stdbuf: add test verifying that execvp is used

Signed-off-by: Etienne Cordonnier <[email protected]>

Author: Ecordonnier

tests/by-util/test_stdbuf.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 dyld dylib setvbuf
+// spell-checker:ignore cmdline dyld dylib PDEATHSIG setvbuf
 #[cfg(target_os = "linux")]
 use uutests::at_and_ucmd;
 use uutests::new_ucmd;
@@ -247,3 +247,64 @@ fn test_stdbuf_non_utf8_paths() {
         .succeeds()
         .stdout_is("test content for stdbuf\n");
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_stdbuf_no_fork_regression() {
+    // Regression test for issue #9066: https://github.com/uutils/coreutils/issues/9066
+    // The original stdbuf implementation used fork+spawn which broke signal handling
+    // and PR_SET_PDEATHSIG. This test verifies that stdbuf uses exec() instead.
+    // With fork: stdbuf process would remain visible in process list
+    // With exec: stdbuf process is replaced by target command (GNU compatible)
+
+    use std::process::{Command, Stdio};
+    use std::thread;
+    use std::time::Duration;
+
+    let scene = TestScenario::new(util_name!());
+
+    // Start stdbuf with a long-running command
+    let mut child = Command::new(&scene.bin_path)
+        .args(["stdbuf", "-o0", "sleep", "3"])
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .spawn()
+        .expect("Failed to start stdbuf");
+
+    let child_pid = child.id();
+
+    // Give it a moment to exec
+    thread::sleep(Duration::from_millis(200));
+
+    // Check what command is actually running at that PID using /proc
+    let cmdline_path = format!("/proc/{child_pid}/cmdline");
+    let cmdline_result = std::fs::read_to_string(&cmdline_path);
+
+    match cmdline_result {
+        Ok(cmdline) => {
+            // cmdline in /proc contains null-separated arguments
+            let cmd_parts: Vec<&str> = cmdline.split('\0').collect();
+            let command_name = cmd_parts.first().map_or("", |v| v);
+
+            if command_name.contains("stdbuf") {
+                child.kill().ok();
+                panic!(
+                    "REGRESSION: Process {child_pid} is still stdbuf - fork() used instead of exec()"
+                );
+            } else if command_name.contains("sleep") {
+                println!("PASS: Process {child_pid} became 'sleep' - exec() used correctly");
+            } else {
+                child.kill().ok();
+                panic!("Unexpected command at PID {child_pid}: {command_name}");
+            }
+        }
+        Err(_) => {
+            // Process may have already exited, which is also fine for this test
+            println!("PASS: Process {child_pid} exited (likely exec worked)");
+        }
+    }
+
+    // Cleanup
+    child.kill().ok();
+    child.wait().ok();
+}