contract/vm: only push NULL after call in dynamic contexts

And determine the need for Null dynamically. For some reason the only dynamic
context is Contract.Call. CALLT is not dynamic and neither is a call from
native contract, go figure...

pkg/core/interop/contract/call.go

@@ -14,6 +14,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/callflag"
 	"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"
 )
 
@@ -67,11 +68,11 @@ func Call(ic *interop.Context) error {
 		return fmt.Errorf("method not found: %s/%d", method, len(args))
 	}
 	hasReturn := md.ReturnType != smartcontract.VoidType
-	return callInternal(ic, cs, method, fs, hasReturn, args, !hasReturn)
+	return callInternal(ic, cs, method, fs, hasReturn, args, true)
 }
 
 func callInternal(ic *interop.Context, cs *state.Contract, name string, f callflag.CallFlag,
-	hasReturn bool, args []stackitem.Item, pushNullOnUnloading bool) error {
+	hasReturn bool, args []stackitem.Item, isDynamic bool) error {
 	md := cs.Manifest.ABI.GetMethod(name, len(args))
 	if md.Safe {
 		f &^= (callflag.WriteStates | callflag.AllowNotify)
@@ -83,12 +84,12 @@ func callInternal(ic *interop.Context, cs *state.Contract, name string, f callfl
 			}
 		}
 	}
-	return callExFromNative(ic, ic.VM.GetCurrentScriptHash(), cs, name, args, f, hasReturn, pushNullOnUnloading, false)
+	return callExFromNative(ic, ic.VM.GetCurrentScriptHash(), cs, name, args, f, hasReturn, isDynamic, false)
 }
 
 // callExFromNative calls a contract with flags using the provided calling hash.
 func callExFromNative(ic *interop.Context, caller util.Uint160, cs *state.Contract,
-	name string, args []stackitem.Item, f callflag.CallFlag, hasReturn bool, pushNullOnUnloading bool, callFromNative bool) error {
+	name string, args []stackitem.Item, f callflag.CallFlag, hasReturn bool, isDynamic bool, callFromNative bool) error {
 	for _, nc := range ic.Natives {
 		if nc.Metadata().Name == nativenames.Policy {
 			var pch = nc.(policyChecker)
@@ -123,7 +124,7 @@ func callExFromNative(ic *interop.Context, caller util.Uint160, cs *state.Contra
 	if wrapped {
 		ic.DAO = ic.DAO.GetPrivate()
 	}
-	onUnload := func(commit bool) error {
+	onUnload := func(ctx *vm.Context, commit bool) error {
 		if wrapped {
 			if commit {
 				_, err := ic.DAO.Persist()
@@ -135,8 +136,13 @@ func callExFromNative(ic *interop.Context, caller util.Uint160, cs *state.Contra
 			}
 			ic.DAO = baseDAO
 		}
-		if pushNullOnUnloading && commit {
-			ic.VM.Context().Estack().PushItem(stackitem.Null{}) // Must use current context stack.
+		if isDynamic && commit {
+			eLen := ctx.Estack().Len()
+			if eLen == 0 && ctx.NumOfReturnVals() == 0 { // No return value and none expected.
+				ic.VM.Context().Estack().PushItem(stackitem.Null{}) // Must use current context stack.
+			} else if eLen > 1 { // 1 or -1 (all) retrun values expected, but only one can be returned.
+				return errors.New("multiple return values in a cross-contract call")
+			} // All other rvcount/stack length mismatches are checked by the VM.
 		}
 		if callFromNative && !commit {
 			return fmt.Errorf("unhandled exception")

pkg/vm/context.go

@@ -73,7 +73,7 @@ type Context struct {
 }
 
 // ContextUnloadCallback is a callback method used on context unloading from istack.
-type ContextUnloadCallback func(commit bool) error
+type ContextUnloadCallback func(ctx *Context, commit bool) error
 
 var errNoInstParam = errors.New("failed to read instruction parameter")
 
@@ -239,6 +239,11 @@ func (c *Context) GetNEF() *nef.File {
 	return c.sc.NEF
 }
 
+// NumOfReturnVals returns the number of return values expected from this context.
+func (c *Context) NumOfReturnVals() int {
+	return c.retCount
+}
+
 // Value implements the stackitem.Item interface.
 func (c *Context) Value() interface{} {
 	return c

pkg/vm/vm.go

@@ -1634,7 +1634,7 @@ func (v *VM) unloadContext(ctx *Context) {
 			ctx.sc.static.ClearRefs(&v.refs)
 		}
 		if ctx.sc.onUnload != nil {
-			err := ctx.sc.onUnload(v.uncaughtException == nil)
+			err := ctx.sc.onUnload(ctx, v.uncaughtException == nil)
 			if err != nil {
 				errMessage := fmt.Sprintf("context unload callback failed: %s", err)
 				if v.uncaughtException != nil {