core: add System.Contract.GetCallFlags interop

Part of #1055.

It returns calling flags of the current context.

pkg/compiler/syscall.go

@@ -25,6 +25,7 @@ var syscalls = map[string]map[string]Syscall{
 		"CreateStandardAccount": {"System.Contract.CreateStandardAccount", false},
 		"Destroy":               {"System.Contract.Destroy", false},
 		"IsStandard":            {"System.Contract.IsStandard", false},
+		"GetCallFlags":          {"System.Contract.GetCallFlags", false},
 		"Update":                {"System.Contract.Update", false},
 	},
 	"crypto": {

pkg/core/interop_system.go

@@ -530,3 +530,9 @@ func contractCreateStandardAccount(ic *interop.Context, v *vm.VM) error {
 	v.Estack().PushVal(p.GetScriptHash().BytesBE())
 	return nil
 }
+
+// contractGetCallFlags returns current context calling flags.
+func contractGetCallFlags(_ *interop.Context, v *vm.VM) error {
+	v.Estack().PushVal(v.Context().GetCallFlags())
+	return nil
+}

pkg/core/interop_system_test.go

@@ -582,3 +582,12 @@ func TestContractUpdate(t *testing.T) {
 		require.Error(t, err)
 	})
 }
+
+func TestContractGetCallFlags(t *testing.T) {
+	v, ic, bc := createVM(t)
+	defer bc.Close()
+
+	v.LoadScriptWithHash([]byte{byte(opcode.RET)}, util.Uint160{1, 2, 3}, smartcontract.All)
+	require.NoError(t, contractGetCallFlags(ic, v))
+	require.Equal(t, int64(smartcontract.All), v.Estack().Pop().Value().(*big.Int).Int64())
+}

pkg/core/interops.go

@@ -93,6 +93,7 @@ var systemInterops = []interop.Function{
 	{Name: "System.Contract.Destroy", Func: contractDestroy, Price: 1000000,
 		AllowedTriggers: trigger.Application, RequiredFlags: smartcontract.AllowModifyStates},
 	{Name: "System.Contract.IsStandard", Func: contractIsStandard, Price: 30000},
+	{Name: "System.Contract.GetCallFlags", Func: contractGetCallFlags, Price: 30000},
 	{Name: "System.Contract.Update", Func: contractUpdate, Price: 0,
 		AllowedTriggers: trigger.Application, RequiredFlags: smartcontract.AllowModifyStates},
 	{Name: "System.Enumerator.Concat", Func: enumerator.Concat, Price: 400},

pkg/interop/contract/contract.go

@@ -49,3 +49,9 @@ func IsStandard(h []byte) bool {
 func CreateStandardAccount(pub []byte) []byte {
 	return nil
 }
+
+// GetCallFlags returns calling flags which execution context was created with.
+// This function uses `System.Contract.GetCallFlags` syscall.
+func GetCallFlags() int64 {
+	return 0
+}