Implement a buffer pool

Downloads in quick succession could cause an OOM when the GC couldn't
  keep up

cmd/comic-hasher/main.go

@@ -74,6 +74,15 @@ var formatValues = map[string]Format{
 	"msgpack": Msgpack,
 }
 
+var bufPool = &sync.Pool{
+	New: func() any {
+		// The Pool's New function should generally only return pointer
+		// types, since a pointer can be put into the return interface
+		// value without an allocation:
+		return new(bytes.Buffer)
+	},
+}
+
 func (f Format) String() string {
 	if name, known := formatNames[f]; known {
 		return name
@@ -799,15 +808,21 @@ func downloadProcessor(chdb ch.CHDB, opts Opts, imagePaths chan cv.Download, ser
 				panic(err)
 			}
 		} else {
-			file = io.NopCloser(bytes.NewBuffer(path.Image))
+			file = io.NopCloser(path.Image)
 		}
 		i, format, err := image.Decode(bufio.NewReader(file))
 		if err != nil {
 			file.Close()
-			log.Println("Reading image failed", path.Dest)
+			log.Println("Reading image failed", path.Dest, err)
+			if path.Image != nil {
+				bufPool.Put(path.Image)
+			}
 			continue // skip this image
 		}
 		file.Close()
+		if path.Image != nil {
+			bufPool.Put(path.Image)
+		}
 		chdb.AddPath(path.Dest) // Add to sqlite db and remove file if opts.deleteHashedImages is true
 
 		im := ch.Im{
@@ -906,7 +921,7 @@ func startServer(opts Opts) {
 		if opts.cv.thumbOnly {
 			imageTypes = append(imageTypes, "thumb_url")
 		}
-		cvdownloader := cv.NewCVDownloader(server.Context, chdb, opts.cv.path, opts.cv.APIKey, imageTypes, opts.cv.keepDownloaded, opts.cv.hashDownloaded, finishedDownloadQueue)
+		cvdownloader := cv.NewCVDownloader(server.Context, bufPool, chdb, opts.cv.path, opts.cv.APIKey, imageTypes, opts.cv.keepDownloaded, opts.cv.hashDownloaded, finishedDownloadQueue)
 		go func() {
 			defer dwg.Done()
 			cv.DownloadCovers(cvdownloader)

cv/cv.go

@@ -30,7 +30,7 @@ type Download struct {
 	URL     string
 	Dest    string
 	IssueID string
-	Image   []byte
+	Image   *bytes.Buffer
 }
 
 type Issue struct {
@@ -80,6 +80,7 @@ type CVDownloader struct {
 	imageDownloads chan download
 	notFound       chan download
 	chdb           ch.CHDB
+	bufPool        *sync.Pool
 }
 
 var (
@@ -375,13 +376,16 @@ func (c *CVDownloader) start_downloader() {
 					}
 
 				} else {
-					image := &bytes.Buffer{}
+					image := c.bufPool.Get().(*bytes.Buffer)
 					log.Println("downloading", dl.dest)
 					_, err = io.Copy(image, resp.Body)
 					if err != nil {
 						log.Println("Failed when downloading image", err)
 						cleanup()
 						os.Remove(dl.dest)
+						if image != nil {
+							c.bufPool.Put(image)
+						}
 						continue
 					}
 
@@ -389,7 +393,7 @@ func (c *CVDownloader) start_downloader() {
 						URL:     dl.url,
 						Dest:    dl.dest,
 						IssueID: strconv.Itoa(dl.issueID),
-						Image:   image.Bytes(),
+						Image:   image,
 					}
 				}
 				cleanup()
@@ -564,15 +568,16 @@ func (c *CVDownloader) cleanDirs() {
 	})
 }
 
-func NewCVDownloader(ctx context.Context, chdb ch.CHDB, workPath, APIKey string, imageTypes []string, keepDownloadedImages, sendExistingImages bool, finishedDownloadQueue chan Download) *CVDownloader {
+func NewCVDownloader(ctx context.Context, bufPool *sync.Pool, chdb ch.CHDB, workPath, APIKey string, imageTypes []string, keepDownloadedImages, sendExistingImages bool, finishedDownloadQueue chan Download) *CVDownloader {
 	return &CVDownloader{
 		Context:               ctx,
 		JSONPath:              filepath.Join(workPath, "_json"),
 		ImagePath:             filepath.Join(workPath, "_image"),
 		APIKey:                APIKey,
-		downloadQueue:         make(chan *CVResult, 1000),
+		downloadQueue:         make(chan *CVResult, 1000), // This is just json it shouldn't take up much more than 122 MB
-		imageDownloads:        make(chan download, 250),
+		imageDownloads:        make(chan download, 250),   // These are just URLs should only take a few MB
-		notFound:              make(chan download, 100),
+		notFound:              make(chan download, 100),   // Same here
+		bufPool:               bufPool,                    // Only used if keepDownloadedImages is false to save space on byte buffers. The buffers get sent back via finishedDownloadQueue
 		FinishedDownloadQueue: finishedDownloadQueue,
 		SendExistingImages:    sendExistingImages,
 		KeepDownloadedImages:  keepDownloadedImages,