tail: fix issue #6543 (--pid when reading from stdin) (#6582)

---------

Co-authored-by: just-an-engineer <Julian.Beltz@zetier.com>
Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

src/uu/tail/src/tail.rs

@@ -65,13 +65,15 @@ fn uu_tail(settings: &Settings) -> UResult<()> {
     // Add `path` and `reader` to `files` map if `--follow` is selected.
     for input in &settings.inputs.clone() {
         match input.kind() {
-            InputKind::File(path) if cfg!(not(unix)) || path != &PathBuf::from(text::DEV_STDIN) => {
-                tail_file(settings, &mut printer, input, path, &mut observer, 0)?;
-            }
-            // File points to /dev/stdin here
-            InputKind::File(_) | InputKind::Stdin => {
+            InputKind::Stdin => {
                 tail_stdin(settings, &mut printer, input, &mut observer)?;
             }
+            InputKind::File(path) if cfg!(unix) && path == &PathBuf::from(text::DEV_STDIN) => {
+                tail_stdin(settings, &mut printer, input, &mut observer)?;
+            }
+            InputKind::File(path) => {
+                tail_file(settings, &mut printer, input, path, &mut observer, 0)?;
+            }
         }
     }
 
@@ -85,7 +87,7 @@ fn uu_tail(settings: &Settings) -> UResult<()> {
         the input file is not a FIFO, pipe, or regular file, it is unspecified whether or
         not the -f option shall be ignored.
         */
-        if !settings.has_only_stdin() {
+        if !settings.has_only_stdin() || settings.pid != 0 {
             follow::follow(observer, settings)?;
         }
     }

tests/by-util/test_tail.rs

@@ -6,6 +6,7 @@
 // spell-checker:ignore (ToDO) abcdefghijklmnopqrstuvwxyz efghijklmnopqrstuvwxyz vwxyz emptyfile file siette ocho nueve diez MULT
 // spell-checker:ignore (libs) kqueue
 // spell-checker:ignore (jargon) tailable untailable datasame runneradmin tmpi
+// spell-checker:ignore (cmd) taskkill
 #![allow(
     clippy::unicode_not_nfc,
     clippy::cast_lossless,
@@ -4822,3 +4823,61 @@ fn test_obsolete_encoding_windows() {
         .stderr_is("tail: bad argument encoding: '-<2D>b'\n")
         .code_is(1);
 }
+
+#[test]
+#[cfg(not(target_vendor = "apple"))] // FIXME: for currently not working platforms
+fn test_following_with_pid() {
+    use std::process::Command;
+
+    let ts = TestScenario::new(util_name!());
+
+    #[cfg(not(windows))]
+    let mut sleep_command = Command::new("sleep")
+        .arg("999d")
+        .spawn()
+        .expect("failed to start sleep command");
+    #[cfg(windows)]
+    let mut sleep_command = Command::new("powershell")
+        .arg("-Command")
+        .arg("Start-Sleep -Seconds 999")
+        .spawn()
+        .expect("failed to start sleep command");
+
+    let sleep_pid = sleep_command.id();
+
+    let at = &ts.fixtures;
+    at.touch("f");
+    // when -f is specified, tail should die after
+    // the pid from --pid also dies
+    let mut child = ts
+        .ucmd()
+        .args(&[
+            "--pid",
+            &sleep_pid.to_string(),
+            "-f",
+            at.plus("f").to_str().unwrap(),
+        ])
+        .stderr_to_stdout()
+        .run_no_wait();
+    child.make_assertion_with_delay(2000).is_alive();
+
+    #[cfg(not(windows))]
+    Command::new("kill")
+        .arg("-9")
+        .arg(sleep_pid.to_string())
+        .output()
+        .expect("failed to kill sleep command");
+    #[cfg(windows)]
+    Command::new("taskkill")
+        .arg("/PID")
+        .arg(sleep_pid.to_string())
+        .arg("/F")
+        .output()
+        .expect("failed to kill sleep command");
+
+    let _ = sleep_command.wait();
+
+    child.make_assertion_with_delay(2000).is_not_alive();
+
+    child.kill();
+}