dance-lessons-coach/pkg/config/config.go

package config

import (
	"context"
	"fmt"
	"os"
	"strings"
	"sync"
	"time"

	"github.com/fsnotify/fsnotify"
	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
	"github.com/spf13/viper"

	"dance-lessons-coach/pkg/version"
)

// SamplerReconfigureFunc is the signature for callbacks invoked when
// telemetry.sampler.type or telemetry.sampler.ratio change via hot-reload.
// The callback receives the new sampler type and ratio values.
// It must be safe to call concurrently — implementations should use their
// own synchronisation if needed. Returns an error if the reconfigure fails.
type SamplerReconfigureFunc func(ctx context.Context, samplerType string, samplerRatio float64) error

// NewZerologWriter creates a zerolog writer based on configuration
func NewZerologWriter() *os.File {
	return os.Stderr
}

// Config represents the application configuration
type Config struct {
	Server    ServerConfig    `mapstructure:"server"`
	Shutdown  ShutdownConfig  `mapstructure:"shutdown"`
	Logging   LoggingConfig   `mapstructure:"logging"`
	Telemetry TelemetryConfig `mapstructure:"telemetry"`
	API       APIConfig       `mapstructure:"api"`
	Auth      AuthConfig      `mapstructure:"auth"`
	Database  DatabaseConfig  `mapstructure:"database"`
	RateLimit RateLimitConfig `mapstructure:"rate_limit"`
	Cache     CacheConfig     `mapstructure:"cache"`

	// viper is the underlying configuration source. Kept (unexported,
	// mapstructure:"-") so hot-reload can re-unmarshal on file changes —
	// see WatchAndApply (ADR-0023 selective hot-reload).
	viper *viper.Viper `mapstructure:"-"`

	// reloadMu serialises Unmarshal during hot-reload so a partial mutation
	// can't be observed mid-flight by getter calls.
	reloadMu sync.RWMutex `mapstructure:"-"`

	// samplerReconfigureCallback is invoked when telemetry.sampler.type or
	// telemetry.sampler.ratio change. nil means no callback registered.
	samplerReconfigureCallback SamplerReconfigureFunc `mapstructure:"-"`

	// prevSamplerType and prevSamplerRatio track the last-seen sampler values
	// to detect changes during hot-reload (ADR-0023 Phase 3).
	prevSamplerType  string  `mapstructure:"-"`
	prevSamplerRatio float64 `mapstructure:"-"`

	// watcherStopped indicates that the config watcher has been stopped via
	// the context being cancelled. This prevents the OnConfigChange handler
	// from processing events after cleanup.
	watcherStopped bool `mapstructure:"-"`
}

// ServerConfig holds server-related configuration
type ServerConfig struct {
	Host string `mapstructure:"host"`
	Port int    `mapstructure:"port"`
}

// ShutdownConfig holds shutdown-related configuration
type ShutdownConfig struct {
	Timeout time.Duration `mapstructure:"timeout"`
}

// LoggingConfig holds logging-related configuration
type LoggingConfig struct {
	JSON   bool   `mapstructure:"json"`
	Level  string `mapstructure:"level"`
	Output string `mapstructure:"output"`
}

// TelemetryConfig holds OpenTelemetry-related configuration
type TelemetryConfig struct {
	Enabled      bool                       `mapstructure:"enabled"`
	OTLPEndpoint string                     `mapstructure:"otlp_endpoint"`
	ServiceName  string                     `mapstructure:"service_name"`
	Insecure     bool                       `mapstructure:"insecure"`
	Sampler      SamplerConfig              `mapstructure:"sampler"`
	Persistence  PersistenceTelemetryConfig `mapstructure:"persistence"`
}

// PersistenceTelemetryConfig holds persistence layer telemetry configuration
type PersistenceTelemetryConfig struct {
	Enabled bool `mapstructure:"enabled"`
}

// APIConfig holds API version configuration
type APIConfig struct {
	V2Enabled bool `mapstructure:"v2_enabled"`
}

// AuthConfig holds authentication configuration
type AuthConfig struct {
	JWTSecret           string    `mapstructure:"jwt_secret"`
	AdminMasterPassword string    `mapstructure:"admin_master_password"`
	JWT                 JWTConfig `mapstructure:"jwt"`
}

// JWTConfig holds JWT-specific configuration
type JWTConfig struct {
	TTL             time.Duration `mapstructure:"ttl"`
	SecretRetention struct {
		RetentionFactor float64       `mapstructure:"retention_factor"`
		MaxRetention    time.Duration `mapstructure:"max_retention"`
		CleanupInterval time.Duration `mapstructure:"cleanup_interval"`
	} `mapstructure:"secret_retention"`
}

// DatabaseConfig holds database configuration
type DatabaseConfig struct {
	Host            string        `mapstructure:"host"`
	Port            int           `mapstructure:"port"`
	User            string        `mapstructure:"user"`
	Password        string        `mapstructure:"password"`
	Name            string        `mapstructure:"name"`
	SSLMode         string        `mapstructure:"ssl_mode"`
	MaxOpenConns    int           `mapstructure:"max_open_conns"`
	MaxIdleConns    int           `mapstructure:"max_idle_conns"`
	ConnMaxLifetime time.Duration `mapstructure:"conn_max_lifetime"`
}

// RateLimitConfig holds rate limiting configuration
type RateLimitConfig struct {
	Enabled           bool `mapstructure:"enabled"`
	RequestsPerMinute int  `mapstructure:"requests_per_minute"`
	BurstSize         int  `mapstructure:"burst_size"`
}

// CacheConfig holds cache configuration
type CacheConfig struct {
	Enabled                bool `mapstructure:"enabled"`
	DefaultTTLSeconds      int  `mapstructure:"default_ttl_seconds"`
	CleanupIntervalSeconds int  `mapstructure:"cleanup_interval_seconds"`
}

// VersionInfo holds application version information
type VersionInfo struct {
	Version   string `mapstructure:"-"` // Set via ldflags
	Commit    string `mapstructure:"-"` // Set via ldflags
	Date      string `mapstructure:"-"` // Set via ldflags
	GoVersion string `mapstructure:"-"` // Set at runtime
}

// VersionCommand handles version display
func (c *Config) VersionCommand() string {
	// This will be enhanced when we integrate with cobra
	return fmt.Sprintf("dance-lessons-coach %s (commit: %s, built: %s, go: %s)",
		version.Version, version.Commit, version.Date, version.GoVersion)
}

// SamplerConfig holds tracing sampler configuration
type SamplerConfig struct {
	Type  string  `mapstructure:"type"`
	Ratio float64 `mapstructure:"ratio"`
}

// peekJSONLogging determines whether JSON logging should be used before the full
// config is loaded, solving the chicken-and-egg problem where the logger format
// must be known before any log is emitted, yet the format is stored in the config.
//
// Resolution order (mirrors Viper's own priority):
//  1. DLC_LOGGING_JSON env var — checked directly via os.Getenv (zero overhead)
//  2. logging.json key in the config file — read with a minimal throwaway Viper
//     instance so we don't parse the whole config twice unnecessarily
func peekJSONLogging() bool {
	// 1. Env var takes highest priority — check it first
	if env := os.Getenv("DLC_LOGGING_JSON"); env != "" {
		return strings.EqualFold(env, "true") || env == "1"
	}

	// 2. Try to read logging.json from the config file
	preV := viper.New()
	preV.SetDefault("logging.json", false)
	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
		preV.SetConfigFile(configFile)
	} else {
		preV.SetConfigName("config")
		preV.SetConfigType("yaml")
		preV.AddConfigPath(".")
	}
	_ = preV.ReadInConfig() // ignore errors — defaults apply on failure
	return preV.GetBool("logging.json")
}

// LoadConfig loads configuration from file, environment variables, and defaults
// Configuration priority: file > environment variables > defaults
// To specify a custom config file path, set DLC_CONFIG_FILE environment variable
func LoadConfig() (*Config, error) {
	// Check if we're in a test environment - this should NOT be called during BDD tests
	if os.Getenv("FEATURE") != "" {
		panic("ERROR: LoadConfig() was called during BDD tests! This should not happen - tests should use createTestConfig() instead.")
	}

	v := viper.New()

	// Configure the logger format before emitting any log output.
	// peekJSONLogging reads the JSON setting early (env var + config file pre-read)
	// so that every log line — including those produced during config loading — is
	// already in the correct format.
	jsonLogging := peekJSONLogging()
	if jsonLogging {
		log.Logger = log.Output(os.Stderr)
	} else {
		log.Logger = log.Output(zerolog.ConsoleWriter{Out: os.Stderr})
	}
	log.Info().Bool("json", jsonLogging).Msg("Logging configured")

	// Set default values
	v.SetDefault("server.host", "0.0.0.0")
	v.SetDefault("server.port", 8080)
	v.SetDefault("shutdown.timeout", 30*time.Second)
	v.SetDefault("logging.json", false)
	v.SetDefault("logging.level", "trace")
	v.SetDefault("logging.output", "")

	// Telemetry defaults
	v.SetDefault("telemetry.enabled", false)
	v.SetDefault("telemetry.otlp_endpoint", "localhost:4317")
	v.SetDefault("telemetry.service_name", "dance-lessons-coach")
	v.SetDefault("telemetry.insecure", true)
	v.SetDefault("telemetry.sampler.type", "parentbased_always_on")
	v.SetDefault("telemetry.sampler.ratio", 1.0)
	v.SetDefault("telemetry.persistence.enabled", false)

	// API defaults
	v.SetDefault("api.v2_enabled", false)

	// Rate limit defaults
	v.SetDefault("rate_limit.enabled", true)
	v.SetDefault("rate_limit.requests_per_minute", 60)
	v.SetDefault("rate_limit.burst_size", 10)

	// Cache defaults
	v.SetDefault("cache.enabled", true)
	v.SetDefault("cache.default_ttl_seconds", 300)
	v.SetDefault("cache.cleanup_interval_seconds", 600)

	// Auth defaults
	v.SetDefault("auth.jwt_secret", "default-secret-key-please-change-in-production")
	v.SetDefault("auth.admin_master_password", "admin123")
	v.SetDefault("auth.jwt.ttl", 1*time.Hour)
	v.SetDefault("auth.jwt.secret_retention.retention_factor", 2.0)
	v.SetDefault("auth.jwt.secret_retention.max_retention", 72*time.Hour)
	v.SetDefault("auth.jwt.secret_retention.cleanup_interval", 1*time.Hour)

	// Check for custom config file path via environment variable
	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
		v.SetConfigFile(configFile)
		log.Info().Str("config_file", configFile).Msg("Using custom config file path")
	} else {
		// Default: look for config.yaml in current directory
		v.SetConfigName("config")
		v.SetConfigType("yaml")
		v.AddConfigPath(".")
	}

	// Read config file if it exists
	if err := v.ReadInConfig(); err != nil {
		if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
			// Config file was found but there was an error reading it
			log.Warn().Err(err).Msg("Error reading config file, using defaults")
		}
		// Config file not found, continue with environment variables and defaults
	} else {
		log.Info().Str("config_file", v.ConfigFileUsed()).Msg("Config file loaded")
	}

	// Bind environment variables
	v.AutomaticEnv()
	v.SetEnvPrefix("DLC") // dance-lessons-coach prefix
	v.BindEnv("server.host", "DLC_SERVER_HOST")
	v.BindEnv("server.port", "DLC_SERVER_PORT")
	v.BindEnv("shutdown.timeout", "DLC_SHUTDOWN_TIMEOUT")
	v.BindEnv("logging.json", "DLC_LOGGING_JSON")
	v.BindEnv("logging.level", "DLC_LOGGING_LEVEL")
	v.BindEnv("logging.output", "DLC_LOGGING_OUTPUT")

	// Telemetry environment variables
	v.BindEnv("telemetry.enabled", "DLC_TELEMETRY_ENABLED")
	v.BindEnv("telemetry.otlp_endpoint", "DLC_TELEMETRY_OTLP_ENDPOINT")
	v.BindEnv("telemetry.service_name", "DLC_TELEMETRY_SERVICE_NAME")
	v.BindEnv("telemetry.insecure", "DLC_TELEMETRY_INSECURE")

	// Auth environment variables
	v.BindEnv("auth.jwt_secret", "DLC_AUTH_JWT_SECRET")
	v.BindEnv("auth.admin_master_password", "DLC_AUTH_ADMIN_MASTER_PASSWORD")
	v.BindEnv("auth.jwt.ttl", "DLC_AUTH_JWT_TTL")
	v.BindEnv("auth.jwt.secret_retention.retention_factor", "DLC_AUTH_JWT_SECRET_RETENTION_FACTOR")
	v.BindEnv("auth.jwt.secret_retention.max_retention", "DLC_AUTH_JWT_SECRET_MAX_RETENTION")
	v.BindEnv("auth.jwt.secret_retention.cleanup_interval", "DLC_AUTH_JWT_SECRET_CLEANUP_INTERVAL")
	v.BindEnv("telemetry.sampler.type", "DLC_TELEMETRY_SAMPLER_TYPE")
	v.BindEnv("telemetry.sampler.ratio", "DLC_TELEMETRY_SAMPLER_RATIO")

	// API environment variables
	v.BindEnv("api.v2_enabled", "DLC_API_V2_ENABLED")

	// Rate limit environment variables
	v.BindEnv("rate_limit.enabled", "DLC_RATE_LIMIT_ENABLED")
	v.BindEnv("rate_limit.requests_per_minute", "DLC_RATE_LIMIT_REQUESTS_PER_MINUTE")
	v.BindEnv("rate_limit.burst_size", "DLC_RATE_LIMIT_BURST_SIZE")

	// Cache environment variables
	v.BindEnv("cache.enabled", "DLC_CACHE_ENABLED")
	v.BindEnv("cache.default_ttl_seconds", "DLC_CACHE_DEFAULT_TTL_SECONDS")
	v.BindEnv("cache.cleanup_interval_seconds", "DLC_CACHE_CLEANUP_INTERVAL_SECONDS")

	// Database environment variables
	v.BindEnv("database.host", "DLC_DATABASE_HOST")
	v.BindEnv("database.port", "DLC_DATABASE_PORT")
	v.BindEnv("database.user", "DLC_DATABASE_USER")
	v.BindEnv("database.password", "DLC_DATABASE_PASSWORD")
	v.BindEnv("database.name", "DLC_DATABASE_NAME")
	v.BindEnv("database.ssl_mode", "DLC_DATABASE_SSL_MODE")

	// Unmarshal into Config struct
	var config Config
	if err := v.Unmarshal(&config); err != nil {
		log.Error().Err(err).Msg("Failed to unmarshal config")
		return nil, fmt.Errorf("config unmarshal error: %w", err)
	}

	// Keep the viper instance for hot-reload (ADR-0023).
	config.viper = v

	// Initialize previous sampler values for hot-reload change detection
	// (ADR-0023 Phase 3).
	config.prevSamplerType = config.Telemetry.Sampler.Type
	config.prevSamplerRatio = config.Telemetry.Sampler.Ratio

	// Setup logging based on configuration (level, output file, time format).
	// The JSON/console format was already applied at the top of LoadConfig via
	// peekJSONLogging, so SetupLogging only needs to handle the remaining knobs.
	config.SetupLogging()

	log.Info().
		Str("host", config.Server.Host).
		Int("port", config.Server.Port).
		Dur("shutdown_timeout", config.Shutdown.Timeout).
		Bool("logging_json", config.Logging.JSON).
		Str("logging_level", config.Logging.Level).
		Str("logging_output", config.Logging.Output).
		Bool("telemetry_enabled", config.Telemetry.Enabled).
		Str("telemetry_service", config.Telemetry.ServiceName).
		Bool("api_v2_enabled", config.API.V2Enabled).
		Dur("jwt_ttl", config.GetJWTTTL()).
		Float64("jwt_retention_factor", config.GetJWTSecretRetentionFactor()).
		Dur("jwt_max_retention", config.GetJWTSecretMaxRetention()).
		Dur("jwt_cleanup_interval", config.GetJWTSecretCleanupInterval()).
		Msg("Configuration loaded")

	return &config, nil
}

// GetServerAddress returns the formatted server address (host:port)
func (c *Config) GetServerAddress() string {
	return fmt.Sprintf("%s:%d", c.Server.Host, c.Server.Port)
}

// GetTelemetryEnabled returns whether telemetry is enabled
func (c *Config) GetTelemetryEnabled() bool {
	return c.Telemetry.Enabled
}

// GetOTLPEndpoint returns the OTLP endpoint for telemetry
func (c *Config) GetOTLPEndpoint() string {
	return c.Telemetry.OTLPEndpoint
}

// GetServiceName returns the service name for telemetry
func (c *Config) GetServiceName() string {
	return c.Telemetry.ServiceName
}

// GetPersistenceTelemetryEnabled returns whether persistence layer telemetry is enabled
func (c *Config) GetPersistenceTelemetryEnabled() bool {
	return c.Telemetry.Enabled && c.Telemetry.Persistence.Enabled
}

// GetTelemetryInsecure returns whether to use insecure connection
func (c *Config) GetTelemetryInsecure() bool {
	return c.Telemetry.Insecure
}

// GetSamplerType returns the sampler type
func (c *Config) GetSamplerType() string {
	return c.Telemetry.Sampler.Type
}

// GetSamplerRatio returns the sampler ratio
func (c *Config) GetSamplerRatio() float64 {
	return c.Telemetry.Sampler.Ratio
}

// SetSamplerReconfigureCallback registers a callback that is invoked when
// telemetry.sampler.type or telemetry.sampler.ratio change via hot-reload.
// The callback receives the new sampler type and ratio values.
// Pass nil to unregister the callback.
func (c *Config) SetSamplerReconfigureCallback(callback SamplerReconfigureFunc) {
	c.reloadMu.Lock()
	defer c.reloadMu.Unlock()
	c.samplerReconfigureCallback = callback
	// Initialize previous values so we can detect changes on first hot-reload
	c.prevSamplerType = c.Telemetry.Sampler.Type
	c.prevSamplerRatio = c.Telemetry.Sampler.Ratio
}

// GetV2Enabled returns whether v2 API is enabled
func (c *Config) GetV2Enabled() bool {
	return c.API.V2Enabled
}

// GetJWTSecret returns the JWT secret
func (c *Config) GetJWTSecret() string {
	return c.Auth.JWTSecret
}

// GetAdminMasterPassword returns the admin master password
func (c *Config) GetAdminMasterPassword() string {
	return c.Auth.AdminMasterPassword
}

// GetJWTTTL returns the JWT TTL
func (c *Config) GetJWTTTL() time.Duration {
	if c.Auth.JWT.TTL == 0 {
		return 1 * time.Hour // Default value
	}
	return c.Auth.JWT.TTL
}

// GetJWTSecretRetentionFactor returns the JWT secret retention factor
func (c *Config) GetJWTSecretRetentionFactor() float64 {
	if c.Auth.JWT.SecretRetention.RetentionFactor == 0 {
		return 2.0 // Default value
	}
	return c.Auth.JWT.SecretRetention.RetentionFactor
}

// GetJWTSecretMaxRetention returns the maximum JWT secret retention period
func (c *Config) GetJWTSecretMaxRetention() time.Duration {
	if c.Auth.JWT.SecretRetention.MaxRetention == 0 {
		return 72 * time.Hour // Default value
	}
	return c.Auth.JWT.SecretRetention.MaxRetention
}

// GetJWTSecretCleanupInterval returns the JWT secret cleanup interval
func (c *Config) GetJWTSecretCleanupInterval() time.Duration {
	if c.Auth.JWT.SecretRetention.CleanupInterval == 0 {
		return 1 * time.Hour // Default value
	}
	return c.Auth.JWT.SecretRetention.CleanupInterval
}

// GetLoggingJSON returns whether JSON logging is enabled
func (c *Config) GetLoggingJSON() bool {
	return c.Logging.JSON
}

// GetLogLevel returns the logging level
func (c *Config) GetLogLevel() string {
	return c.Logging.Level
}

// GetLogOutput returns the log output path
func (c *Config) GetLogOutput() string {
	return c.Logging.Output
}

// GetRateLimitEnabled returns whether rate limiting is enabled
func (c *Config) GetRateLimitEnabled() bool {
	return c.RateLimit.Enabled
}

// GetRateLimitRequestsPerMinute returns the requests per minute limit
func (c *Config) GetRateLimitRequestsPerMinute() int {
	if c.RateLimit.RequestsPerMinute <= 0 {
		return 60
	}
	return c.RateLimit.RequestsPerMinute
}

// GetRateLimitBurstSize returns the burst size for rate limiting
func (c *Config) GetRateLimitBurstSize() int {
	if c.RateLimit.BurstSize <= 0 {
		return 10
	}
	return c.RateLimit.BurstSize
}

// GetCacheEnabled returns whether cache is enabled
func (c *Config) GetCacheEnabled() bool {
	return c.Cache.Enabled
}

// GetCacheDefaultTTLSeconds returns the default TTL in seconds for cache items
func (c *Config) GetCacheDefaultTTLSeconds() int {
	if c.Cache.DefaultTTLSeconds <= 0 {
		return 300
	}
	return c.Cache.DefaultTTLSeconds
}

// GetCacheCleanupIntervalSeconds returns the cleanup interval in seconds for cache
func (c *Config) GetCacheCleanupIntervalSeconds() int {
	if c.Cache.CleanupIntervalSeconds <= 0 {
		return 600
	}
	return c.Cache.CleanupIntervalSeconds
}

// GetDatabaseHost returns the database host
func (c *Config) GetDatabaseHost() string {
	if c.Database.Host == "" {
		return "localhost"
	}
	return c.Database.Host
}

// GetDatabasePort returns the database port
func (c *Config) GetDatabasePort() int {
	if c.Database.Port == 0 {
		return 5432
	}
	return c.Database.Port
}

// GetDatabaseUser returns the database user
func (c *Config) GetDatabaseUser() string {
	if c.Database.User == "" {
		return "postgres"
	}
	return c.Database.User
}

// GetDatabasePassword returns the database password
func (c *Config) GetDatabasePassword() string {
	return c.Database.Password
}

// GetDatabaseName returns the database name
func (c *Config) GetDatabaseName() string {
	if c.Database.Name == "" {
		return "dance_lessons_coach"
	}
	return c.Database.Name
}

// GetDatabaseSSLMode returns the database SSL mode
func (c *Config) GetDatabaseSSLMode() string {
	if c.Database.SSLMode == "" {
		return "disable"
	}
	return c.Database.SSLMode
}

// GetDatabaseMaxOpenConns returns the maximum number of open connections
func (c *Config) GetDatabaseMaxOpenConns() int {
	if c.Database.MaxOpenConns == 0 {
		return 25
	}
	return c.Database.MaxOpenConns
}

// GetDatabaseMaxIdleConns returns the maximum number of idle connections
func (c *Config) GetDatabaseMaxIdleConns() int {
	if c.Database.MaxIdleConns == 0 {
		return 5
	}
	return c.Database.MaxIdleConns
}

// GetDatabaseConnMaxLifetime returns the maximum lifetime of connections
func (c *Config) GetDatabaseConnMaxLifetime() time.Duration {
	if c.Database.ConnMaxLifetime == 0 {
		return time.Hour
	}
	return c.Database.ConnMaxLifetime
}

// SetupLogging configures zerolog based on the configuration
func (c *Config) SetupLogging() {
	// Parse log level
	level := parseLogLevel(c.GetLogLevel())
	zerolog.SetGlobalLevel(level)

	// Setup log output
	c.setupLogOutput()

	zerolog.TimeFieldFormat = zerolog.TimeFormatUnix
}

// parseLogLevel converts a string log level to zerolog.Level
func parseLogLevel(level string) zerolog.Level {
	switch strings.ToLower(level) {
	case "trace":
		return zerolog.TraceLevel
	case "debug":
		return zerolog.DebugLevel
	case "info":
		return zerolog.InfoLevel
	case "warn", "warning":
		return zerolog.WarnLevel
	case "error":
		return zerolog.ErrorLevel
	case "fatal":
		return zerolog.FatalLevel
	case "panic":
		return zerolog.PanicLevel
	default:
		log.Warn().Str("level", level).Msg("Unknown log level, defaulting to trace")
		return zerolog.TraceLevel
	}
}

// setupLogOutput configures the log output based on configuration
func (c *Config) setupLogOutput() {
	output := c.GetLogOutput()
	if output == "" {
		// Use stderr by default
		return
	}

	// Open the log file
	file, err := os.OpenFile(output, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
	if err != nil {
		log.Error().Err(err).Str("output", output).Msg("Failed to open log file, using stderr")
		return
	}

	// Set the log output
	log.Logger = log.Output(file)
	log.Trace().Str("output", output).Msg("Logging to file")
}

// WatchAndApply starts watching the config file for changes and applies the
// hot-reloadable subset on every change (ADR-0023 selective hot-reload).
//
// Phases shipped:
//   - Phase 1: logging.level — re-applied via SetupLogging on every change.
//   - Phase 2: auth.jwt.ttl — picked up automatically because the userService
//     reads it via JWTConfig.GetTTL (a method value capturing this *Config).
//     The reloaded TTL is used on the NEXT token generation; tokens issued
//     before the change keep their original expiry.
//   - Phase 3: telemetry.sampler.type + telemetry.sampler.ratio — triggers
//     the callback set via SetSamplerReconfigureCallback if the values change.
//
// The other fields listed in ADR-0023 (api.v2_enabled) remain restart-only
// until their handlers land in subsequent phases.
//
// Stops when ctx is cancelled. Safe to call once at server startup.
// If the config file is absent (ConfigFileNotFoundError at load time), this
// becomes a no-op and logs a single warning.
func (c *Config) WatchAndApply(ctx context.Context) {
	if c.viper == nil {
		log.Warn().Msg("Config hot-reload disabled: no viper instance attached")
		return
	}
	if c.viper.ConfigFileUsed() == "" {
		log.Info().Msg("Config hot-reload disabled: no config file in use (env-only or defaults)")
		return
	}

	c.viper.OnConfigChange(func(in fsnotify.Event) {
		// Skip processing if watcher has been stopped
		c.reloadMu.Lock()
		if c.watcherStopped {
			c.reloadMu.Unlock()
			return
		}
		c.reloadMu.Unlock()

		log.Info().Str("event", in.Op.String()).Str("file", in.Name).Msg("Config file changed, reloading hot-reloadable fields")
		c.reloadMu.Lock()
		defer c.reloadMu.Unlock()

		if err := c.viper.Unmarshal(c); err != nil {
			log.Error().Err(err).Msg("Hot-reload: failed to unmarshal new config, keeping previous values")
			return
		}

		// Apply hot-reloadable fields. Order matters: logging first so the
		// rest of the reload is logged at the right level.
		c.SetupLogging()

		// Check if sampler config changed and invoke callback if registered
		samplerChanged := c.prevSamplerType != c.Telemetry.Sampler.Type ||
			c.prevSamplerRatio != c.Telemetry.Sampler.Ratio
		if samplerChanged && c.samplerReconfigureCallback != nil {
			if err := c.samplerReconfigureCallback(context.Background(),
				c.Telemetry.Sampler.Type,
				c.Telemetry.Sampler.Ratio); err != nil {
				log.Error().Err(err).Msg("Hot-reload: sampler reconfigure callback failed")
			} else {
				// Update previous values only after successful callback
				c.prevSamplerType = c.Telemetry.Sampler.Type
				c.prevSamplerRatio = c.Telemetry.Sampler.Ratio
				log.Info().
					Str("sampler_type", c.prevSamplerType).
					Float64("sampler_ratio", c.prevSamplerRatio).
					Msg("Hot-reload applied: telemetry sampler reconfigured")
			}
		} else if samplerChanged {
			// No callback registered, just update tracking values
			c.prevSamplerType = c.Telemetry.Sampler.Type
			c.prevSamplerRatio = c.Telemetry.Sampler.Ratio
		}

		log.Info().
			Str("logging_level", c.GetLogLevel()).
			Dur("jwt_ttl", c.GetJWTTTL()).
			Msg("Hot-reload applied (logging.level + auth.jwt.ttl)")
	})
	c.viper.WatchConfig()

	log.Info().Str("file", c.viper.ConfigFileUsed()).Msg("Config hot-reload watcher started (ADR-0023 Phase 1)")

	// Stop the watcher on context cancel — we set a flag that the
	// OnConfigChange handler checks, avoiding the race with viper's
	// internal state that would occur if we called OnConfigChange again.
	// We deliberately do NOT log here: viper's internal watcher goroutine
	// has no public Stop, so it can outlive ctx, and a zerolog call here
	// would race with the next test's LoadConfig → SetupLogging →
	// zerolog.SetGlobalLevel under -race (observed 2026-05-05).
	go func() {
		<-ctx.Done()
		c.reloadMu.Lock()
		c.watcherStopped = true
		c.reloadMu.Unlock()
	}()
}