tests(cat,stdbuf): Add broken-pipe robustness tests (#4627)

Add test coverage for cat and stdbuf broken pipe handling:

**cat tests:**
- test_cat_broken_pipe_nonzero_and_message: Verify cat handles SIGPIPE
  without hanging or crashing and exits with nonzero status

**stdbuf tests:**
- test_permission_external_missing_lib: Handle missing external libstdbuf
- test_no_such_external_missing_lib: Error handling in external lib mode
- Guard existing tests with #[cfg(not(feature = "feat_external_libstdbuf"))]

These tests address write-errors.sh from GNU test suite (#4627) and improve
cross-platform robustness for stdbuf feat_external_libstdbuf builds.

tests/by-util/test_cat.rs

@@ -18,6 +18,32 @@ use uutests::util::TestScenario;
 use uutests::util::vec_of_size;
 use uutests::util_name;
 
+#[cfg(unix)]
+// Verify cat handles a broken pipe on stdout without hanging or crashing and exits nonzero
+#[test]
+fn test_cat_broken_pipe_nonzero_and_message() {
+    use std::fs::File;
+    use std::os::unix::io::FromRawFd;
+    use uutests::new_ucmd;
+
+    unsafe {
+        let mut fds: [libc::c_int; 2] = [0, 0];
+        assert_eq!(libc::pipe(fds.as_mut_ptr()), 0, "Failed to create pipe");
+        // Close the read end to simulate a broken pipe on stdout
+        let read_end = File::from_raw_fd(fds[0]);
+        // Explicitly drop the read-end so writers see EPIPE instead of blocking on a full pipe
+        std::mem::drop(read_end);
+        let write_end = File::from_raw_fd(fds[1]);
+
+        let content = (0..10000).map(|_| "x").collect::<String>();
+        // On Unix, SIGPIPE should lead to a non-zero exit; ensure process exits and fails
+        new_ucmd!()
+            .set_stdout(write_end)
+            .pipe_in(content.as_bytes())
+            .fails();
+    }
+}
+
 #[test]
 fn test_output_simple() {
     new_ucmd!()

tests/by-util/test_stdbuf.rs

@@ -15,6 +15,7 @@ fn invalid_input() {
     new_ucmd!().arg("-/").fails_with_code(125);
 }
 
+#[cfg(not(feature = "feat_external_libstdbuf"))]
 #[test]
 fn test_permission() {
     new_ucmd!()
@@ -24,6 +25,23 @@ fn test_permission() {
         .stderr_contains("Permission denied");
 }
 
+// TODO: Tests below are brittle when feat_external_libstdbuf is enabled and libstdbuf is not installed.
+// Align stdbuf with GNU search order to enable deterministic testing without installation:
+// 1) search for libstdbuf next to the stdbuf binary, 2) then in LIBSTDBUF_DIR, 3) then system locations.
+// After implementing this, rework tests to provide a temporary symlink rather than depending on system state.
+
+#[cfg(feature = "feat_external_libstdbuf")]
+#[test]
+fn test_permission_external_missing_lib() {
+    // When built with external libstdbuf, running stdbuf fails early if lib is not installed
+    new_ucmd!()
+        .arg("-o1")
+        .arg(".")
+        .fails_with_code(1)
+        .stderr_contains("External libstdbuf not found");
+}
+
+#[cfg(not(feature = "feat_external_libstdbuf"))]
 #[test]
 fn test_no_such() {
     new_ucmd!()
@@ -33,6 +51,17 @@ fn test_no_such() {
         .stderr_contains("No such file or directory");
 }
 
+#[cfg(feature = "feat_external_libstdbuf")]
+#[test]
+fn test_no_such_external_missing_lib() {
+    // With external lib mode and missing installation, stdbuf fails before spawning the command
+    new_ucmd!()
+        .arg("-o1")
+        .arg("no_such")
+        .fails_with_code(1)
+        .stderr_contains("External libstdbuf not found");
+}
+
 // Disabled on x86_64-unknown-linux-musl because the cross-rs Docker image for this target
 // does not provide musl-compiled system utilities (like head), leading to dynamic linker errors
 // when preloading musl-compiled libstdbuf.so into glibc-compiled binaries. Same thing for FreeBSD.