examples: add TCP echo server using os.notify (#10481)

examples/tcp_notify_echo_server.v

@@ -0,0 +1,73 @@
+import os
+import os.notify
+import net
+import time
+
+// This example demonstrates a single threaded TCP server using os.notify to be
+// notified of events on file descriptors. You can connect to the server using
+// netcat in separate shells, for example: `nc localhost 9001`
+
+fn main() {
+	$if !linux {
+		eprintln('This example only works on Linux')
+		exit(1)
+	}
+
+	// create TCP listener
+	mut listener := net.listen_tcp(.ip, 'localhost:9001') ?
+	defer {
+		listener.close() or {}
+	}
+	addr := listener.addr() ?
+	eprintln('Listening on $addr')
+	eprintln('Type `stop` to stop the server')
+
+	// create file descriptor notifier
+	mut notifier := notify.new() ?
+	defer {
+		notifier.close() or {}
+	}
+	notifier.add(os.stdin().fd, .read) ?
+	notifier.add(listener.sock.handle, .read) ?
+
+	for {
+		for event in notifier.wait(time.infinite) {
+			match event.fd {
+				listener.sock.handle {
+					// someone is trying to connect
+					eprint('trying to connect.. ')
+					if conn := listener.accept() {
+						if _ := notifier.add(conn.sock.handle, .read | .peer_hangup) {
+							eprintln('connected')
+						} else {
+							eprintln('error adding to notifier: $err')
+						}
+					} else {
+						eprintln('unable to accept: $err')
+					}
+				}
+				0 {
+					// stdin
+					s, _ := os.fd_read(event.fd, 10)
+					if s == 'stop\n' {
+						eprintln('stopping')
+						return
+					}
+				}
+				else {
+					// remote connection
+					if event.kind.has(.peer_hangup) {
+						if _ := notifier.remove(event.fd) {
+							eprintln('remote disconnected')
+						} else {
+							eprintln('error removing from notifier: $err')
+						}
+					} else {
+						s, _ := os.fd_read(event.fd, 10)
+						os.fd_write(event.fd, s)
+					}
+				}
+			}
+		}
+	}
+}