pub const NOSTR: ::nl_item = 0x50003;
 
+f! {
+    pub fn FD_ISSET(fd: ::c_int, set: *const fd_set) -> bool {
+        let set = &*set;
+        let n = set.__nfds;
+        return set.__fds[..n].iter().any(|p| *p == fd)
+    }
+
+    pub fn FD_SET(fd: ::c_int, set: *mut fd_set) -> () {
+        let set = &mut *set;
+        let n = set.__nfds;
+        if !set.__fds[..n].iter().any(|p| *p == fd) {
+            set.__nfds = n + 1;
+            set.__fds[n] = fd;
+        }
+    }
+
+    pub fn FD_ZERO(set: *mut fd_set) -> () {
+        (*set).__nfds = 0;
+        return
+    }
+}
+
 #[cfg_attr(
     feature = "rustc-dep-of-std",
     link(
@@ -737,6 +764,14 @@ extern "C" {
     pub fn nl_langinfo(item: ::nl_item) -> *mut ::c_char;
     pub fn nl_langinfo_l(item: ::nl_item, loc: ::locale_t) -> *mut ::c_char;
 
+    pub fn select(
+        nfds: c_int,
+        readfds: *mut fd_set,
+        writefds: *mut fd_set,
+        errorfds: *mut fd_set,
+        timeout: *const timeval,
+    ) -> c_int;
+
     pub fn __wasilibc_register_preopened_fd(fd: c_int, path: *const c_char) -> c_int;
     pub fn __wasilibc_fd_renumber(fd: c_int, newfd: c_int) -> c_int;
     pub fn __wasilibc_unlinkat(fd: c_int, path: *const c_char) -> c_int;