filer: count reader on cacher waitgroup under the map lock
The read's wg.Add(1) ran after ReadChunkAt released the ReaderCache lock, so a
concurrent destroy() (error eviction, LRU, or UnCache) could start wg.Wait() on
a zero counter and then race the Add - a WaitGroup reuse that trips -race and
can panic. Move the Add under the lock, before the cacher can leave the
downloaders map, so a destroy is always ordered against a counted read.
Chris LuandGitHub
ece4f42ecd