Query maximum vector count on Linux and macOS

Both Linux and MacOS enforce limits on the vector count when performing vectored
I/O via the readv and writev system calls and return EINVAL when these limits
are exceeded. This changes the standard library to handle those limits as short
reads and writes to avoid forcing its users to query these limits using
platform specific mechanisms.

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

@@ -26,6 +26,27 @@ const READ_LIMIT: usize = c_int::MAX as usize - 1;
 #[cfg(not(target_os = "macos"))]
 const READ_LIMIT: usize = libc::ssize_t::MAX as usize;
 
+#[cfg(any(target_os = "linux", target_os = "macos"))]
+fn max_iov() -> c_int {
+    let ret = unsafe {
+        libc::sysconf(
+            #[cfg(target_os = "linux")]
+            libc::_SC_IOV_MAX,
+            #[cfg(target_os = "macos")]
+            libc::_SC_UIO_MAXIOV,
+        )
+    };
+
+    // 1024 is the default value on modern Linux systems
+    // and hopefully more useful than `c_int::MAX`.
+    if ret > 0 { ret as c_int } else { 1024 }
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "macos")))]
+fn max_iov() -> c_int {
+    c_int::MAX
+}
+
 impl FileDesc {
     pub fn new(fd: c_int) -> FileDesc {
         FileDesc { fd }
@@ -54,7 +75,7 @@ impl FileDesc {
             libc::readv(
                 self.fd,
                 bufs.as_ptr() as *const libc::iovec,
-                cmp::min(bufs.len(), c_int::MAX as usize) as c_int,
+                cmp::min(bufs.len(), max_iov() as usize) as c_int,
             )
         })?;
         Ok(ret as usize)
@@ -111,7 +132,7 @@ impl FileDesc {
             libc::writev(
                 self.fd,
                 bufs.as_ptr() as *const libc::iovec,
-                cmp::min(bufs.len(), c_int::MAX as usize) as c_int,
+                cmp::min(bufs.len(), max_iov() as usize) as c_int,
             )
         })?;
         Ok(ret as usize)
@@ -256,3 +277,16 @@ impl Drop for FileDesc {
         let _ = unsafe { libc::close(self.fd) };
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{FileDesc, IoSlice};
+
+    #[test]
+    fn limit_vector_count() {
+        let stdout = FileDesc { fd: 1 };
+        let bufs = (0..1500).map(|_| IoSlice::new(&[])).collect::<Vec<_>>();
+
+        assert!(stdout.write_vectored(&bufs).is_ok());
+    }
+}