skip attempts to set baud rate of 0 via ioctl

On macOS, attempts to set the baud rate of pseudo terminals via normal
ioctl apis appears to be problematic. To support users targeting these
pseudo terminals, this commit adds the ability to use a `baud_rate` of 0
to skip the attempt altogether.

src/posix/termios.rs

@@ -100,7 +100,13 @@ pub(crate) fn get_termios(fd: RawFd) -> Result<Termios> {
 pub(crate) fn set_termios(fd: RawFd, termios: &libc::termios, baud_rate: u32) -> Result<()> {
     let res = unsafe { libc::tcsetattr(fd, libc::TCSANOW, termios) };
     nix::errno::Errno::result(res)?;
-    crate::posix::ioctl::iossiospeed(fd, &(baud_rate as libc::speed_t))?;
+
+    // Note: attempting to set the baud rate on a pseudo terminal via this ioctl call will faill
+    // with the `ENOTTY` error.
+    if baud_rate > 0 {
+        crate::posix::ioctl::iossiospeed(fd, &(baud_rate as libc::speed_t))?;
+    }
+
     Ok(())
 }
 

src/posix/tty.rs

@@ -37,6 +37,21 @@ fn close(fd: RawFd) {
 /// should not be instantiated directly by using `TTYPort::open()`, instead use
 /// the cross-platform `serialport::open()` or
 /// `serialport::open_with_settings()`.
+///
+/// Note: on macOS, when connecting to a pseudo-terminal (`pty` opened via
+/// `posix_openpt`), the `baud_rate` should be set to 0; this will be used to
+/// explicitly _skip_ an attempt to set the baud rate of the file descriptor
+/// that would otherwise happen via an `ioctl` command.
+///
+/// ```
+/// use serialport::{TTYPort, SerialPort};
+///
+/// let (mut master, slave) = TTYPort::pair().expect("Unable to create ptty pair");
+///
+/// // ... elsewhere
+///
+/// let mut port = TTYPort::open(&serialport::new(slave.name().unwrap(), 0)).expect("unable to open");
+/// ```
 #[derive(Debug)]
 pub struct TTYPort {
     fd: RawFd,

tests/test_tty.rs

@@ -90,6 +90,29 @@ fn test_ttyport_timeout() {
     }
 }
 
+#[test]
+#[cfg(any(target_os = "ios", target_os = "macos"))]
+fn test_osx_pty_pair() {
+    #![allow(unused_variables)]
+    let (mut master, slave) = TTYPort::pair().expect("Unable to create ptty pair");
+    let (output_sink, output_stream) = std::sync::mpsc::sync_channel(1);
+    let name = slave.name().unwrap();
+
+    master.write("12".as_bytes()).expect("");
+
+    let reader_thread = std::thread::spawn(move || {
+        let mut port = TTYPort::open(&serialport::new(&name, 0)).expect("unable to open");
+        let mut buffer = [0u8; 2];
+        let amount = port.read_exact(&mut buffer);
+        output_sink
+            .send(String::from_utf8(buffer.to_vec()).expect("buffer not read as valid utf-8"))
+            .expect("unable to send from thread");
+    });
+
+    reader_thread.join().expect("unable to join sink thread");
+    assert_eq!(output_stream.recv().unwrap(), "12");
+}
+
 // On Mac this should work (in fact used to in b77768a) but now fails. It's not functionality that
 // should be required, and the ptys work otherwise. So going to just diable this test instead.
 #[test]