Don't call poll when keyseq_timeout is set to 0 to avoid broken poll implementation for devices on MacOS.

[PaulJuliusMartinez]

The poll system call is broken on MacOS and doesn't work properly for devices, such as /dev/tty.

As a result, reading single escapes is broken when using Behavior::PreferTerm, which causes rustyline to read from /dev/tty. After reading a single escape character, the subsequent call to poll will always return Ok(1), so rustyline will try to parse an escape sequence.

This PR makes a small change to at least improve the behavior when keyseq_timeout is set to Some(0). In this case, we bypass the call to poll altogether, and instead check whether there is any buffered input to determine whether a single escape keypress has been entered or if an escape sequence has been entered, which is a pretty effective heuristic.

I don't know if there's any good workaround for the broken poll implementation when keyseq_timeout is non-zero, but this is at least an improvement.

This should resolve #607.

[gwenn]

I manage to reproduce the issue.
Only with this patch to debug:

diff --git a/src/tty/unix.rs b/src/tty/unix.rs
index 08d7548..264fc3a 100644
--- a/src/tty/unix.rs
+++ b/src/tty/unix.rs
@@ -707,8 +707,10 @@ impl PosixRawReader {
         if n > 0 {
             return Ok(n as i32);
         }
+        debug!(target: "rustyline", "poll with: {:?}", timeout_ms);
         let mut fds = [poll::PollFd::new(self.as_fd(), PollFlags::POLLIN)];
         let r = poll::poll(&mut fds, timeout_ms);
+        debug!(target: "rustyline", "poll returns: {:?}", r);
         match r {
             Ok(n) => Ok(n),
             Err(Errno::EINTR) => {

And this example (timeout <> 0 + Behavior::PreferTerm):

use rustyline::{Config, DefaultEditor, Result};

fn main() -> Result<()> {
    env_logger::init();
    let config = Config::builder()
        .behavior(rustyline::Behavior::PreferTerm)
        .keyseq_timeout(Some(50))
        .build();
    let mut rl = DefaultEditor::with_config(config)?;

    let _ = rl.readline("> ")?;
    Ok(())
}

RUST_LOG=rustyline=debug cargo run --example issue 2> debug.log

gives

[DEBUG rustyline] poll with: PollTimeout(50)
[DEBUG rustyline] poll returns: Ok(1)

on MacOS as you say.
But we cannot rely on timeout.

No issue with Behavior::Stdio on MacOs or with Behavior::PreferTerm on Linux:

[DEBUG rustyline] poll with: PollTimeout(50)
[DEBUG rustyline] poll returns: Ok(0)
[DEBUG rustyline] c: '\u{1b}' => key: KeyEvent(Esc, Modifiers(0x0))
[DEBUG rustyline] Emacs command: Abort

[gwenn]

See #803