config/config.go

package config

import (
	"fmt"
	"path/filepath"
	"strings"
	"sync"

	"github.com/go-playground/validator/v10"
	"github.com/spf13/viper"
)

// Manager is the top-level configuration schema and viper instance.
type Manager[T any] struct {
	// PreLoad is an optional function that is called before loading the
	// configuration file.
	PreLoad func(*Manager[T]) error

	// PostLoad is an optional function that is called after loading the
	// configuration file.
	PostLoad func(*Manager[T]) error

	// R is a raw access to the configuration schema. WARNING: This does not
	// perform ANY validation or unmarshalling, so it should only be used if you
	// have already manually called the Load method and are sure that no errors
	// occured.
	R *T

	// loaded indicates whether the configuration has been loaded.
	loaded bool
	// mu is a mutex to ensure thread-safe access to the configuration.
	mu sync.RWMutex

	viper *viper.Viper
}

// NewManager creates a new configuration manager for schema T. Fields in T are
// responsible for including any necessary `mapstructure` and `validate` tags
// to ensure proper unmarshalling and validation of the configuration schema.
func NewManager[T any](configName, configType, configPath string) *Manager[T] {
	m := &Manager[T]{
		viper: viper.New(),
	}

	m.viper.SetConfigName(configName)
	m.viper.SetConfigType(configType)
	m.viper.AddConfigPath(configPath)

	return m
}

// WithPrefix sets the environment variable prefix for the viper instance. Must
// be set in order to use environment variables for configuration.
func (m *Manager[T]) WithPrefix(prefix string) *Manager[T] {
	m.viper.SetEnvPrefix(prefix)
	m.viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	m.viper.AutomaticEnv()
	return m
}

// C returns the configuration schema. If the configuration has not been
// loaded, C will load the configuration file before returning the schema.
// Any errors encountered during loading will result in a panic.
func (m *Manager[T]) C() *T {
	// First, try to acquire a read lock
	m.mu.RLock()
	if m.loaded {
		m.mu.RUnlock()
		return m.R // Already loaded, return the cached instance
	}
	m.mu.RUnlock()

	// Need to load the configuration, so acquire a write lock
	m.mu.Lock()
	defer m.mu.Unlock()

	// Double-check if loaded after acquiring the write lock
	if m.loaded {
		return m.R // Already loaded, return the cached instance
	}

	// Proceed to load the configuration
	if err := m.loadUnsafe(); err != nil {
		panic(fmt.Errorf("failed to load configuration: %w", err))
	}

	return m.R // Return the loaded configuration
}

// ConfigPath returns the absolute path to the configuration file that was used.
// If absolute path cannot be determined, the relative path is returned.
func (m *Manager[T]) ConfigPath() string {
	if m.viper.ConfigFileUsed() == "" {
		return ""
	}

	absPath, err := filepath.Abs(m.viper.ConfigFileUsed())
	if err != nil {
		return m.viper.ConfigFileUsed()
	}

	return absPath
}

// Viper returns the viper instance used by the configuration manager.
func (m *Manager[T]) Viper() *viper.Viper {
	return m.viper
}

// Load reads and validates the configuration file, returning an error if any.
func (m *Manager[T]) Load() error {
	// Make sure we have lock before checking if loaded
	m.mu.Lock()
	defer m.mu.Unlock()

	if m.loaded {
		return nil // Already loaded, no need to load again
	}

	return m.loadUnsafe()
}

// loadUnsafe performs the actual loading without acquiring locks. Caller MUST
// hold the write lock.
func (m *Manager[T]) loadUnsafe() error {
	// Initialize R to a new instance of T
	m.R = new(T)

	// Run the pre-load function if it is set
	if m.PreLoad != nil {
		if err := m.PreLoad(m); err != nil {
			return fmt.Errorf("pre-load function failed: %w", err)
		}
	}

	// Load the configuration file using viper
	if err := m.viper.ReadInConfig(); err != nil {
		return fmt.Errorf("viper failed to read config file: %w", err)
	}
	if err := m.viper.Unmarshal(m.R); err != nil {
		return fmt.Errorf("viper failed to unmarshal config file: %w", err)
	}

	// Validate the configuration schema using the validator package
	validate := validator.New(validator.WithRequiredStructEnabled())
	if err := validate.Struct(m.R); err != nil {
		return fmt.Errorf("validator failed to validate config file: %w", err)
	}

	// Run the post-load function if it is set
	if m.PostLoad != nil {
		if err := m.PostLoad(m); err != nil {
			return fmt.Errorf("post-load function failed: %w", err)
		}
	}

	m.loaded = true
	return nil
}