Add support for Ctrl-C aborting the current command on the CLI (#528) r=rnewman

tools/cli/src/mentat_cli/input.rs

@@ -14,6 +14,7 @@ use linefeed::{
     DefaultTerminal,
     Reader,
     ReadResult,
+    Signal,
 };
 
 use self::InputResult::*;
@@ -51,11 +52,26 @@ pub struct InputReader {
     in_process_cmd: Option<Command>,
 }
 
+enum UserAction {
+    // We've received some text that we should interpret as a new command, or
+    // as part of the current command.
+    TextInput(String),
+    // We were interrupted, if we have a current command we should clear it,
+    // otherwise we should exit. Currently can only be generated by reading from
+    // a terminal (and not by reading from stdin).
+    Interrupt,
+    // We hit the end of the file, there was an error getting user input, or
+    // something else happened that means we should exit.
+    Quit,
+}
+
 impl InputReader {
     /// Constructs a new `InputReader` reading from `stdin`.
     pub fn new() -> InputReader {
         let r = match Reader::new("mentat") {
             Ok(mut r) => {
+                // Handle SIGINT (Ctrl-C)
+                r.set_report_signal(Signal::Interrupt, true);
                 r.set_word_break_chars(" \t\n!\"#$%&'(){}*+,-./:;<=>?@[\\]^`");
                 Some(r)
             },
@@ -80,8 +96,16 @@ impl InputReader {
     pub fn read_input(&mut self) -> Result<InputResult, cli::Error> {
         let prompt = if self.in_process_cmd.is_some() { MORE_PROMPT } else { DEFAULT_PROMPT };
         let line = match self.read_line(prompt) {
-            Some(s) => s,
-            None => return Ok(Eof),
+            UserAction::TextInput(s) => s,
+            UserAction::Interrupt if self.in_process_cmd.is_some() => {
+                self.in_process_cmd = None;
+                self.buffer.clear();
+                // Move to the next line, so that our next prompt isn't on top
+                // of the previous.
+                println!();
+                String::new()
+            },
+            _ => return Ok(Eof),
         };
 
         if !self.buffer.is_empty() {
@@ -142,14 +166,16 @@ impl InputReader {
         }
     }
 
-    fn read_line(&mut self, prompt: &str) -> Option<String> {
+    fn read_line(&mut self, prompt: &str) -> UserAction {
         match self.reader {
             Some(ref mut r) => {
                 r.set_prompt(prompt);
-                r.read_line().ok().and_then(|line|
+                r.read_line().ok().map_or(UserAction::Quit, |line|
                     match line {
-                        ReadResult::Input(s) => Some(s),
-                        _ => None
+                        ReadResult::Input(s) => UserAction::TextInput(s),
+                        ReadResult::Signal(Signal::Interrupt) =>
+                            UserAction::Interrupt,
+                        _ => UserAction::Quit,
                     })
 
             },
@@ -157,12 +183,12 @@ impl InputReader {
         }
     }
 
-    fn read_stdin(&self) -> Option<String> {
+    fn read_stdin(&self) -> UserAction {
         let mut s = String::new();
 
         match stdin().read_line(&mut s) {
-            Ok(0) | Err(_) => None,
-            Ok(_) => Some(s)
+            Ok(0) | Err(_) => UserAction::Quit,
+            Ok(_) => UserAction::TextInput(s)
         }
     }