code: func (ix *Index) noteBlobIndexed(br blob.Ref) {
  	for _, needer := range ix.neededBy[br] {
  		newNeeds := blobsFilteringOut(ix.needs[needer], br)
  		if len(newNeeds) == 0 {
  			ix.readyReindex[needer] = true
  			delete(ix.needs, needer)
  			ix.reindexWg.Add(1)
  			go ix.indexReadyBlobs(context.Background())
  		} else {
  			ix.needs[needer] = newNeeds
  		}
  	}
  	delete(ix.neededBy, br)
  }
docs: // noteBlobIndexed checks if the recent indexing of br now allows the blobs that
  // were depending on br, to be indexed in turn. If yes, they're reindexed
  // asynchronously by indexReadyBlobs.