From ff38fe2a780f46777bf5ed91729ec99d01e88918 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Polomsk=C3=BD?=
 <polomsky@users.noreply.github.com>
Date: Sat, 9 May 2020 08:34:28 +0200
Subject: [PATCH] add support for drain

Tested on Mac OS, Linux and Windows with UART emulated on USB.

I wrote general BSD code, because it should work, but I am not able to test
it.

On Mac OS I am quite sure that drain behaves as expected because it fixed
an issue with buffer overflow (which OS does not report but fails).

For Windows it seems that drain is actually part of `Write`, because
`Write` was long and `Drain` was fast. But I have not found any mention in
Win32 docs about buffering and asynchronous write, so I put there flushing
code to be sure, and this code did not break anything.

For Linux `Drain` is also taking more time than writing so it looks working
too.
---
 serial.go         |  3 +++
 serial_bsd.go     | 10 ++++++++++
 serial_linux.go   | 10 ++++++++++
 serial_windows.go |  4 ++++
 4 files changed, 27 insertions(+)
 create mode 100644 serial_bsd.go

diff --git a/serial.go b/serial.go
index 2bb7f3e..4420100 100644
--- a/serial.go
+++ b/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
 
diff --git a/serial_bsd.go b/serial_bsd.go
new file mode 100644
index 0000000..4db5b3d
--- /dev/null
+++ b/serial_bsd.go
@@ -0,0 +1,10 @@
+//go:build darwin || dragonfly || freebsd || netbsd || openbsd
+// +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)
+}
diff --git a/serial_linux.go b/serial_linux.go
index 116474d..0ac0f39 100644
--- a/serial_linux.go
+++ b/serial_linux.go
@@ -85,3 +85,13 @@ func setTermSettingsBaudrate(speed int, settings *unix.Termios) (error, bool) {
 	settings.Ospeed = toTermiosSpeedType(baudrate)
 	return nil, false
 }
+
+func (port *unixPort) Drain() error {
+	// simulate drain with change settings with TCSETSW
+	settings, err := port.getTermSettings()
+	if err != nil {
+		return err
+	}
+
+	return unix.IoctlSetTermios(port.handle, unix.TCSETSW, settings)
+}
diff --git a/serial_windows.go b/serial_windows.go
index e393d4e..5c4a576 100644
--- a/serial_windows.go
+++ b/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