tail: accept obsolete -cf and allow only one trailing f

src/uu/tail/src/args.rs

@@ -428,13 +428,15 @@ fn parse_num(src: &str) -> Result<Signum, ParseSizeError> {
 pub fn parse_args(args: impl uucore::Args) -> UResult<Settings> {
     let args_vec: Vec<OsString> = args.collect();
     let clap_args = uu_app().try_get_matches_from(args_vec.clone());
+    let mut settings_failed = false;
     let clap_result = match clap_args {
         // Kept for the caret in size diagnostics, which needs the value as
         // typed.
-        Ok(matches) => Ok(Settings::from_with_diagnostics(
+        Ok(matches) => Settings::from_with_diagnostics(
             &matches,
             uucore::diagnostics::capture(&args_vec).as_deref(),
-        )?),
+        )
+        .inspect_err(|_| settings_failed = true),
         Err(err) => Err(err.into()),
     };
 
@@ -465,6 +467,14 @@ pub fn parse_args(args: impl uucore::Args) -> UResult<Settings> {
     if clap_result.is_ok() && possible_obsolete_args.as_encoded_bytes().first() != Some(&b'+') {
         return clap_result;
     }
+    // `tail -cf` is obsolete syntax even though clap reads it as `-c f`,
+    // but in `tail -c f` the separate argument is always the count.
+    // A bare short option such as `-c` is two bytes long: no value is
+    // attached to it, so the value came from the next argument.
+    let no_value_attached = possible_obsolete_args.len() <= "-c".len();
+    if settings_failed && no_value_attached {
+        return clap_result;
+    }
     match parse_obsolete(possible_obsolete_args, args_vec.get(2))? {
         Some(settings) => Ok(settings),
         None => clap_result,

src/uu/tail/src/parse.rs

@@ -68,9 +68,9 @@ pub fn parse_obsolete(src: &OsString) -> Option<Result<ObsoleteArgs, ParseError>
         'l'
     };
 
-    let follow = rest.contains('f');
-    if !rest.chars().all(|f| f == 'f') {
-        // GNU allows an arbitrary amount of following fs, but nothing else
+    let follow = rest == "f";
+    if !follow && !rest.is_empty() {
+        // At most one trailing 'f' is allowed
         if sign == '-' && has_num {
             return Some(Err(ParseError::Context));
         }
@@ -168,11 +168,17 @@ mod tests {
             parse_obsolete(&OsString::from("-1vzqvq")), // spell-checker:disable-line
             Some(Err(ParseError::Context))
         );
+        assert_eq!(
+            parse_obsolete(&OsString::from("-5ff")),
+            Some(Err(ParseError::Context))
+        );
     }
     #[test]
     fn test_parse_obsolete_no_match() {
         assert_eq!(parse_obsolete(&OsString::from("-k")), None);
         assert_eq!(parse_obsolete(&OsString::from("asd")), None);
         assert_eq!(parse_obsolete(&OsString::from("-cc")), None);
+        assert_eq!(parse_obsolete(&OsString::from("-cff")), None);
+        assert_eq!(parse_obsolete(&OsString::from("+ff")), None);
     }
 }

tests/by-util/test_tail.rs

@@ -5141,6 +5141,47 @@
     p.kill().make_assertion().with_all_output().no_output();
 }
 
+#[test]
+#[cfg_attr(wasi_runner, ignore = "WASI: tail follow mode disabled")]
+fn test_tail_obsolete_bytes_follow_stdin() {
+    // `-f` is ignored when stdin is a pipe, so tail exits after printing.
+    new_ucmd!()
+        .arg("-cf")
+        .pipe_in("abcdefghijklmnop\n")
+        .succeeds()
+        .stdout_only("hijklmnop\n");
+}
+
+#[test]
+#[cfg_attr(wasi_runner, ignore = "WASI: tail follow mode disabled")]
+fn test_tail_obsolete_bytes_follow() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    at.write("file", "abcdefghijklmnop\n");
+    let mut p = scene.ucmd().args(&["-cf", "file"]).run_no_wait();
+    p.make_assertion_with_delay(500).is_alive();
+    p.kill()
+        .make_assertion()
+        .with_all_output()
+        .stdout_only("hijklmnop\n");
+
+    scene
+        .ucmd()
+        .arg("-c")
+        .arg("f")
+        .fails_with_code(1)
+        .no_stdout()
+        .stderr_is("tail: invalid number of bytes: 'f'\n");
+
+    scene
+        .ucmd()
+        .args(&["-5ff", "file"])
+        .fails_with_code(1)
+        .no_stdout()
+        .stderr_is("tail: option used in invalid context -- 5\n");
+}
+
 #[test]
 #[cfg(unix)]
 #[cfg_attr(wasi_runner, ignore = "WASI: argv/filenames must be valid UTF-8")]
@@ -5506,7 +5547,7 @@
         .fails_with_code(1)
         .stderr_is("tail: invalid number of bytes: '0fb'\n");
     new_ucmd!()
         .args(&["-n", "000ff", "/dev/null"])
         .fails_with_code(1)
         .stderr_is("tail: invalid number of lines: '000ff'\n");
     // The sign is not put back with them: GNU reports `-c-0fb` as '0fb'.