Fix hang on TTY read

In some cases, when a underlying error happens to a TTY port between poll::wait_read_fd and unistd::read, the read function would hang waiting for some data that is never received.

This commit sets the port to non-canonical mode, with VMIN = VTIME = 0. With this change, it has the effect of making reads non-blocking, returning right away.

The timeout behaviour is maintained, as prior to reading we call unix::poll through poll::wait_read_fd.

Fixes: serialport#7

src/posix/termios.rs

@@ -5,6 +5,7 @@ use crate::{DataBits, FlowControl, Parity, Result, StopBits};
 use nix::libc;
 
 use std::os::unix::prelude::*;
+use std::time::Duration;
 
 cfg_if! {
     if #[cfg(any(
@@ -183,6 +184,12 @@ pub(crate) fn set_flow_control(termios: &mut Termios, flow_control: FlowControl)
     };
 }
 
+pub(crate) fn set_timeout(termios: &mut Termios, timeout: Duration) {
+    let timeout = u128::min(timeout.as_millis() / 100, u8::MAX as u128) as u8;
+    termios.c_cc[libc::VMIN as usize] = 0;
+    termios.c_cc[libc::VTIME as usize] = timeout;
+}
+
 pub(crate) fn set_data_bits(termios: &mut Termios, data_bits: DataBits) {
     let size = match data_bits {
         DataBits::Five => libc::CS5,

src/posix/tty.rs

@@ -134,7 +134,9 @@ impl TTYPort {
         nix::errno::Errno::result(unsafe { tcgetattr(fd.0, termios.as_mut_ptr()) })?;
         let mut termios = unsafe { termios.assume_init() };
 
-        // setup TTY for binary serial port access
+        // Setup TTY for binary serial port access
+        // Enable non-canonical mode
+        termios.c_lflag &= !libc::ICANON;
         // Enable reading from the port and ignore all modem control lines
         termios.c_cflag |= libc::CREAD | libc::CLOCAL;
         // Enable raw mode which disables any implicit processing of the input or output data streams
@@ -175,6 +177,8 @@ impl TTYPort {
         termios::set_flow_control(&mut termios, builder.flow_control);
         termios::set_data_bits(&mut termios, builder.data_bits);
         termios::set_stop_bits(&mut termios, builder.stop_bits);
+        // Set termios read to non-blocking, as we handle blocking ourselves (with unix::poll)
+        termios::set_timeout(&mut termios, Duration::from_millis(0));
         #[cfg(not(any(target_os = "ios", target_os = "macos")))]
         termios::set_baud_rate(&mut termios, builder.baud_rate);
         #[cfg(any(target_os = "ios", target_os = "macos"))]