repository: store oversized blobs in separate pack files

Store oversized blobs in separate pack files as the blobs is large
enough to warrant its own pack file. This simplifies the garbage
collection of such blobs and keeps the cache smaller, as oversize (tree)
blobs only have to be downloaded if they are actually used.

internal/repository/packer_manager_test.go

@@ -89,6 +89,24 @@ func testPackerManager(t testing.TB) int64 {
 	return int64(bytes)
 }
 
+func TestPackerManagerWithOversizeBlob(t *testing.T) {
+	packFiles := int(0)
+	sizeLimit := uint(512 * 1024)
+	pm := newPackerManager(crypto.NewRandomKey(), restic.DataBlob, sizeLimit, func(ctx context.Context, tp restic.BlobType, p *Packer) error {
+		packFiles++
+		return nil
+	})
+
+	for _, i := range []uint{sizeLimit / 2, sizeLimit, sizeLimit / 3} {
+		_, err := pm.SaveBlob(context.TODO(), restic.DataBlob, restic.ID{}, make([]byte, i), 0)
+		test.OK(t, err)
+	}
+	test.OK(t, pm.Flush(context.TODO()))
+
+	// oversized blob must be stored in a separate packfile
+	test.Equals(t, packFiles, 2)
+}
+
 func BenchmarkPackerManager(t *testing.B) {
 	// Run testPackerManager if it hasn't run already, to set totalSize.
 	once.Do(func() {