Fix fcntl_setpipe_size return value

`fcntl` man page says that:
> F_SETPIPE_SZ ... The actual capacity (in bytes) that is set is
> returned as the function result.

But the current `fcntl_setpipe_size` function assumes that the return
value is 0 (success) or negative (failure). This makes the function
panic due to `debug_assert!` in debug mode, and it returns `Err` on
success in release mode.

This commit fixes the return value type and its checking, and adds a
test for it.

src/backend/libc/pipe/syscalls.rs

@@ -118,8 +118,8 @@ pub(crate) fn fcntl_getpipe_sz(fd: BorrowedFd<'_>) -> io::Result<usize> {
 
 #[cfg(linux_kernel)]
 #[inline]
-pub(crate) fn fcntl_setpipe_sz(fd: BorrowedFd<'_>, size: usize) -> io::Result<()> {
+pub(crate) fn fcntl_setpipe_sz(fd: BorrowedFd<'_>, size: usize) -> io::Result<usize> {
     let size: c::c_int = size.try_into().map_err(|_| io::Errno::PERM)?;
 
-    unsafe { ret(c::fcntl(borrowed_fd(fd), c::F_SETPIPE_SZ, size)) }
+    unsafe { ret_c_int(c::fcntl(borrowed_fd(fd), c::F_SETPIPE_SZ, size)).map(|size| size as usize) }
 }

src/backend/linux_raw/pipe/syscalls.rs

@@ -111,12 +111,12 @@ pub(crate) fn fcntl_getpipe_sz(fd: BorrowedFd<'_>) -> io::Result<usize> {
 }
 
 #[inline]
-pub(crate) fn fcntl_setpipe_sz(fd: BorrowedFd<'_>, size: usize) -> io::Result<()> {
+pub(crate) fn fcntl_setpipe_sz(fd: BorrowedFd<'_>, size: usize) -> io::Result<usize> {
     let size: c::c_int = size.try_into().map_err(|_| io::Errno::PERM)?;
 
     #[cfg(target_pointer_width = "32")]
     unsafe {
-        ret(syscall_readonly!(
+        ret_usize(syscall_readonly!(
             __NR_fcntl64,
             fd,
             c_uint(F_SETPIPE_SZ),
@@ -125,7 +125,7 @@ pub(crate) fn fcntl_setpipe_sz(fd: BorrowedFd<'_>, size: usize) -> io::Result<()
     }
     #[cfg(target_pointer_width = "64")]
     unsafe {
-        ret(syscall_readonly!(
+        ret_usize(syscall_readonly!(
             __NR_fcntl,
             fd,
             c_uint(F_SETPIPE_SZ),

src/pipe.rs

@@ -215,6 +215,6 @@ pub fn fcntl_getpipe_size<Fd: AsFd>(fd: Fd) -> io::Result<usize> {
 /// [Linux]: https://man7.org/linux/man-pages/man2/fcntl.2.html
 #[cfg(linux_kernel)]
 #[inline]
-pub fn fcntl_setpipe_size<Fd: AsFd>(fd: Fd, size: usize) -> io::Result<()> {
+pub fn fcntl_setpipe_size<Fd: AsFd>(fd: Fd, size: usize) -> io::Result<usize> {
     backend::pipe::syscalls::fcntl_setpipe_sz(fd.as_fd(), size)
 }

tests/pipe/fcntl.rs

@@ -0,0 +1,31 @@
+#[cfg(linux_kernel)]
+#[test]
+fn test_fcntl_getpipe_size() {
+    use rustix::pipe::fcntl_getpipe_size;
+
+    let (reader, writer) = rustix::pipe::pipe().unwrap();
+
+    let reader_size = fcntl_getpipe_size(&reader).unwrap();
+    let writer_size = fcntl_getpipe_size(&writer).unwrap();
+    assert_eq!(reader_size, writer_size);
+}
+
+#[cfg(linux_kernel)]
+#[test]
+fn test_fcntl_setpipe_size() {
+    use rustix::pipe::{fcntl_getpipe_size, fcntl_setpipe_size};
+
+    let (reader, writer) = rustix::pipe::pipe().unwrap();
+
+    let new_size = 4096 * 2;
+    let reader_size = fcntl_setpipe_size(&reader, new_size).unwrap();
+    let writer_size = fcntl_getpipe_size(&writer).unwrap();
+    assert_eq!(reader_size, new_size);
+    assert_eq!(reader_size, writer_size);
+
+    let new_size = 4096 * 16;
+    let reader_size = fcntl_setpipe_size(&reader, new_size).unwrap();
+    let writer_size = fcntl_getpipe_size(&writer).unwrap();
+    assert_eq!(reader_size, new_size);
+    assert_eq!(reader_size, writer_size);
+}

tests/pipe/main.rs

@@ -6,3 +6,4 @@
 mod basic;
 mod splice;
 mod tee;
+mod fcntl;