Feature/drain support

serial.go

@@ -26,6 +26,9 @@ type Port interface {
 	// Returns the number of bytes written.
 	Write(p []byte) (n int, err error)
 
+	// Wait until all data in the buffer are sent
+	Drain() error
+
 	// ResetInputBuffer Purges port read buffer
 	ResetInputBuffer() error
 

serial_bsd.go

@@ -0,0 +1,9 @@
+//go:build darwin || dragonfly || freebsd || netbsd || openbsd
+
+package serial
+
+import "golang.org/x/sys/unix"
+
+func (port *unixPort) Drain() error {
+	return unix.IoctlSetInt(port.handle, unix.TIOCDRAIN, 0)
+}

serial_linux.go

@@ -85,3 +85,10 @@ func setTermSettingsBaudrate(speed int, settings *unix.Termios) (error, bool) {
 	settings.Ospeed = toTermiosSpeedType(baudrate)
 	return nil, false
 }
+
+func (port *unixPort) Drain() error {
+	// It's not super well documented, but this is the same as calling tcdrain:
+	// - https://git.musl-libc.org/cgit/musl/tree/src/termios/tcdrain.c
+	// - https://elixir.bootlin.com/linux/v6.2.8/source/drivers/tty/tty_io.c#L2673
+	return unix.IoctlSetInt(port.handle, unix.TCSBRK, 1)
+}

serial_windows.go

@@ -119,6 +119,10 @@ func (port *windowsPort) Write(p []byte) (int, error) {
 	return int(writed), err
 }
 
+func (port *windowsPort) Drain() (err error) {
+	return syscall.FlushFileBuffers(port.handle)
+}
+
 const (
 	purgeRxAbort uint32 = 0x0002
 	purgeRxClear        = 0x0008