Terminate REPL if not connected to tty input (#6480)

* Terminate REPL if not connected to tty input

If the standard input stream is not a TTY abort the REPL execution.

Solves a problem as the current REPL tries to be IO fault tolerant and
would indefinetely fail when crossterm tries to handle the STDIN.

Fixes nushell/nushell#6452

* Improve the error message

Cargo.lock

@@ -2630,6 +2630,7 @@ dependencies = [
 name = "nu-cli"
 version = "0.67.1"
 dependencies = [
+ "atty",
  "chrono",
  "crossterm 0.24.0",
  "fancy-regex",

crates/nu-cli/Cargo.toml

@@ -22,6 +22,7 @@ nu-ansi-term = "0.46.0"
 nu-color-config = { path = "../nu-color-config", version = "0.67.1"  }
 reedline = { version = "0.10.0", features = ["bashisms", "sqlite"]}
 
+atty = "0.2.14"
 chrono = "0.4.21"
 crossterm = "0.24.0"
 fancy-regex = "0.10.0"

crates/nu-cli/src/repl.rs

@@ -43,6 +43,16 @@ pub fn evaluate_repl(
 ) -> Result<()> {
     use reedline::{FileBackedHistory, Reedline, Signal};
 
+    // Guard against invocation without a connected terminal.
+    // reedline / crossterm event polling will fail without a connected tty
+    if !atty::is(atty::Stream::Stdin) {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::NotFound,
+            "Nushell launched as interactive REPL but STDIN is not a TTY, either launch in a valid terminal or provide arguments to invoke a script!",
+        ))
+        .into_diagnostic();
+    }
+
     let mut entry_num = 0;
 
     let mut nu_prompt = NushellPrompt::new();
@@ -498,6 +508,10 @@ pub fn evaluate_repl(
                 let message = err.to_string();
                 if !message.contains("duration") {
                     println!("Error: {:?}", err);
+                    // TODO: Identify possible error cases where a hard failure is preferable
+                    // Ignoring and reporting could hide bigger problems
+                    // e.g. https://github.com/nushell/nushell/issues/6452
+                    // Alternatively only allow that expected failures let the REPL loop
                 }
                 if shell_integration {
                     run_ansi_sequence(&get_command_finished_marker(stack, engine_state))?;