Fix dup2_stdout et al to handle fd aliasing. (#1069)

* Fix `dup2_stdout` et al to handle fd aliasing.

Fix `dup2_stdout` et al to work in the case where the file descriptor
they are passed is the same as the file descriptor they implicitly
operate on, including on targets that don't have a `dup2` syscall and
implement it using `dup3`, such as aarch64.

Fixes #1067.

* rustfmt and fix test compilation.

Author: sunfishcode

src/stdio.rs

@@ -14,7 +14,11 @@ use backend::c;
 use backend::fd::{BorrowedFd, FromRawFd, RawFd};
 
 #[cfg(not(any(windows, target_os = "wasi")))]
-use {crate::io, backend::fd::AsFd, core::mem::forget};
+use {
+    crate::io,
+    backend::fd::{AsFd, AsRawFd},
+    core::mem::forget,
+};
 
 /// `STDIN_FILENO`—Standard input, borrowed.
 ///
@@ -476,11 +480,13 @@ pub const fn raw_stderr() -> RawFd {
 #[allow(clippy::mem_forget)]
 #[inline]
 pub fn dup2_stdin<Fd: AsFd>(fd: Fd) -> io::Result<()> {
-    // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
-    // dropped.
-    let mut target = unsafe { take_stdin() };
-    backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
-    forget(target);
+    if fd.as_fd().as_raw_fd() != c::STDIN_FILENO {
+        // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
+        // dropped.
+        let mut target = unsafe { take_stdin() };
+        backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
+        forget(target);
+    }
     Ok(())
 }
 
@@ -489,11 +495,13 @@ pub fn dup2_stdin<Fd: AsFd>(fd: Fd) -> io::Result<()> {
 #[allow(clippy::mem_forget)]
 #[inline]
 pub fn dup2_stdout<Fd: AsFd>(fd: Fd) -> io::Result<()> {
-    // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
-    // dropped.
-    let mut target = unsafe { take_stdout() };
-    backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
-    forget(target);
+    if fd.as_fd().as_raw_fd() != c::STDOUT_FILENO {
+        // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
+        // dropped.
+        let mut target = unsafe { take_stdout() };
+        backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
+        forget(target);
+    }
     Ok(())
 }
 
@@ -502,10 +510,12 @@ pub fn dup2_stdout<Fd: AsFd>(fd: Fd) -> io::Result<()> {
 #[allow(clippy::mem_forget)]
 #[inline]
 pub fn dup2_stderr<Fd: AsFd>(fd: Fd) -> io::Result<()> {
-    // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
-    // dropped.
-    let mut target = unsafe { take_stderr() };
-    backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
-    forget(target);
+    if fd.as_fd().as_raw_fd() != c::STDERR_FILENO {
+        // SAFETY: We pass the returned `OwnedFd` to `forget` so that it isn't
+        // dropped.
+        let mut target = unsafe { take_stderr() };
+        backend::io::syscalls::dup2(fd.as_fd(), &mut target)?;
+        forget(target);
+    }
     Ok(())
 }

tests/stdio/dup2_stdio.rs

@@ -0,0 +1,20 @@
+use rustix::io::fcntl_getfd;
+use rustix::stdio::{dup2_stdout, stdout};
+
+#[test]
+fn dup2_stdin_stdin() {
+    let _ = dup2_stdout(stdout());
+    fcntl_getfd(stdout()).unwrap();
+}
+
+#[test]
+fn dup2_stdout_stdout() {
+    let _ = dup2_stdout(stdout());
+    fcntl_getfd(stdout()).unwrap();
+}
+
+#[test]
+fn dup2_stderr_stderr() {
+    let _ = dup2_stdout(stdout());
+    fcntl_getfd(stdout()).unwrap();
+}

tests/stdio/main.rs

@@ -1,5 +1,11 @@
 //! Tests for [`rustix::stdio`].
 
+#![cfg(feature = "stdio")]
+
+#[cfg(not(feature = "rustc-dep-of-std"))]
+#[cfg(not(windows))]
+#[cfg(not(target_os = "wasi"))]
+mod dup2_stdio;
 #[cfg(not(feature = "rustc-dep-of-std"))]
 #[cfg(not(windows))]
 #[cfg(not(target_os = "wasi"))]