keep using poll as fast path and only use fcntl as fallback

this minimizes the amount of syscalls performed during startup

Author: the8472

library/std/src/lib.rs

@@ -239,6 +239,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exhaustive_patterns)]
 #![feature(intra_doc_pointers)]
+#![feature(label_break_value)]
 #![feature(lang_items)]
 #![feature(let_chains)]
 #![feature(linkage)]

library/std/src/sys/unix/mod.rs

@@ -67,25 +67,70 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
  args::init(argc, argv);
 
  unsafe fn sanitize_standard_fds() {
- cfg_if::cfg_if! {
- if #[cfg(not(any(
- // The standard fds are always available in Miri.
- miri,
- target_os = "emscripten",
- target_os = "fuchsia",
- target_os = "vxworks",
- target_os = "l4re",
- )))] {
- use crate::sys::os::errno;
- for fd in 0..3 {
- if libc::fcntl(fd, libc::F_GETFD) == -1 && errno() == libc::EBADF {
- if libc::open("/dev/null\0".as_ptr().cast(), libc::O_RDWR, 0) == -1 {
- // If the stream is closed but we failed to reopen it, abort the
- // process. Otherwise we wouldn't preserve the safety of
- // operations on the corresponding Rust object Stdin, Stdout, or
- // Stderr.
- libc::abort();
- }
+ // fast path with a single syscall for systems with poll()
+ #[cfg(not(any(
+ miri,
+ target_os = "emscripten",
+ target_os = "fuchsia",
+ target_os = "vxworks",
+ // The poll on Darwin doesn't set POLLNVAL for closed fds.
+ target_os = "macos",
+ target_os = "ios",
+ target_os = "redox",
+ target_os = "l4re",
+ )))]
+ 'poll: {
+ use crate::sys::os::errno;
+ let pfds: &mut [_] = &mut [
+ libc::pollfd { fd: 0, events: 0, revents: 0 },
+ libc::pollfd { fd: 1, events: 0, revents: 0 },
+ libc::pollfd { fd: 2, events: 0, revents: 0 },
+ ];
+
+ while libc::poll(pfds.as_mut_ptr(), 3, 0) == -1 {
+ if errno() == libc::EINTR {
+ continue;
+ }
+ if errno() == libc::EINVAL {
+ // RLIMIT_NOFILE may be preventing use of poll()
+ break 'poll;
+ }
+ libc::abort();
+ }
+ for pfd in pfds {
+ if pfd.revents & libc::POLLNVAL == 0 {
+ continue;
+ }
+ if libc::open("/dev/null\0".as_ptr().cast(), libc::O_RDWR, 0) == -1 {
+ // If the stream is closed but we failed to reopen it, abort the
+ // process. Otherwise we wouldn't preserve the safety of
+ // operations on the corresponding Rust object Stdin, Stdout, or
+ // Stderr.
+ libc::abort();
+ }
+ }
+ return;
+ }
+
+ // fallback in case poll isn't available or limited by RLIMIT_NOFILE
+ #[cfg(not(any(
+ // The standard fds are always available in Miri.
+ miri,
+ target_os = "emscripten",
+ target_os = "fuchsia",
+ target_os = "vxworks",
+ target_os = "l4re",
+ )))]
+ {
+ use crate::sys::os::errno;
+ for fd in 0..3 {
+ if libc::fcntl(fd, libc::F_GETFD) == -1 && errno() == libc::EBADF {
+ if libc::open("/dev/null\0".as_ptr().cast(), libc::O_RDWR, 0) == -1 {
+ // If the stream is closed but we failed to reopen it, abort the
+ // process. Otherwise we wouldn't preserve the safety of
+ // operations on the corresponding Rust object Stdin, Stdout, or
+ // Stderr.
+ libc::abort();
  }
  }
  }