[#242] container: allow to fetch total amount of countainers

There are 2 approaches:
1. Use `storage.Find` to enumerate all containers.
2. Store a counter by a separate key.

Here we implemented 1, because this method is readonly (thus GAS cost it
not a problem) and simpler to implement.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

container/config.yml

@@ -1,5 +1,5 @@
 name: "NeoFS Container"
-safemethods: ["get", "owner", "list", "eACL", "getContainerSize", "listContainerSizes", "version"]
+safemethods: ["count", "get", "owner", "list", "eACL", "getContainerSize", "listContainerSizes", "version"]
 permissions:
   - methods: ["update", "addKey", "transferX",
                "addRoot", "register", "addRecord", "deleteRecords"]

container/container_contract.go

@@ -387,6 +387,21 @@ func Owner(containerID []byte) []byte {
 	return owner
 }
 
+// Count method returns the number of registered containers.
+func Count() int {
+	count := 0
+	ctx := storage.GetReadOnlyContext()
+	it := storage.Find(ctx, []byte{}, storage.KeysOnly)
+	for iterator.Next(it) {
+		key := iterator.Value(it).([]byte)
+		// V2 format
+		if len(key) == containerIDSize {
+			count++
+		}
+	}
+	return count
+}
+
 // List method returns a list of all container IDs owned by the specified owner.
 func List(owner []byte) [][]byte {
 	ctx := storage.GetReadOnlyContext()

tests/container_test.go

@@ -79,6 +79,39 @@ func dummyContainer(owner neotest.Signer) testContainer {
 	}
 }
 
+func TestContainerCount(t *testing.T) {
+	c, cBal, _ := newContainerInvoker(t)
+
+	checkCount := func(t *testing.T, expected int64) {
+		s, err := c.TestInvoke(t, "count")
+		require.NoError(t, err)
+		bi := s.Pop().BigInt()
+		require.True(t, bi.IsInt64())
+		require.Equal(t, int64(expected), bi.Int64())
+	}
+
+	checkCount(t, 0)
+	acc1, cnt1 := addContainer(t, c, cBal)
+	checkCount(t, 1)
+
+	_, cnt2 := addContainer(t, c, cBal)
+	checkCount(t, 2)
+
+	// Same owner.
+	cnt3 := dummyContainer(acc1)
+	balanceMint(t, cBal, acc1, containerFee*1, []byte{})
+	c.Invoke(t, stackitem.Null{}, "put", cnt3.value, cnt3.sig, cnt3.pub, cnt3.token)
+
+	c.Invoke(t, stackitem.Null{}, "delete", cnt1.id[:], cnt1.sig, cnt1.token)
+	checkCount(t, 2)
+
+	c.Invoke(t, stackitem.Null{}, "delete", cnt2.id[:], cnt2.sig, cnt2.token)
+	checkCount(t, 1)
+
+	c.Invoke(t, stackitem.Null{}, "delete", cnt3.id[:], cnt3.sig, cnt3.token)
+	checkCount(t, 0)
+}
+
 func TestContainerPut(t *testing.T) {
 	c, cBal, _ := newContainerInvoker(t)