feat: config and config loader implemented

internal/config/config.go

@@ -0,0 +1,138 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+	"net/url"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/knadh/koanf/parsers/toml/v2"
+	"github.com/knadh/koanf/providers/confmap"
+	"github.com/knadh/koanf/providers/file"
+	"github.com/knadh/koanf/v2"
+)
+
+type Config struct {
+	MQTT struct {
+		URL      string `koanf:"url"`
+		ClientID string `koanf:"client_id"`
+		Username string `koanf:"username"`
+		Password string `koanf:"password"`
+		DeviceID string `koanf:"device_id"`
+	} `koanf:"mqtt"`
+	App struct {
+		ConnectTimeout string `koanf:"connect_timeout"`
+	} `koanf:"app"`
+}
+
+// Keep duration input as text; validate/parse it explicitly after merging.
+func (c Config) ConnectTimeout() time.Duration {
+	d, _ := time.ParseDuration(c.App.ConnectTimeout) // Load already validated it.
+	return d
+}
+
+func Load(args []string) (Config, error) {
+	var result Config
+	flags := flag.NewFlagSet("mqtt5-learning", flag.ContinueOnError)
+	filename := flags.String("config", "configs/lab.toml", "TOML file")
+	broker := flags.String("broker", "", "broker URL override")
+	clientID := flags.String("client-id", "", "client ID override")
+	username := flags.String("username", "", "lab username override")
+	deviceID := flags.String("device-id", "", "device identity override")
+	timeout := flags.String("connect-timeout", "", "startup deadline override")
+	if err := flags.Parse(args); err != nil {
+		return result, err
+	}
+	if flags.NArg() != 0 {
+		return result, fmt.Errorf("unexpected positional arguments")
+	}
+
+	k := koanf.New(".")
+	defaults := map[string]any{
+		"mqtt.url":       "mqtt://127.0.0.1:18883",
+		"mqtt.client_id": "lab-backend",
+		"mqtt.username":  "", "mqtt.password": "",
+		"mqtt.device_id":      "device-001",
+		"app.connect_timeout": "10s",
+	}
+	if err := k.Load(confmap.Provider(defaults, "."), nil); err != nil {
+		return result, fmt.Errorf("defaults: %w", err)
+	}
+	// A requested/missing config file is an error, not a silent fallback.
+	if err := k.Load(file.Provider(*filename), toml.Parser()); err != nil {
+		return result, fmt.Errorf("TOML: %w", err)
+	}
+
+	// Explicit mapping preserves underscores inside names such as client_id.
+	mapping := map[string]string{
+		"TEST_MQTT5_MQTT_URL":            "mqtt.url",
+		"TEST_MQTT5_MQTT_CLIENT_ID":      "mqtt.client_id",
+		"TEST_MQTT5_MQTT_USERNAME":       "mqtt.username",
+		"TEST_MQTT5_MQTT_PASSWORD":       "mqtt.password",
+		"TEST_MQTT5_MQTT_DEVICE_ID":      "mqtt.device_id",
+		"TEST_MQTT5_APP_CONNECT_TIMEOUT": "app.connect_timeout",
+	}
+	for name, key := range mapping {
+		if value, exists := os.LookupEnv(name); exists {
+			if err := k.Set(key, value); err != nil {
+				return result, err
+			}
+		}
+	}
+	overrides := map[string]string{}
+	flags.Visit(func(f *flag.Flag) {
+		switch f.Name {
+		case "broker":
+			overrides["mqtt.url"] = *broker
+		case "client-id":
+			overrides["mqtt.client_id"] = *clientID
+		case "username":
+			overrides["mqtt.username"] = *username
+		case "device-id":
+			overrides["mqtt.device_id"] = *deviceID
+		case "connect-timeout":
+			overrides["app.connect_timeout"] = *timeout
+		}
+	})
+	for key, value := range overrides {
+		if err := k.Set(key, value); err != nil {
+			return result, err
+		}
+	}
+	if err := k.Unmarshal("", &result); err != nil {
+		return result, fmt.Errorf("decode configuration: %w", err)
+	}
+	if err := result.Validate(); err != nil {
+		return result, err
+	}
+	return result, nil
+}
+
+func (c Config) Validate() error {
+	u, err := url.Parse(c.MQTT.URL)
+	if err != nil || u == nil || u.Hostname() == "" || u.Port() == "" {
+		return fmt.Errorf("mqtt.url must contain a valid host and explicit port")
+	}
+	// Early lab only. TLS configuration is deliberately introduced later.
+	if u.Scheme != "mqtt" || u.User != nil || u.RawQuery != "" || u.Fragment != "" || u.Path != "" {
+		return fmt.Errorf("early lab requires mqtt://host:port without credentials or extra URL components")
+	}
+	if u.Hostname() != "127.0.0.1" && u.Hostname() != "localhost" {
+		return fmt.Errorf("plaintext learning broker must be on loopback")
+	}
+	for _, value := range []string{c.MQTT.ClientID, c.MQTT.Username, c.MQTT.Password} {
+		if strings.TrimSpace(value) == "" {
+			return fmt.Errorf("client ID and credentials are required")
+		}
+	}
+	if c.MQTT.DeviceID == "" || strings.ContainsAny(c.MQTT.DeviceID, "/+# \t\r\n") {
+		return fmt.Errorf("device ID must be a nonempty single topic segment")
+	}
+	d, err := time.ParseDuration(c.App.ConnectTimeout)
+	if err != nil || d <= 0 || d > time.Minute {
+		return fmt.Errorf("connect timeout must be a duration greater than zero and at most 1m")
+	}
+	return nil
+}