cat: Do not assume complete writes

src/uu/cat/src/splice.rs

@@ -44,21 +44,23 @@ pub(super) fn write_fast_using_splice<R: FdReadable>(
     }
 }
 
-/// Caller must ensure that `num_bytes <= BUF_SIZE`, otherwise this function
-/// will panic. The way we use this function in `write_fast_using_splice`
-/// above is safe because `splice` is set to write at most `BUF_SIZE` to the
-/// pipe.
-#[inline]
+/// Move exactly `num_bytes` bytes from `read_fd` to `write_fd`.
+///
+/// Panics if not enough bytes can be read.
 fn copy_exact(read_fd: RawFd, write_fd: RawFd, num_bytes: usize) -> nix::Result<()> {
     let mut left = num_bytes;
     let mut buf = [0; BUF_SIZE];
-    loop {
-        let read = unistd::read(read_fd, &mut buf[..left])?;
-        let written = unistd::write(write_fd, &buf[..read])?;
-        left -= written;
-        if left == 0 {
-            break;
+    while left > 0 {
+        let read = unistd::read(read_fd, &mut buf)?;
+        assert_ne!(read, 0, "unexpected end of pipe");
+        let mut written = 0;
+        while written < read {
+            match unistd::write(write_fd, &buf[written..read])? {
+                0 => panic!(),
+                n => written += n,
             }
         }
+        left -= read;
+    }
     Ok(())
 }