Use fcntl(fd, F_GETFD) to detect if standard streams are open

In the previous implementation, if the standard streams were open,
but the RLIMIT_NOFILE value was below three, the poll would fail
with EINVAL:

> ERRORS: EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

Switch to the existing fcntl based implementation to avoid the issue.

Author: tmiasko

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

@@ -67,48 +67,23 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
     args::init(argc, argv);
 
     unsafe fn sanitize_standard_fds() {
-        #[cfg(not(miri))]
-        // The standard fds are always available in Miri.
         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",
-                // The poll on Darwin doesn't set POLLNVAL for closed fds.
-                target_os = "macos",
-                target_os = "ios",
-                target_os = "redox",
                 target_os = "l4re",
             )))] {
-                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;
-                    }
-                    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();
-                    }
-                }
-            } else if #[cfg(any(target_os = "macos", target_os = "ios", target_os = "redox"))] {
                 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();
                         }
                     }

src/test/ui/process/nofile-limit.rs

@@ -0,0 +1,46 @@
+// Check that statically linked binary executes successfully
+// with RLIMIT_NOFILE resource lowered to zero. Regression
+// test for issue #96621.
+//
+// run-pass
+// dont-check-compiler-stderr
+// only-linux
+// no-prefer-dynamic
+// compile-flags: -Ctarget-feature=+crt-static -Crpath=no
+#![feature(exit_status_error)]
+#![feature(rustc_private)]
+extern crate libc;
+
+use std::os::unix::process::CommandExt;
+use std::process::Command;
+
+fn main() {
+    let mut args = std::env::args();
+    let this = args.next().unwrap();
+    match args.next().as_deref() {
+        None => {
+            let mut cmd = Command::new(this);
+            cmd.arg("Ok!");
+            unsafe {
+                cmd.pre_exec(|| {
+                    let rlim = libc::rlimit {
+                        rlim_cur: 0,
+                        rlim_max: 0,
+                    };
+                    if libc::setrlimit(libc::RLIMIT_NOFILE, &rlim) == -1 {
+                        Err(std::io::Error::last_os_error())
+                    } else {
+                        Ok(())
+                    }
+                })
+            };
+            let output = cmd.output().unwrap();
+            println!("{:?}", output);
+            output.status.exit_ok().unwrap();
+            assert!(output.stdout.starts_with(b"Ok!"));
+        }
+        Some(word) => {
+            println!("{}", word);
+        }
+    }
+}