tail: Add gnu's warning when following stdin is ineffective. If no pid support set to 0 in Observer.

Cargo.lock

@@ -2984,6 +2984,7 @@ dependencies = [
 name = "uu_tail"
 version = "0.0.16"
 dependencies = [
+ "atty",
  "clap",
  "libc",
  "memchr",

src/uu/tail/Cargo.toml

@@ -22,6 +22,7 @@ memchr = "2.5.0"
 notify = { version = "=5.0.0", features=["macos_kqueue"]}
 uucore = { version=">=0.0.16", package="uucore", path="../../uucore", features=["ringbuffer", "lines"] }
 same-file = "1.0.6"
+atty = "0.2"
 
 [target.'cfg(windows)'.dependencies]
 windows-sys = { version = "0.42.0", default-features = false, features = ["Win32_System_Threading", "Win32_Foundation"] }

src/uu/tail/src/args.rs

@@ -7,8 +7,10 @@
 
 use crate::paths::Input;
 use crate::{parse, platform, Quotable};
+use atty::Stream;
 use clap::crate_version;
 use clap::{parser::ValueSource, Arg, ArgAction, ArgMatches, Command};
+use same_file::Handle;
 use std::collections::VecDeque;
 use std::ffi::OsString;
 use std::time::Duration;
@@ -120,11 +122,6 @@ pub enum VerificationResult {
     NoOutput,
 }
 
-pub enum CheckResult {
-    Ok,
-    NoPidSupport,
-}
-
 #[derive(Debug, Default)]
 pub struct Settings {
     pub follow: Option<FollowMode>,
@@ -202,6 +199,7 @@ impl Settings {
                             format!("invalid PID: {}", pid_str.quote()),
                         ));
                     }
+
                     settings.pid = pid;
                 }
                 Err(e) => {
@@ -243,7 +241,11 @@ impl Settings {
         self.inputs.iter().any(|input| input.is_stdin())
     }
 
-    pub fn check_warnings(&self) -> CheckResult {
+    pub fn num_inputs(&self) -> usize {
+        self.inputs.len()
+    }
+
+    pub fn check_warnings(&self) {
         if self.retry {
             if self.follow.is_none() {
                 show_warning!("--retry ignored; --retry is useful only when following");
@@ -257,11 +259,24 @@ impl Settings {
                 show_warning!("PID ignored; --pid=PID is useful only when following");
             } else if !platform::supports_pid_checks(self.pid) {
                 show_warning!("--pid=PID is not supported on this system");
-                return CheckResult::NoPidSupport;
             }
         }
 
-        CheckResult::Ok
+        if self.follow.is_some() && self.has_stdin() {
+            let blocking_stdin = self.pid == 0
+                && self.follow == Some(FollowMode::Descriptor)
+                && self.num_inputs() == 1
+                && Handle::stdin().map_or(false, |handle| {
+                    handle
+                        .as_file()
+                        .metadata()
+                        .map_or(false, |meta| !meta.is_file())
+                });
+
+            if !blocking_stdin && atty::is(Stream::Stdin) {
+                show_warning!("following standard input indefinitely is ineffective");
+            }
+        }
     }
 
     pub fn verify(&self) -> VerificationResult {

src/uu/tail/src/follow/watch.rs

@@ -107,6 +107,12 @@ impl Observer {
         files: FileHandling,
         pid: platform::Pid,
     ) -> Self {
+        let pid = if platform::supports_pid_checks(pid) {
+            pid
+        } else {
+            0
+        };
+
         Self {
             retry,
             follow,

src/uu/tail/src/tail.rs

@@ -42,12 +42,7 @@ use uucore::{show, show_error};
 pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     let settings = parse_args(args)?;
 
-    let mut observer = Observer::from(&settings);
-
-    match settings.check_warnings() {
-        args::CheckResult::NoPidSupport => observer.pid = 0,
-        args::CheckResult::Ok => {}
-    }
+    settings.check_warnings();
 
     match settings.verify() {
         args::VerificationResult::CannotFollowStdinByName => {
@@ -62,11 +57,12 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         args::VerificationResult::Ok => {}
     }
 
-    uu_tail(&settings, observer)
+    uu_tail(&settings)
 }
 
-fn uu_tail(settings: &Settings, mut observer: Observer) -> UResult<()> {
+fn uu_tail(settings: &Settings) -> UResult<()> {
     let mut printer = HeaderPrinter::new(settings.verbose, true);
+    let mut observer = Observer::from(settings);
 
     observer.start(settings)?;
     // Do an initial tail print of each path's content.

src/uu/tail/src/text.rs

@@ -19,3 +19,5 @@ pub static BACKEND: &str = "kqueue";
 #[cfg(target_os = "windows")]
 pub static BACKEND: &str = "ReadDirectoryChanges";
 pub static FD0: &str = "/dev/fd/0";
+pub static IS_A_DIRECTORY: &str = "Is a directory";
+pub static DEV_TTY: &str = "/dev/tty";