monitor: refactor for architecture-specific connection implementations

Signed-off-by: wardn <wardn@users.noreply.github.com>

wgengine/monitor/monitor_freebsd.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package monitor
+
+import (
+	"bufio"
+	"fmt"
+	"net"
+	"strings"
+)
+
+type devdConn struct {
+	conn net.Conn
+}
+
+func NewConn() (Conn, error) {
+	conn, err := net.Dial("unixpacket", "/var/run/devd.seqpacket.pipe")
+	if err != nil {
+		return nil, fmt.Errorf("devd dial error: %v", err)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("dialing devd socket: %v", err)
+	}
+	return &devdConn{conn}, nil
+}
+
+func (c *devdConn) Close() error {
+	return c.conn.Close()
+}
+
+func (c *devdConn) Receive() (Message, error) {
+	for {
+		msg, err := bufio.NewReader(c.conn).ReadString('\n')
+		if err != nil {
+			return nil, fmt.Errorf("reading devd socket: %v", err)
+		}
+		// Only return messages related to the network subsystem.
+		if !strings.Contains(msg, "system=IFNET") {
+			continue
+		}
+		// TODO(]|[): this is where the devd-specific message would
+		// get converted into a "standard" event message and returned.
+		return nil, nil
+	}
+}