feat(cli): return immediately when stdin is empty

With this change, calling `bitcli` (or `bitcli shorten`) without any
arguments will no longer wait for the standard input indefinitely if
it's empty and its handle does not refer to a terminal/tty.

src/io.rs

@@ -1,4 +1,6 @@
 use std::error::Error;
+use std::io::IsTerminal as _;
+use std::os::fd::AsFd as _;
 use std::str::FromStr;
 
 use async_stream::try_stream;
@@ -7,13 +9,22 @@ use tokio::io::{self, AsyncBufReadExt as _, BufReader};
 
 use crate::error::Result;
 
-pub fn read_input<T>() -> impl TryStream<Item = Result<T>>
+/// Read standard input as a [`TryStream`] of parsed lines of type `T`.
+///
+/// Returns `None` if the stdin handle does not refer to a terminal/tty.
+pub fn read_input<T>() -> Option<impl TryStream<Item = Result<T>>>
 where
     T: FromStr + 'static,
     <T as FromStr>::Err: Error + Send + Sync,
 {
-    try_stream! {
-        let mut stdin = BufReader::new(io::stdin());
+    let stdin = io::stdin();
+
+    if stdin.as_fd().is_terminal() {
+        return None;
+    }
+
+    Some(try_stream! {
+        let mut stdin = BufReader::new(stdin);
         let mut buf = String::new();
 
         loop {
@@ -27,7 +38,7 @@ where
 
             buf.clear();
         }
-    }
+    })
 }
 
 #[inline]

src/main.rs

@@ -39,9 +39,10 @@ async fn main() {
     match cmd {
         Command::Shorten(args) => {
             let urls = if args.urls.is_empty() {
-                io::read_input::<url::Url>()
-                    .map(|url| crash_if_err!(url))
-                    .boxed()
+                let Some(urls) = io::read_input::<url::Url>() else {
+                    return;
+                };
+                urls.map(|url| crash_if_err!(url)).boxed()
             } else {
                 stream::iter(args.urls).boxed()
             };