suggestion.go

// UnionFetcher fetches from east and west and combines the result. If one
// fetcher fails only the reply of the other fetcher is returned.
type UnionFetcher struct {
	East, West PathFetcher
}

// GetPaths gets paths from both fetchers.
func (f *UnionFetcher) GetPaths(ctx context.Context, req *sciond.PathReq,
	earlyReplyInterval time.Duration, logger log.Logger) (*sciond.PathReply, error) {

	results:=make(chan *sciond.PathReply,2) //space for two non-blocking reads
	for _, target:= {f.east,f.west}{
		go func(){
			r,err:= <- target.GetPaths(ctx, req, earlyReplyInterval, logger)
			if err!=nil{ //logme}
			results <- r // non-blocking because size 2, max ctx timeout then goroutine exits
		}()
	}
	

	return mergeReplies(<-result, <-result) //max-time the ctx both goroutine have returned
}

func mergeReplies(e, w *sciond.PathReply) (*sciond.PathReply, error) {

	entries := []sciond.PathReplyEntry{}
	if e.ErrorCode == sciond.ErrorOk {
		entries = append(entries, e.Entries...)
	}

	if w.ErrorCode == sciond.ErrorOk {
		entries = append(entries, w.Entries...)
	}

	//happy path
	return &sciond.PathReply{
		Entries:   entries,
		ErrorCode: sciond.ErrorOk,
	}, nil

}