cat: fix #5186 by adding explicit flush. (#5256)

* fix #5186 by adding explicit flush.
* make test machine-independent

---------

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>
Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

src/uu/cat/src/cat.rs

@@ -467,6 +467,13 @@ fn write_fast<R: FdReadable>(handle: &mut InputHandle<R>) -> CatResult<()> {
         }
         stdout_lock.write_all(&buf[..n])?;
     }
+
+    // If the splice() call failed and there has been some data written to
+    // stdout via while loop above AND there will be second splice() call
+    // that will succeed, data pushed through splice will be output before
+    // the data buffered in stdout.lock. Therefore additional explicit flush
+    // is required here.
+    stdout_lock.flush()?;
     Ok(())
 }
 

tests/by-util/test_cat.rs

@@ -2,7 +2,7 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore NOFILE nonewline
+// spell-checker:ignore NOFILE nonewline cmdline
 
 #[cfg(not(windows))]
 use crate::common::util::vec_of_size;
@@ -529,6 +529,21 @@ fn test_dev_full_show_all() {
     proc.kill();
 }
 
+// For some reason splice() on first of those files fails, resulting in
+// fallback inside `write_fast`, the other splice succeeds, in effect
+// without additional flush output gets reversed.
+#[test]
+#[cfg(target_os = "linux")]
+fn test_write_fast_fallthrough_uses_flush() {
+    const PROC_INIT_CMDLINE: &str = "/proc/1/cmdline";
+    let cmdline = std::fs::read_to_string(PROC_INIT_CMDLINE).unwrap();
+
+    new_ucmd!()
+        .args(&[PROC_INIT_CMDLINE, "alpha.txt"])
+        .succeeds()
+        .stdout_only(format!("{cmdline}abcde\nfghij\nklmno\npqrst\nuvwxyz\n")); // spell-checker:disable-line
+}
+
 #[test]
 #[cfg(unix)]
 #[ignore]