[#42] container: Make GAS costs more predictable in Delete()

Persisting a transaction is done in 2 stages:
1. TestInvoke
2. Sign and send to the network.
3. At some point the tx is persisted.
Some time passes between 1 and 3, this could lead to different GAS
costs. It is a known issue for container delete: different epoch can
have different size in bytes and thus different cost to store.
Here we introduce fixed-length encoding for integers, so that the
problem can be avoided.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

container/container_contract.go

@@ -343,6 +343,11 @@ type DelInfo struct {
 	Epoch int
 }
 
+type delInfo struct {
+	Owner []byte
+	Epoch []byte
+}
+
 // DeletionInfo method returns container deletion info.
 // If the container had never existed, NotFoundError is throwed.
 // It can be used to check whether non-existing container was indeed deleted
@@ -354,7 +359,12 @@ func DeletionInfo(containerID []byte) DelInfo {
 	if data == nil {
 		panic(NotFoundError)
 	}
-	return std.Deserialize(data).(DelInfo)
+
+	d := std.Deserialize(data).(delInfo)
+	return DelInfo{
+		Owner: d.Owner,
+		Epoch: common.FromFixedWidth64(d.Epoch),
+	}
 }
 
 // Get method returns a structure that contains a stable marshaled Container structure,
@@ -631,9 +641,9 @@ func removeContainer(ctx storage.Context, id []byte, owner []byte) {
 	graveKey := append([]byte{graveKeyPrefix}, id...)
 	netmapContractAddr := storage.Get(ctx, netmapContractKey).(interop.Hash160)
 	epoch := contract.Call(netmapContractAddr, "epoch", contract.ReadOnly).(int)
-	common.SetSerialized(ctx, graveKey, DelInfo{
+	common.SetSerialized(ctx, graveKey, delInfo{
 		Owner: owner,
-		Epoch: epoch,
+		Epoch: common.ToFixedWidth64(epoch),
 	})
 }
 

tests/container_test.go

@@ -283,8 +283,6 @@ func TestContainerDelete(t *testing.T) {
 	})
 
 	t.Run("gas costs are the same for different epochs", func(t *testing.T) {
-		t.Skip()
-
 		_, cnt2 := addContainer(t, c, cBal)
 		args := []interface{}{cnt2.id[:], cnt2.sig, cnt2.pub, cnt2.token}