netwatch/backend/internal/config/config.go

// Package config loads application configuration from
// environment variables, .env files, and config files.
package config

import (
	"errors"
	"fmt"
	"log/slog"
	"math"
	"net/netip"
	"net/url"
	"strconv"
	"strings"

	"sneak.berlin/go/netwatch/internal/globals"
	"sneak.berlin/go/netwatch/internal/logger"

	_ "github.com/joho/godotenv/autoload" // loads .env file
	"github.com/spf13/viper"
	"go.uber.org/fx"
)

// defaultTrustedProxies lists the networks whose forwarded
// headers are honoured by default: IPv4 and IPv6 loopback,
// for a reverse proxy on the same host, and the RFC1918
// ranges. The container image does not use it:
// bin/entrypoint.sh gives the server 127.0.0.1/32, since
// nginx is its only client there.
const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"

// Default limits on stored reports; backend/README.md gives the
// reasons for these values.
const (
	defaultReportsPerMinute = 60
	defaultDataDirMaxBytes  = 1 << 30 // 1 GiB
)

var (
	errNotPositive = errors.New("must be a positive whole number")
	errNotOrigin   = errors.New(
		"must be an origin, scheme://host with an optional port",
	)
	errNotPort = errors.New("must be a port number, 1 to 65535")
	errNotBool = errors.New("must be true or false")
	errNotIP   = errors.New("must be an IP address, or empty")

	errMetricsCredentials = errors.New(
		"METRICS_USERNAME and METRICS_PASSWORD must be set together, " +
			"or neither",
	)
	errMetricsUsernameColon = errors.New(
		"METRICS_USERNAME must not contain \":\", " +
			"which basic auth cannot carry in a user name",
	)
)

// Params defines the dependencies for Config.
type Params struct {
	fx.In

	Globals *globals.Globals
	Logger  *logger.Logger
}

// Config holds the resolved application configuration.
type Config struct {
	BindAddress        string
	CORSAllowedOrigins []string
	DataDir            string
	DataDirMaxBytes    int64
	Debug              bool
	MetricsPassword    string
	MetricsUsername    string
	Port               int
	ReportsPerMinute   int
	SentryDSN          string
	TrustedProxies     []string
	log                *slog.Logger
	params             *Params
}

// New loads configuration from env, .env files, and config
// files, returning a fully resolved Config. It fails, with an error
// naming the setting, on a value the server cannot use, and on a
// config file it finds but cannot read.
func New(
	_ fx.Lifecycle,
	params Params,
) (*Config, error) {
	log := params.Logger.Get()
	name := params.Globals.Appname

	viper.SetConfigName(name)
	viper.SetConfigType("yaml")
	viper.AddConfigPath("/etc/" + name)
	viper.AddConfigPath("$HOME/.config/" + name)

	viper.AutomaticEnv()

	// An empty CORS_ALLOWED_ORIGINS allows no other origin.
	viper.SetDefault("CORS_ALLOWED_ORIGINS", "")
	viper.SetDefault("DATA_DIR", "./data/reports")
	viper.SetDefault("DATA_DIR_MAX_BYTES", defaultDataDirMaxBytes)
	viper.SetDefault("DEBUG", "false")
	// An empty BIND_ADDRESS listens on every interface.
	viper.SetDefault("BIND_ADDRESS", "")
	viper.SetDefault("PORT", "8080")
	viper.SetDefault("REPORTS_PER_MINUTE", defaultReportsPerMinute)
	viper.SetDefault("SENTRY_DSN", "")
	viper.SetDefault("METRICS_USERNAME", "")
	viper.SetDefault("METRICS_PASSWORD", "")
	viper.SetDefault("TRUSTED_PROXIES", defaultTrustedProxies)

	err := viper.ReadInConfig()
	if err != nil {
		var notFound viper.ConfigFileNotFoundError
		if !errors.As(err, &notFound) {
			return nil, fmt.Errorf("config file %s: %w",
				viper.ConfigFileUsed(), err)
		}
	}

	// Read with strconv: viper's GetInt and GetBool would read a value
	// they cannot parse as 0 or false instead of failing.
	port, err := strconv.Atoi(viper.GetString("PORT"))
	if err != nil || port < 1 || port > math.MaxUint16 {
		return nil, fmt.Errorf("PORT %q: %w",
			viper.GetString("PORT"), errNotPort)
	}

	debug, err := strconv.ParseBool(viper.GetString("DEBUG"))
	if err != nil {
		return nil, fmt.Errorf("DEBUG %q: %w",
			viper.GetString("DEBUG"), errNotBool)
	}

	s := &Config{
		BindAddress:        viper.GetString("BIND_ADDRESS"),
		CORSAllowedOrigins: splitList(viper.GetString("CORS_ALLOWED_ORIGINS")),
		DataDir:            viper.GetString("DATA_DIR"),
		DataDirMaxBytes:    viper.GetInt64("DATA_DIR_MAX_BYTES"),
		Debug:              debug,
		MetricsPassword:    viper.GetString("METRICS_PASSWORD"),
		MetricsUsername:    viper.GetString("METRICS_USERNAME"),
		Port:               port,
		ReportsPerMinute:   viper.GetInt("REPORTS_PER_MINUTE"),
		SentryDSN:          viper.GetString("SENTRY_DSN"),
		TrustedProxies:     splitList(viper.GetString("TRUSTED_PROXIES")),
		log:                log,
		params:             &params,
	}

	err = s.check()
	if err != nil {
		return nil, err
	}

	if s.Debug {
		params.Logger.EnableDebugLogging()
		s.log = params.Logger.Get()
	}

	return s, nil
}

// check fails with an error naming the first setting here whose value
// the server cannot use. New checks PORT and DEBUG as it reads them,
// and the middleware checks TRUSTED_PROXIES as it parses it.
func (s *Config) check() error {
	// viper reads a value that is not a number as 0, so this also
	// catches a mistyped setting.
	if s.ReportsPerMinute <= 0 {
		return fmt.Errorf("REPORTS_PER_MINUTE %q: %w",
			viper.GetString("REPORTS_PER_MINUTE"), errNotPositive)
	}

	if s.DataDirMaxBytes <= 0 {
		return fmt.Errorf("DATA_DIR_MAX_BYTES %q: %w",
			viper.GetString("DATA_DIR_MAX_BYTES"), errNotPositive)
	}

	if s.BindAddress != "" {
		_, err := netip.ParseAddr(s.BindAddress)
		if err != nil {
			return fmt.Errorf("BIND_ADDRESS %q: %w", s.BindAddress, errNotIP)
		}
	}

	// The server records and serves metrics only with both set, so
	// one alone is a mistake that would otherwise go unnoticed.
	if (s.MetricsUsername == "") != (s.MetricsPassword == "") {
		return errMetricsCredentials
	}

	// Basic auth splits the credentials at the first ":", so with one
	// in the user name every request to /metrics would get 401.
	if strings.Contains(s.MetricsUsername, ":") {
		return errMetricsUsernameColon
	}

	return checkOrigins(s.CORSAllowedOrigins)
}

// checkOrigins fails on the first CORS_ALLOWED_ORIGINS entry that is
// not a plain origin, scheme://host with an optional port, as browsers
// send it; anything more, such as a trailing "/", would match no page.
// go-chi/cors reads a "*" anywhere in an entry as a wildcard, so no
// entry may contain one.
func checkOrigins(origins []string) error {
	for _, origin := range origins {
		u, err := url.Parse(origin)
		if err != nil || u.Scheme == "" || u.Host == "" ||
			strings.Contains(origin, "*") ||
			origin != u.Scheme+"://"+u.Host {
			return fmt.Errorf("CORS_ALLOWED_ORIGINS %q: %w",
				origin, errNotOrigin)
		}
	}

	return nil
}

// splitList turns a comma-separated setting into a trimmed
// slice, dropping empty entries.
func splitList(raw string) []string {
	parts := strings.Split(raw, ",")

	out := make([]string, 0, len(parts))
	for _, p := range parts {
		p = strings.TrimSpace(p)
		if p != "" {
			out = append(out, p)
		}
	}

	return out
}