only check rate limit when there is actual download

internal/fetcher/fetcher.go

@@ -147,6 +147,7 @@ thread:
 
 // downloadFile in a thread.
 func (f *fetcher) downloadFile(bookID int64, format file.Format, share driver.Share) error {
+	f.progress.TakeRateLimit()
 	log.Debugf("Start download book id %d, format %s, share %v.", bookID, format, share)
 	// Create the file writer.
 	writer, err := f.creator.NewWriter(bookID, f.progress.Size(), share.FileName, share.SubPath, format, share.Size)

internal/progress/progress.go

@@ -20,6 +20,9 @@ var (
 )
 
 type Progress interface {
+	// TakeRateLimit would wait until the rate limit is available.
+	TakeRateLimit()
+
 	// AcquireBookID would find the book id from the assign array.
 	AcquireBookID() int64
 
@@ -127,14 +130,16 @@ func loadStorage(file *os.File) (*bitset.BitSet, error) {
 	return set, nil
 }
 
+// TakeRateLimit block until the rate meets the given config.
+func (storage *bitProgress) TakeRateLimit() {
+	storage.limit.Take()
+}
+
 // AcquireBookID would find the book id from the assign array.
 func (storage *bitProgress) AcquireBookID() int64 {
 	storage.lock.Lock()
 	defer storage.lock.Unlock()
 
-	// Block until the rate meets the given config.
-	storage.limit.Take()
-
 	for i := uint(0); i < storage.assigned.Len(); i++ {
 		if !storage.assigned.Test(i) {
 			storage.assigned.Set(i)