Singleflight cache fill to prevent thundering herd

                  package cache

import (
  "context"
  "sync"
  "time"

  "golang.org/x/sync/singleflight"
)

type item struct {
  v       string
  expires time.Time
}

type Cache struct {
  mu    sync.Mutex
  data  map[string]item
  group singleflight.Group
}

func New() *Cache { return &Cache{data: make(map[string]item)} }

func (c *Cache) Get(ctx context.Context, key string, ttl time.Duration, loader func(context.Context) (string, error)) (string, error) {
  c.mu.Lock()
  if it, ok := c.data[key]; ok && time.Now().Before(it.expires) {
    v := it.v
    c.mu.Unlock()
    return v, nil
  }
  c.mu.Unlock()

  v, err, _ := c.group.Do(key, func() (any, error) {
    val, err := loader(ctx)
    if err != nil {
      return "", err
    }
    c.mu.Lock()
    c.data[key] = item{v: val, expires: time.Now().Add(ttl)}
    c.mu.Unlock()
    return val, nil
  })
  if err != nil {
    return "", err
  }
  return v.(string), nil
}