core: implement System.Callback.CreateFromMethod interop

Support creating callbacks from contract methods.

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

pkg/core/interop/callback/method.go

@@ -0,0 +1,60 @@
+package callback
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/nspcc-dev/neo-go/pkg/core/interop"
+	"github.com/nspcc-dev/neo-go/pkg/core/state"
+	"github.com/nspcc-dev/neo-go/pkg/smartcontract/manifest"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neo-go/pkg/vm"
+	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
+)
+
+// MethodCallback represents callback for contract method.
+type MethodCallback struct {
+	contract *state.Contract
+	method   *manifest.Method
+}
+
+var _ Callback = (*MethodCallback)(nil)
+
+// ArgCount implements Callback interface.
+func (s *MethodCallback) ArgCount() int {
+	return len(s.method.Parameters)
+}
+
+// LoadContext implements Callback interface.
+func (s *MethodCallback) LoadContext(v *vm.VM, args []stackitem.Item) {
+	v.Estack().PushVal(args)
+	v.Estack().PushVal(s.method.Name)
+	v.Estack().PushVal(s.contract.ScriptHash().BytesBE())
+}
+
+// CreateFromMethod creates callback for a contract method.
+func CreateFromMethod(ic *interop.Context, v *vm.VM) error {
+	rawHash := v.Estack().Pop().Bytes()
+	h, err := util.Uint160DecodeBytesBE(rawHash)
+	if err != nil {
+		return err
+	}
+	cs, err := ic.DAO.GetContractState(h)
+	if err != nil {
+		return errors.New("contract not found")
+	}
+	method := string(v.Estack().Pop().Bytes())
+	if strings.HasPrefix(method, "_") {
+		return errors.New("invalid method name")
+	}
+	currCs, err := ic.DAO.GetContractState(v.GetCurrentScriptHash())
+	if err == nil && !currCs.Manifest.CanCall(&cs.Manifest, method) {
+		return errors.New("method call is not allowed")
+	}
+	md := cs.Manifest.ABI.GetMethod(method)
+	v.Estack().PushVal(stackitem.NewInterop(&MethodCallback{
+		contract: cs,
+		method:   md,
+	}))
+	return nil
+}