mcp-framework/config/config.go

package config

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

const (
	CurrentVersion = 1
	DefaultFile    = "config.json"
)

var (
	ErrFutureVersion      = errors.New("future config version")
	ErrUnsupportedVersion = errors.New("unsupported config version")
	ErrInvalidConfig      = errors.New("invalid config")
)

type FileConfig[T any] struct {
	Version        int          `json:"version"`
	CurrentProfile string       `json:"current_profile"`
	Profiles       map[string]T `json:"profiles"`
}

type ValidationIssue struct {
	Path    string
	Message string
}

type ValidationError struct {
	Issues []ValidationIssue
}

func (e *ValidationError) Error() string {
	parts := make([]string, 0, len(e.Issues))
	for _, issue := range e.Issues {
		if issue.Path == "" {
			parts = append(parts, issue.Message)
			continue
		}
		parts = append(parts, fmt.Sprintf("%s: %s", issue.Path, issue.Message))
	}

	return fmt.Sprintf("invalid config: %s", strings.Join(parts, "; "))
}

func (e *ValidationError) Unwrap() error {
	return ErrInvalidConfig
}

type Migration func(doc map[string]json.RawMessage) error

type Validator[T any] func(FileConfig[T]) []ValidationIssue

type Options[T any] struct {
	FileName   string
	Version    int
	Migrations map[int]Migration
	Validator  Validator[T]
}

type Store[T any] struct {
	dirName    string
	fileName   string
	version    int
	migrations map[int]Migration
	validator  Validator[T]
}

func NewStore[T any](dirName string) Store[T] {
	return NewStoreWithOptions[T](dirName, Options[T]{})
}

func NewStoreWithFile[T any](dirName, fileName string) Store[T] {
	return NewStoreWithOptions[T](dirName, Options[T]{FileName: fileName})
}

func NewStoreWithOptions[T any](dirName string, options Options[T]) Store[T] {
	fileName := options.FileName
	if fileName == "" {
		fileName = DefaultFile
	}

	version := options.Version
	if version <= 0 {
		version = CurrentVersion
	}

	return Store[T]{
		dirName:    dirName,
		fileName:   fileName,
		version:    version,
		migrations: buildMigrations(version, options.Migrations),
		validator:  options.Validator,
	}
}

func (s Store[T]) Default() FileConfig[T] {
	return FileConfig[T]{
		Version:  s.version,
		Profiles: map[string]T{},
	}
}

func (s Store[T]) ConfigPath() (string, error) {
	userConfigDir, err := os.UserConfigDir()
	if err != nil {
		return "", fmt.Errorf("resolve user config dir: %w", err)
	}

	return filepath.Join(userConfigDir, s.dirName, s.fileName), nil
}

func (s Store[T]) Load(path string) (FileConfig[T], error) {
	data, err := os.ReadFile(path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return s.Default(), nil
		}
		return FileConfig[T]{}, fmt.Errorf("read config %s: %w", path, err)
	}

	if len(data) == 0 {
		return s.Default(), nil
	}

	doc, err := parseDocument(path, data)
	if err != nil {
		return FileConfig[T]{}, err
	}

	if err := s.migrateDocument(path, doc); err != nil {
		return FileConfig[T]{}, err
	}

	cfg := s.Default()
	data, err = json.Marshal(doc)
	if err != nil {
		return FileConfig[T]{}, fmt.Errorf("encode config %s: %w", path, err)
	}
	if err := json.Unmarshal(data, &cfg); err != nil {
		return FileConfig[T]{}, fmt.Errorf("parse config %s: %w", path, err)
	}

	s.normalize(&cfg)
	if err := s.validate(cfg); err != nil {
		return FileConfig[T]{}, fmt.Errorf("validate config %s: %w", path, err)
	}

	return cfg, nil
}

func (s Store[T]) LoadDefault() (FileConfig[T], string, error) {
	path, err := s.ConfigPath()
	if err != nil {
		return FileConfig[T]{}, "", err
	}

	cfg, err := s.Load(path)
	if err != nil {
		return FileConfig[T]{}, "", err
	}

	return cfg, path, nil
}

func (s Store[T]) Save(path string, cfg FileConfig[T]) error {
	s.normalize(&cfg)
	if err := s.validate(cfg); err != nil {
		return fmt.Errorf("validate config %s: %w", path, err)
	}

	dir := filepath.Dir(path)
	if err := os.MkdirAll(dir, 0o700); err != nil {
		return fmt.Errorf("create config dir %s: %w", dir, err)
	}
	if err := os.Chmod(dir, 0o700); err != nil {
		return fmt.Errorf("set config dir permissions %s: %w", dir, err)
	}

	data, err := json.MarshalIndent(cfg, "", "  ")
	if err != nil {
		return fmt.Errorf("encode config: %w", err)
	}
	data = append(data, '\n')

	tmpFile, err := os.CreateTemp(dir, "config-*.json")
	if err != nil {
		return fmt.Errorf("create temp config in %s: %w", dir, err)
	}

	tmpPath := tmpFile.Name()
	cleanup := true
	defer func() {
		_ = tmpFile.Close()
		if cleanup {
			_ = os.Remove(tmpPath)
		}
	}()

	if err := tmpFile.Chmod(0o600); err != nil {
		return fmt.Errorf("set temp config permissions %s: %w", tmpPath, err)
	}
	if _, err := tmpFile.Write(data); err != nil {
		return fmt.Errorf("write temp config %s: %w", tmpPath, err)
	}
	if err := tmpFile.Close(); err != nil {
		return fmt.Errorf("close temp config %s: %w", tmpPath, err)
	}
	if err := os.Rename(tmpPath, path); err != nil {
		return fmt.Errorf("replace config %s: %w", path, err)
	}
	if err := os.Chmod(path, 0o600); err != nil {
		return fmt.Errorf("set config permissions %s: %w", path, err)
	}

	cleanup = false
	return nil
}

func (s Store[T]) SaveDefault(cfg FileConfig[T]) (string, error) {
	path, err := s.ConfigPath()
	if err != nil {
		return "", err
	}

	if err := s.Save(path, cfg); err != nil {
		return "", err
	}

	return path, nil
}

func (s Store[T]) normalize(cfg *FileConfig[T]) {
	if cfg.Version == 0 {
		cfg.Version = s.version
	}
	if cfg.Profiles == nil {
		cfg.Profiles = map[string]T{}
	}
}

func (s Store[T]) validate(cfg FileConfig[T]) error {
	issues := make([]ValidationIssue, 0, 2)
	if cfg.Version != s.version {
		issues = append(issues, ValidationIssue{
			Path:    "version",
			Message: fmt.Sprintf("must be %d", s.version),
		})
	}
	if cfg.CurrentProfile != "" {
		if _, ok := cfg.Profiles[cfg.CurrentProfile]; !ok {
			issues = append(issues, ValidationIssue{
				Path:    "current_profile",
				Message: fmt.Sprintf("references unknown profile %q", cfg.CurrentProfile),
			})
		}
	}
	if s.validator != nil {
		issues = append(issues, s.validator(cfg)...)
	}
	if len(issues) == 0 {
		return nil
	}

	return &ValidationError{Issues: issues}
}

func (s Store[T]) migrateDocument(path string, doc map[string]json.RawMessage) error {
	version, err := documentVersion(doc)
	if err != nil {
		return fmt.Errorf("parse config %s: %w", path, err)
	}
	if version > s.version {
		return fmt.Errorf("%w: config version %d is newer than supported version %d", ErrFutureVersion, version, s.version)
	}

	for version < s.version {
		migration, ok := s.migrations[version]
		if !ok {
			return fmt.Errorf("%w: no migration registered from version %d to %d", ErrUnsupportedVersion, version, version+1)
		}
		if err := migration(doc); err != nil {
			return fmt.Errorf("migrate config %s from version %d to %d: %w", path, version, version+1, err)
		}
		version++
		if err := setDocumentVersion(doc, version); err != nil {
			return fmt.Errorf("set config version %s to %d: %w", path, version, err)
		}
	}

	return nil
}

func buildMigrations(version int, custom map[int]Migration) map[int]Migration {
	migrations := map[int]Migration{}
	if version >= 1 {
		migrations[0] = func(doc map[string]json.RawMessage) error {
			return nil
		}
	}
	for fromVersion, migration := range custom {
		migrations[fromVersion] = migration
	}

	return migrations
}

func parseDocument(path string, data []byte) (map[string]json.RawMessage, error) {
	var doc map[string]json.RawMessage
	if err := json.Unmarshal(data, &doc); err != nil {
		return nil, fmt.Errorf("parse config %s: %w", path, err)
	}
	return doc, nil
}

func documentVersion(doc map[string]json.RawMessage) (int, error) {
	rawVersion, ok := doc["version"]
	if !ok || len(rawVersion) == 0 {
		return 0, nil
	}

	var version int
	if err := json.Unmarshal(rawVersion, &version); err != nil {
		return 0, fmt.Errorf("decode version: %w", err)
	}
	if version < 0 {
		return 0, fmt.Errorf("%w: version must be non-negative", ErrUnsupportedVersion)
	}

	return version, nil
}

func setDocumentVersion(doc map[string]json.RawMessage, version int) error {
	rawVersion, err := json.Marshal(version)
	if err != nil {
		return err
	}
	doc["version"] = rawVersion
	return nil
}