Save new packs in index atomically

This commit fixes a situation reported by a user where two indexes
contained information about the same pack without overlap, e.g.:

Index 3e6a32 contained:

    {
      "id": "c02e3b",
      "blobs": [
        {
          "id": "8114b1",
          "type": "data",
          "offset": 0,
          "length": 530107
        }
      ]
    }

And index 62da5f contained:

    {
      "id": "c02e3b",
      "blobs": [
        {
          "id": "e344f8",
          "type": "data",
          "offset": 1975848,
          "length": 3426468
        },
        {
          "id": "939ed9",
          "type": "data",
          "offset": 530107,
          "length": 1445741
        }
      ]
    }

This commit adds all blobs in a pack in one atomic operation so that
intermediate such as these do not happen.

repository/index.go

@@ -109,6 +109,22 @@ func (idx *Index) Store(blob PackedBlob) {
 	idx.store(blob)
 }
 
+// StoreBlobs saves information about the blobs to the index in one atomic transaction.
+func (idx *Index) StoreBlobs(blobs []PackedBlob) {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+
+	if idx.final {
+		panic("store new item in finalized index")
+	}
+
+	debug.Log("Index.StoreBlobs", "stored %d blobs", len(blobs))
+
+	for _, blob := range blobs {
+		idx.store(blob)
+	}
+}
+
 // Lookup queries the index for the blob ID and returns a PackedBlob.
 func (idx *Index) Lookup(id backend.ID) (pb PackedBlob, err error) {
 	idx.m.Lock()

repository/repository.go

@@ -268,9 +268,9 @@ func (r *Repository) savePacker(p *pack.Packer) error {
 	debug.Log("Repo.savePacker", "saved as %v", sid.Str())
 
 	// update blobs in the index
+	var packedBlobs []PackedBlob
 	for _, b := range p.Blobs() {
-		debug.Log("Repo.savePacker", "  updating blob %v to pack %v", b.ID.Str(), sid.Str())
-		r.idx.Current().Store(PackedBlob{
+		packedBlobs = append(packedBlobs, PackedBlob{
 			Type:   b.Type,
 			ID:     b.ID,
 			PackID: sid,
@@ -279,6 +279,7 @@ func (r *Repository) savePacker(p *pack.Packer) error {
 		})
 		r.idx.RemoveFromInFlight(b.ID)
 	}
+	r.idx.Current().StoreBlobs(packedBlobs)
 
 	return nil
 }