Feature flag snapshot with periodic refresh and atomic reads

                  package flags

import (
  "context"
  "encoding/json"
  "sync/atomic"
  "time"
)

type Snapshot map[string]bool

type Store struct {
  v atomic.Value
  load func(context.Context) ([]byte, error)
}

func New(load func(context.Context) ([]byte, error)) *Store {
  s := &Store{load: load}
  s.v.Store(Snapshot{})
  return s
}

func (s *Store) Enabled(name string) bool {
  snap := s.v.Load().(Snapshot)
  return snap[name]
}

func (s *Store) Run(ctx context.Context) {
  t := time.NewTicker(30 * time.Second)
  defer t.Stop()
  for {
    select {
    case <-ctx.Done():
      return
    case <-t.C:
      c, cancel := context.WithTimeout(ctx, 2*time.Second)
      b, err := s.load(c)
      cancel()
      if err != nil {
        continue
      }
      var snap Snapshot
      if json.Unmarshal(b, &snap) == nil {
        s.v.Store(snap)
      }
    }
  }
}