From ddcb8fc600c0fad3d57f0fc008925181e2aa5339 Mon Sep 17 00:00:00 2001
From: hermes <hermes@obahan.xyz>
Date: Wed, 30 Sep 2026 21:59:32 +0200
Subject: [PATCH] add internal/infra/mqtt.go

---
 internal/infra/mqtt.go | 122 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 122 insertions(+)
 create mode 100644 internal/infra/mqtt.go

diff --git a/internal/infra/mqtt.go b/internal/infra/mqtt.go
new file mode 100644
index 0000000..9c8868b
--- /dev/null
+++ b/internal/infra/mqtt.go
@@ -0,0 +1,122 @@
+package infra
+
+import (
+	"bytes"
+	"fmt"
+	"net"
+	"time"
+)
+
+// Minimal MQTT 3.1.1 client (QoS 0 only) — enough for now-playing
+// announcements without pulling in a third-party dependency.
+//
+// Wire format references: OASIS MQTT v3.1.1, sections 2-3.
+// All multi-byte integers are big-endian; strings are 2-byte length prefixed.
+
+// mqttEncodeRemainingLength encodes the variable-length integer used in every
+// MQTT fixed header (7 bits per byte, high bit = "more bytes follow").
+func mqttEncodeRemainingLength(n int) []byte {
+	var out []byte
+	for {
+		b := byte(n % 128)
+		n /= 128
+		if n > 0 {
+			b |= 0x80
+		}
+		out = append(out, b)
+		if n == 0 {
+			return out
+		}
+	}
+}
+
+// mqttStr encodes a UTF-8 string field (2-byte length prefix + bytes).
+func mqttStr(s string) []byte {
+	return append([]byte{byte(len(s) >> 8), byte(len(s))}, s...)
+}
+
+// mqttConnect builds a CONNECT packet (protocol 3.1.1, clean session,
+// 30s keepalive). Username/password fields are included only when non-empty.
+func mqttConnect(clientID, user, pass string) []byte {
+	var v bytes.Buffer
+	v.Write(mqttStr("MQTT"))  // protocol name
+	v.WriteByte(4)            // protocol level 3.1.1
+	flags := byte(0x02)       // clean session
+	if user != "" {
+		flags |= 0x80
+	}
+	if pass != "" {
+		flags |= 0x40
+	}
+	v.WriteByte(flags)
+	v.Write([]byte{0x00, 30}) // keepalive 30s
+	v.Write(mqttStr(clientID))
+	if user != "" {
+		v.Write(mqttStr(user))
+	}
+	if pass != "" {
+		v.Write(mqttStr(pass))
+	}
+	body := v.Bytes()
+	return append([]byte{0x10}, append(mqttEncodeRemainingLength(len(body)), body...)...)
+}
+
+// mqttPublish builds a QoS 0 PUBLISH packet (no packet id on the wire).
+func mqttPublish(topic string, payload []byte) []byte {
+	var v bytes.Buffer
+	v.Write(mqttStr(topic))
+	v.Write(payload)
+	body := v.Bytes()
+	return append([]byte{0x30}, append(mqttEncodeRemainingLength(len(body)), body...)...)
+}
+
+// MQTTNowPlaying publishes now-playing JSON to a broker, one short-lived
+// connection per publish (fire-and-forget, QoS 0). Safe to call every song.
+type MQTTNowPlaying struct {
+	Addr     string // host:port
+	User     string
+	Pass     string
+	ClientID string
+	Topic    string // e.g. "airstudio/nowplaying"
+}
+
+// Publish sends payload to the configured topic. Returns a descriptive error
+// on any network failure; callers should log and continue — the stream must
+// never stop because MQTT is down.
+func (m *MQTTNowPlaying) Publish(payload []byte) error {
+	conn, err := net.DialTimeout("tcp", m.Addr, 5*time.Second)
+	if err != nil {
+		return fmt.Errorf("mqtt dial %s: %w", m.Addr, err)
+	}
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Second))
+
+	if _, err := conn.Write(mqttConnect(m.ClientID, m.User, m.Pass)); err != nil {
+		return fmt.Errorf("mqtt connect: %w", err)
+	}
+	// read CONNACK (4 bytes); verify return code 0
+	ack := make([]byte, 4)
+	if _, err := readFull(conn, ack); err != nil {
+		return fmt.Errorf("mqtt connack: %w", err)
+	}
+	if ack[0] != 0x20 || ack[3] != 0x00 {
+		return fmt.Errorf("mqtt broker refused, code %d", ack[3])
+	}
+	if _, err := conn.Write(mqttPublish(m.Topic, payload)); err != nil {
+		return fmt.Errorf("mqtt publish: %w", err)
+	}
+	return nil
+}
+
+// readFull fills buf exactly or errors.
+func readFull(r net.Conn, buf []byte) (int, error) {
+	got := 0
+	for got < len(buf) {
+		n, err := r.Read(buf[got:])
+		got += n
+		if err != nil {
+			return got, err
+		}
+	}
+	return got, nil
+}