From 29cda5112a41f4aeb4ace1bd8722e9535e605049 Mon Sep 17 00:00:00 2001
From: Roman Khimov <roman@nspcc.ru>
Date: Fri, 19 Nov 2021 20:02:32 +0300
Subject: [PATCH] vm: simplify NEF loading by providing a special method

Offsets are still handled outside of VM to avoid introducing manifest
dependency (that's likely to be circular).
---
 pkg/core/blockchain.go            | 14 +++++++-------
 pkg/core/interop/contract/call.go | 19 +++++++++----------
 pkg/vm/vm.go                      | 21 ++++++++++++++++++---
 3 files changed, 34 insertions(+), 20 deletions(-)

diff --git a/pkg/core/blockchain.go b/pkg/core/blockchain.go
index 6e8541f73..df7c1adfd 100644
--- a/pkg/core/blockchain.go
+++ b/pkg/core/blockchain.go
@@ -2185,14 +2185,14 @@ func (bc *Blockchain) InitVerificationVM(v *vm.VM, getContract func(util.Uint160
 		if md == nil || md.ReturnType != smartcontract.BoolType {
 			return ErrInvalidVerificationContract
 		}
-		initMD := cs.Manifest.ABI.GetMethod(manifest.MethodInit, 0)
-		v.LoadScriptWithHash(cs.NEF.Script, hash, callflag.ReadOnly)
-		v.Context().NEF = &cs.NEF
-		v.Context().Jump(md.Offset)
-
-		if initMD != nil {
-			v.Call(initMD.Offset)
+		verifyOffset := md.Offset
+		initOffset := -1
+		md = cs.Manifest.ABI.GetMethod(manifest.MethodInit, 0)
+		if md != nil {
+			initOffset = md.Offset
 		}
+		v.LoadNEFMethod(&cs.NEF, util.Uint160{}, hash, callflag.ReadOnly,
+			true, verifyOffset, initOffset)
 	}
 	if len(witness.InvocationScript) != 0 {
 		err := vm.IsScriptCorrect(witness.InvocationScript, nil)
diff --git a/pkg/core/interop/contract/call.go b/pkg/core/interop/contract/call.go
index c9c7bbc5a..484a857cb 100644
--- a/pkg/core/interop/contract/call.go
+++ b/pkg/core/interop/contract/call.go
@@ -112,20 +112,19 @@ func callExFromNative(ic *interop.Context, caller util.Uint160, cs *state.Contra
 		return fmt.Errorf("invalid argument count: %d (expected %d)", len(args), len(md.Parameters))
 	}
 
+	methodOff := md.Offset
+	initOff := -1
+	md = cs.Manifest.ABI.GetMethod(manifest.MethodInit, 0)
+	if md != nil {
+		initOff = md.Offset
+	}
 	ic.VM.Invocations[cs.Hash]++
-	ic.VM.LoadScriptWithCallingHash(caller, cs.NEF.Script, cs.Hash, ic.VM.Context().GetCallFlags()&f, hasReturn)
-	ic.VM.Context().NEF = &cs.NEF
+	ic.VM.LoadNEFMethod(&cs.NEF, caller, cs.Hash, ic.VM.Context().GetCallFlags()&f,
+		hasReturn, methodOff, initOff)
+
 	for e, i := ic.VM.Estack(), len(args)-1; i >= 0; i-- {
 		e.PushItem(args[i])
 	}
-	// Move IP to the target method.
-	ic.VM.Context().Jump(md.Offset)
-
-	md = cs.Manifest.ABI.GetMethod(manifest.MethodInit, 0)
-	if md != nil {
-		ic.VM.Call(md.Offset)
-	}
-
 	return nil
 }
 
diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index 855aa7cb4..42209fb5f 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -297,12 +297,27 @@ func (v *VM) LoadScriptWithFlags(b []byte, f callflag.CallFlag) {
 // each other.
 func (v *VM) LoadScriptWithHash(b []byte, hash util.Uint160, f callflag.CallFlag) {
 	shash := v.GetCurrentScriptHash()
-	v.LoadScriptWithCallingHash(shash, b, hash, f, true)
+	v.loadScriptWithCallingHash(shash, b, hash, f, true)
 }
 
-// LoadScriptWithCallingHash is similar to LoadScriptWithHash but sets calling hash explicitly.
+// LoadNEFMethod allows to create a context to execute a method from the NEF
+// file with specified caller and executing hash, call flags, return value,
+// method and _initialize offsets.
+func (v *VM) LoadNEFMethod(exe *nef.File, caller util.Uint160, hash util.Uint160, f callflag.CallFlag,
+	hasReturn bool, methodOff int, initOff int) {
+	v.loadScriptWithCallingHash(caller, exe.Script, hash, f, hasReturn)
+	ctx := v.Context()
+	ctx.NEF = exe
+	// Move IP to the target method.
+	ctx.Jump(methodOff)
+	if initOff >= 0 {
+		v.Call(initOff)
+	}
+}
+
+// loadScriptWithCallingHash is similar to LoadScriptWithHash but sets calling hash explicitly.
 // It should be used for calling from native contracts.
-func (v *VM) LoadScriptWithCallingHash(caller util.Uint160, b []byte, hash util.Uint160,
+func (v *VM) loadScriptWithCallingHash(caller util.Uint160, b []byte, hash util.Uint160,
 	f callflag.CallFlag, hasReturn bool) {
 	v.LoadScriptWithFlags(b, f)
 	ctx := v.Context()