fix resumable downloads

glob returns files in lexical order which is not appropriate when
rebuilding the parts list

server/download.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -39,9 +40,18 @@ type blobDownload struct {
 }
 
 type blobDownloadPart struct {
+	N         int
 	Offset    int64
 	Size      int64
 	Completed int64
+
+	*blobDownload `json:"-"`
+}
+
+func (p *blobDownloadPart) Name() string {
+	return strings.Join([]string{
+		p.blobDownload.Name, "partial", strconv.Itoa(p.N),
+	}, "-")
 }
 
 func (b *blobDownload) Prepare(ctx context.Context, requestURL *url.URL, opts *RegistryOptions) error {
@@ -78,14 +88,10 @@ func (b *blobDownload) Prepare(ctx context.Context, requestURL *url.URL, opts *R
 				size = b.Total - offset
 			}
 
-			partName := b.Name + "-partial-" + strconv.Itoa(len(b.Parts))
-			part := blobDownloadPart{Offset: offset, Size: size}
-			if err := b.writePart(partName, &part); err != nil {
+			if err := b.newPart(offset, size); err != nil {
 				return err
 			}
 
-			b.Parts = append(b.Parts, &part)
-
 			offset += size
 		}
 	}
@@ -161,7 +167,6 @@ func (b *blobDownload) Run(ctx context.Context, requestURL *url.URL, opts *Regis
 func (b *blobDownload) downloadChunk(ctx context.Context, requestURL *url.URL, i int, opts *RegistryOptions) error {
 	part := b.Parts[i]
 
-	partName := b.File.Name() + "-" + strconv.Itoa(i)
 	offset := part.Offset + part.Completed
 	w := io.NewOffsetWriter(b.File, offset)
 
@@ -181,7 +186,7 @@ func (b *blobDownload) downloadChunk(ctx context.Context, requestURL *url.URL, i
 	}
 
 	part.Completed += n
-	if err := b.writePart(partName, part); err != nil {
+	if err := b.writePart(part.Name(), part); err != nil {
 		return err
 	}
 
@@ -189,6 +194,16 @@ func (b *blobDownload) downloadChunk(ctx context.Context, requestURL *url.URL, i
 	return err
 }
 
+func (b *blobDownload) newPart(offset, size int64) error {
+	part := blobDownloadPart{blobDownload: b, Offset: offset, Size: size, N: len(b.Parts)}
+	if err := b.writePart(part.Name(), &part); err != nil {
+		return err
+	}
+
+	b.Parts = append(b.Parts, &part)
+	return nil
+}
+
 func (b *blobDownload) readPart(partName string) (*blobDownloadPart, error) {
 	var part blobDownloadPart
 	partFile, err := os.Open(partName)
@@ -201,6 +216,7 @@ func (b *blobDownload) readPart(partName string) (*blobDownloadPart, error) {
 		return nil, err
 	}
 
+	part.blobDownload = b
 	return &part, nil
 }