cache: fix worker scale down

Ensure that calling scaleWorkers will create/destroy the right amount of
workers.

backend/cache/handle.go

@@ -49,12 +49,13 @@ type Handle struct {
 	offset         int64
 	seenOffsets    map[int64]bool
 	mu             sync.Mutex
+	workersWg      sync.WaitGroup
 	confirmReading chan bool
-
-	UseMemory bool
-	workers   []*worker
-	closed    bool
-	reading   bool
+	workers        int
+	maxWorkerID    int
+	UseMemory      bool
+	closed         bool
+	reading        bool
 }
 
 // NewObjectHandle returns a new Handle for an existing Object
@@ -95,7 +96,7 @@ func (r *Handle) String() string {
 
 // startReadWorkers will start the worker pool
 func (r *Handle) startReadWorkers() {
-	if r.hasAtLeastOneWorker() {
+	if r.workers > 0 {
 		return
 	}
 	totalWorkers := r.cacheFs().opt.TotalWorkers
@@ -117,26 +118,27 @@ func (r *Handle) startReadWorkers() {
 
 // scaleOutWorkers will increase the worker pool count by the provided amount
 func (r *Handle) scaleWorkers(desired int) {
-	current := len(r.workers)
+	current := r.workers
 	if current == desired {
 		return
 	}
 	if current > desired {
 		// scale in gracefully
-		for i := 0; i < current-desired; i++ {
+		for r.workers > desired {
 			r.preloadQueue <- -1
+			r.workers--
 		}
 	} else {
 		// scale out
-		for i := 0; i < desired-current; i++ {
+		for r.workers < desired {
 			w := &worker{
 				r:  r,
-				ch: r.preloadQueue,
-				id: current + i,
+				id: r.maxWorkerID,
 			}
+			r.workersWg.Add(1)
+			r.workers++
+			r.maxWorkerID++
 			go w.run()
-
-			r.workers = append(r.workers, w)
 		}
 	}
 	// ignore first scale out from 0
@@ -148,7 +150,7 @@ func (r *Handle) scaleWorkers(desired int) {
 func (r *Handle) confirmExternalReading() {
 	// if we have a max value of workers
 	// then we skip this step
-	if len(r.workers) > 1 ||
+	if r.workers > 1 ||
 		!r.cacheFs().plexConnector.isConfigured() {
 		return
 	}
@@ -178,7 +180,7 @@ func (r *Handle) queueOffset(offset int64) {
 			}
 		}
 
-		for i := 0; i < len(r.workers); i++ {
+		for i := 0; i < r.workers; i++ {
 			o := r.preloadOffset + int64(r.cacheFs().opt.ChunkSize)*int64(i)
 			if o < 0 || o >= r.cachedObject.Size() {
 				continue
@@ -193,16 +195,6 @@ func (r *Handle) queueOffset(offset int64) {
 	}
 }
 
-func (r *Handle) hasAtLeastOneWorker() bool {
-	oneWorker := false
-	for i := 0; i < len(r.workers); i++ {
-		if r.workers[i].isRunning() {
-			oneWorker = true
-		}
-	}
-	return oneWorker
-}
-
 // getChunk is called by the FS to retrieve a specific chunk of known start and size from where it can find it
 // it can be from transient or persistent cache
 // it will also build the chunk from the cache's specific chunk boundaries and build the final desired chunk in a buffer
@@ -243,7 +235,7 @@ func (r *Handle) getChunk(chunkStart int64) ([]byte, error) {
 	// not found in ram or
 	// the worker didn't managed to download the chunk in time so we abort and close the stream
 	if err != nil || len(data) == 0 || !found {
-		if !r.hasAtLeastOneWorker() {
+		if r.workers == 0 {
 			fs.Errorf(r, "out of workers")
 			return nil, io.ErrUnexpectedEOF
 		}
@@ -304,14 +296,7 @@ func (r *Handle) Close() error {
 	close(r.preloadQueue)
 	r.closed = true
 	// wait for workers to complete their jobs before returning
-	waitCount := 3
-	for i := 0; i < len(r.workers); i++ {
-		waitIdx := 0
-		for r.workers[i].isRunning() && waitIdx < waitCount {
-			time.Sleep(time.Second)
-			waitIdx++
-		}
-	}
+	r.workersWg.Wait()
 	r.memory.db.Flush()
 
 	fs.Debugf(r, "cache reader closed %v", r.offset)
@@ -348,12 +333,9 @@ func (r *Handle) Seek(offset int64, whence int) (int64, error) {
 }
 
 type worker struct {
-	r       *Handle
-	ch      <-chan int64
-	rc      io.ReadCloser
-	id      int
-	running bool
-	mu      sync.Mutex
+	r  *Handle
+	rc io.ReadCloser
+	id int
 }
 
 // String is a representation of this worker
@@ -398,33 +380,19 @@ func (w *worker) reader(offset, end int64, closeOpen bool) (io.ReadCloser, error
 	})
 }
 
-func (w *worker) isRunning() bool {
-	w.mu.Lock()
-	defer w.mu.Unlock()
-	return w.running
-}
-
-func (w *worker) setRunning(f bool) {
-	w.mu.Lock()
-	defer w.mu.Unlock()
-	w.running = f
-}
-
 // run is the main loop for the worker which receives offsets to preload
 func (w *worker) run() {
 	var err error
 	var data []byte
-	defer w.setRunning(false)
 	defer func() {
 		if w.rc != nil {
 			_ = w.rc.Close()
-			w.setRunning(false)
 		}
+		w.r.workersWg.Done()
 	}()
 
 	for {
-		chunkStart, open := <-w.ch
-		w.setRunning(true)
+		chunkStart, open := <-w.r.preloadQueue
 		if chunkStart < 0 || !open {
 			break
 		}