compiler/engine: add dynamic APPCALL generation, fix #914

Previously we could generate dynamic appcall with a kludge of
    AppCall([]byte{/* 20 zeroes */, realScriptHash, args...)

Now there is a separate function for this.

pkg/compiler/analysis.go

@@ -16,7 +16,7 @@ var (
 		"SHA1", "Hash256", "Hash160",
 		"VerifySignature", "AppCall",
 		"FromAddress", "Equals",
-		"panic",
+		"panic", "DynAppCall",
 	}
 )
 

pkg/compiler/codegen.go

@@ -1085,14 +1085,23 @@ func (c *codegen) convertBuiltin(expr *ast.CallExpr) {
 		emit.Opcode(c.prog.BinWriter, opcode.HASH160)
 	case "VerifySignature":
 		emit.Opcode(c.prog.BinWriter, opcode.VERIFY)
-	case "AppCall":
-		numArgs := len(expr.Args) - 1
+	case "AppCall", "DynAppCall":
+		numArgs := len(expr.Args)
+		if name == "AppCall" {
+			numArgs--
+		}
 		c.emitReverse(numArgs)
 
 		emit.Opcode(c.prog.BinWriter, opcode.APPCALL)
-		buf := c.getByteArray(expr.Args[0])
-		if len(buf) != 20 {
-			c.prog.Err = errors.New("invalid script hash")
+		var buf []byte
+		if name == "AppCall" {
+			buf = c.getByteArray(expr.Args[0])
+			if len(buf) != 20 {
+				c.prog.Err = errors.New("invalid script hash")
+			}
+		} else {
+			// Zeroes for DynAppCall.
+			buf = make([]byte, 20)
 		}
 
 		c.prog.WriteBytes(buf)

pkg/compiler/interop_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
 	"github.com/nspcc-dev/neo-go/pkg/encoding/address"
 	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neo-go/pkg/vm"
 	"github.com/stretchr/testify/require"
 )
 
@@ -52,6 +53,19 @@ func TestFromAddress(t *testing.T) {
 }
 
 func TestAppCall(t *testing.T) {
+	const srcDynApp = `
+	package foo
+	import "github.com/nspcc-dev/neo-go/pkg/interop/engine"
+	func Main(h []byte) []byte {
+		x := []byte{1, 2}
+		y := []byte{3, 4}
+		result := engine.DynAppCall(h, x, y)
+		return result.([]byte)
+	}
+`
+
+	var hasDynamicInvoke bool
+
 	srcInner := `
 	package foo
 	func Main(a []byte, b []byte) []byte {
@@ -62,14 +76,31 @@ func TestAppCall(t *testing.T) {
 	inner, err := compiler.Compile(strings.NewReader(srcInner))
 	require.NoError(t, err)
 
+	dynapp, err := compiler.Compile(strings.NewReader(srcDynApp))
+	require.NoError(t, err)
+
 	ih := hash.Hash160(inner)
+	dh := hash.Hash160(dynapp)
 	getScript := func(u util.Uint160) ([]byte, bool) {
 		if u.Equals(ih) {
 			return inner, true
 		}
+		if u.Equals(dh) {
+			return dynapp, hasDynamicInvoke
+		}
 		return nil, false
 	}
 
+	dynEntryScript := `
+	package foo
+	import "github.com/nspcc-dev/neo-go/pkg/interop/engine"
+	func Main(h []byte) interface{} {
+		return engine.AppCall(` + fmt.Sprintf("%#v", dh.BytesBE()) + `, h)
+	}
+`
+	dynentry, err := compiler.Compile(strings.NewReader(dynEntryScript))
+	require.NoError(t, err)
+
 	t.Run("valid script", func(t *testing.T) {
 		src := getAppCallScript(fmt.Sprintf("%#v", ih.BytesBE()))
 		v := vmAndCompile(t, src)
@@ -118,6 +149,38 @@ func TestAppCall(t *testing.T) {
 
 		assertResult(t, v, []byte{1, 2, 3, 4})
 	})
+
+	t.Run("dynamic", func(t *testing.T) {
+		t.Run("valid script", func(t *testing.T) {
+			hasDynamicInvoke = true
+			v := vm.New()
+			v.Load(dynentry)
+			v.SetScriptGetter(getScript)
+			v.Estack().PushVal(ih.BytesBE())
+
+			require.NoError(t, v.Run())
+
+			assertResult(t, v, []byte{1, 2, 3, 4})
+		})
+		t.Run("invalid script", func(t *testing.T) {
+			hasDynamicInvoke = true
+			v := vm.New()
+			v.Load(dynentry)
+			v.SetScriptGetter(getScript)
+			v.Estack().PushVal([]byte{1})
+
+			require.Error(t, v.Run())
+		})
+		t.Run("no dynamic invoke", func(t *testing.T) {
+			hasDynamicInvoke = false
+			v := vm.New()
+			v.Load(dynentry)
+			v.SetScriptGetter(getScript)
+			v.Estack().PushVal(ih.BytesBE())
+
+			require.Error(t, v.Run())
+		})
+	})
 }
 
 func getAppCallScript(h string) string {

pkg/interop/engine/engine.go

@@ -54,3 +54,13 @@ func GetEntryScriptHash() []byte {
 func AppCall(scriptHash []byte, args ...interface{}) interface{} {
 	return nil
 }
+
+// DynAppCall executes previously deployed blockchain contract with specified
+// hash (160 bit in BE form represented as 20-byte slice) using provided
+// arguments. It returns whatever this contract returns. It differs from AppCall
+// in that you can use it for truly dynamic scriptHash values, but at the same
+// time using it requires HasDynamicInvoke property set for a contract doing
+// this call. This function uses `APPCALL` opcode.
+func DynAppCall(scriptHash []byte, args ...interface{}) interface{} {
+	return nil
+}