fancy-dns/internal/config/store.go

package config

import (
	"sync"
	"sync/atomic"
)

// Store is the process-wide, hot-reloadable configuration holder. Readers take
// a snapshot pointer (cheap, no lock) and writers swap a fully-built *Config
// in atomically, so a request never observes a half-applied config.
type Store struct {
	cur atomic.Pointer[Config]
	// base is the configuration as the file (or an API write) defined it. cur
	// is base with the overlay applied; without an overlay the two are the
	// same pointer. Writers that persist back to the file must start from
	// base, or they would write overlay content into the file.
	base    atomic.Pointer[Config]
	overlay atomic.Pointer[func(*Config) *Config]
	mu      sync.Mutex // serialises writers so LastError/Version stay coherent
	// path is the file the config was loaded from, used by the API to
	// persist mutations.
	path string
	// lastError records why the most recent reload attempt failed, so the
	// health endpoint can report a stale-but-running state.
	lastErr atomic.Pointer[string]
	// generation increments on every successful swap.
	generation atomic.Uint64
	// hooks run after a successful swap (server re-binds listeners etc).
	hooksMu sync.RWMutex
	hooks   []func(*Config)
}

// NewStore wraps cfg.
func NewStore(cfg *Config, path string) *Store {
	s := &Store{path: path}
	s.cur.Store(cfg)
	s.base.Store(cfg)
	return s
}

// Base returns the configuration without the overlay: what the file says.
func (s *Store) Base() *Config {
	if b := s.base.Load(); b != nil {
		return b
	}
	return s.cur.Load()
}

// SetOverlay installs fn, which derives the effective configuration from the
// base (a cluster source adding groups and lists, say). fn must not modify its
// argument. Call Reapply afterwards, and again whenever fn's inputs change.
func (s *Store) SetOverlay(fn func(*Config) *Config) {
	if fn == nil {
		s.overlay.Store(nil)
		return
	}
	s.overlay.Store(&fn)
}

func (s *Store) effective(base *Config) *Config {
	if fn := s.overlay.Load(); fn != nil {
		if out := (*fn)(base); out != nil {
			return out
		}
	}
	return base
}

// Reapply recomputes the effective configuration from the base and swaps it
// in, running the hooks as for any other swap.
func (s *Store) Reapply() {
	s.mu.Lock()
	next := s.effective(s.Base())
	s.cur.Store(next)
	s.generation.Add(1)
	s.lastErr.Store(nil)
	s.mu.Unlock()
	s.runHooks(next)
}

// Get returns the current configuration snapshot.
func (s *Store) Get() *Config { return s.cur.Load() }

// Path is the file this config was loaded from ("" when running without one).
func (s *Store) Path() string { return s.path }

// Generation counts successful swaps; the cluster uses it to detect
// divergence between instances.
func (s *Store) Generation() uint64 { return s.generation.Load() }

// LastError returns the error from the most recent failed reload, or nil.
func (s *Store) LastError() string {
	if p := s.lastErr.Load(); p != nil {
		return *p
	}
	return ""
}

// Swap installs cfg as the current configuration. Callers are responsible for
// validating first; Swap itself only guards structural invariants that would
// otherwise panic subsystems.
func (s *Store) Swap(cfg *Config) error {
	if cfg == nil {
		return errNilConfig
	}
	if cfg.Cache.Shards <= 0 {
		cfg.Cache.Shards = 32
	}
	s.mu.Lock()
	s.base.Store(cfg)
	eff := s.effective(cfg)
	s.cur.Store(eff)
	s.generation.Add(1)
	s.lastErr.Store(nil)
	s.mu.Unlock()
	s.runHooks(eff)
	return nil
}

// Reload re-reads the backing file and swaps it in on success. On failure the
// previous configuration stays active and the error is recorded.
func (s *Store) Reload() error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.path == "" {
		return errNoPath
	}
	cfg, err := Load(s.path)
	if err != nil {
		msg := err.Error()
		s.lastErr.Store(&msg)
		return err
	}
	// Preserve the working dir of the running instance so relative paths keep
	// resolving the same way even if the file moved.
	cfg.Server.WorkingDir = s.Base().Server.WorkingDir
	s.base.Store(cfg)
	eff := s.effective(cfg)
	s.cur.Store(eff)
	s.generation.Add(1)
	s.lastErr.Store(nil)
	s.runHooks(eff)
	return nil
}

// OnSwap registers a callback invoked with the new config after each swap.
// Hooks run outside the writer lock; a panicking hook is contained so one bad
// subsystem cannot take down a reload.
func (s *Store) OnSwap(fn func(*Config)) {
	if fn == nil {
		return
	}
	s.hooksMu.Lock()
	s.hooks = append(s.hooks, fn)
	s.hooksMu.Unlock()
}

func (s *Store) runHooks(cfg *Config) {
	s.hooksMu.RLock()
	hooks := s.hooks
	s.hooksMu.RUnlock()
	for _, h := range hooks {
		func() {
			defer func() { _ = recover() }()
			h(cfg)
		}()
	}
}

type storeError string

func (e storeError) Error() string { return string(e) }

const (
	errNilConfig = storeError("config: refusing to swap a nil configuration")
	errNoPath    = storeError("config: no config file path known, cannot reload")
)