who: fix panic in -q/--count on stdout write error

src/uu/who/src/platform/unix.rs

@@ -209,8 +209,9 @@ impl Who {
                 .filter(UtmpxRecord::is_user_process)
                 .map(|ut| ut.user())
                 .collect::<Vec<_>>();
-            println!("{}", users.join(" "));
-            println!("{}", translate!("who-user-count", "count" => users.len()));
+            let mut out = stdout().lock();
+            writeln!(out, "{}", users.join(" "))?;
+            writeln!(out, "{}", translate!("who-user-count", "count" => users.len()))?;
         } else {
             let records = utmpx::Utmpx::iter_all_records_from(f);
 

tests/by-util/test_who.rs

@@ -292,3 +292,22 @@ fn test_piped_to_dev_full() {
         .fails()
         .stderr_is("who: No space left on device\n");
 }
+
+#[cfg(target_os = "linux")]
+#[test]
+fn test_count_write_error() {
+    // Before the fix, `who -q` used `println!` which panics on a write error.
+    // The short-list branch must propagate stdout failures gracefully (#13388).
+    let ts = TestScenario::new(util_name!());
+
+    let dev_full = std::fs::OpenOptions::new()
+        .write(true)
+        .open("/dev/full")
+        .unwrap();
+
+    ts.ucmd()
+        .arg("--count")
+        .set_stdout(dev_full)
+        .fails()
+        .stderr_is("who: No space left on device\n");
+}