code: func GlobalCache(c context.Context, namespace string) BlobCache {
  	if p, _ := c.Value(&globalCacheKey).(BlobCacheProvider); p != nil {
  		return p(namespace)
  	}
  	return nil
  }
docs: // GlobalCache returns a global cache targeting the given namespace or nil if
  // the global cache is not available in the current environment.
  //
  // Libraries that can live without a global cache mush handle its absence
  // gracefully (e.g by falling back to using only the process cache).