compiler: fix handling of struct, array and interface values.

As outlined in #661, the compiler fails to generate code to copy a struct/array value for an assignment when the target's underlying type is an interface type, whether for explicit variable assignments, implicit function/method parameters etc. Instead, taking the example of explicit variable assignment, the interface variable is assigned a value that contains the same pointer to the source struct/array val (we're in Javascript world, so everything is a pointer). This means that changes to the struct/array value via the source variable are, incorrectly, visible via the target variable. #661 gives a simple example. There is a further issue when interface values are assigned to interface-typed variables: struct/array values are not copied when they should be. Fixes #661.
gopherjs · myitcv · Jul 31, 2017 · Apr 15, 2018 · Apr 20, 2018 · Apr 21, 2018
commit e8d039e22128550f1be5fcdd5dc4388b37314efe
diff --git a/compiler/expressions.go b/compiler/expressions.go
@@ -812,6 +812,11 @@ func (c *funcContext) makeReceiver(e *ast.SelectorExpr) *expression {
 
 	_, isPointer := recvType.Underlying().(*types.Pointer)
 	methodsRecvType := sel.Obj().Type().(*types.Signature).Recv().Type()
+
+	if _, isInterface := methodsRecvType.Underlying().(*types.Interface); isInterface {
+		return c.formatExpr("$copyInterfaceVal(%e)", x)
+	}
+
 	_, pointerExpected := methodsRecvType.(*types.Pointer)
 	if !isPointer && pointerExpected {
 		recvType = types.NewPointer(recvType)
@@ -825,6 +830,7 @@ func (c *funcContext) makeReceiver(e *ast.SelectorExpr) *expression {
 	if isWrapped(recvType) {
 		recv = c.formatExpr("new %s(%s)", c.typeName(methodsRecvType), recv)
 	}
+
 	return recv
 }
 
@@ -1130,12 +1136,17 @@ func (c *funcContext) translateImplicitConversion(expr ast.Expr, desiredType typ
 			// wrap JS object into js.Object struct when converting to interface
 			return c.formatExpr("new $jsObjectPtr(%e)", expr)
 		}
+
+		switch exprType.Underlying().(type) {
+		case *types.Array:
+			return c.formatExpr("new %1s($clone(%e, %1s))", c.typeName(exprType), expr)
+		case *types.Struct:
+			return c.formatExpr("new %1e.constructor.elem($clone(%1e, %s))", expr, c.typeName(exprType))
+		}
+
 		if isWrapped(exprType) {
 			return c.formatExpr("new %s(%e)", c.typeName(exprType), expr)
 		}
-		if _, isStruct := exprType.Underlying().(*types.Struct); isStruct {
-			return c.formatExpr("new %1e.constructor.elem(%1e)", expr)
-		}
 	}
 
 	return c.translateExpr(expr)

diff --git a/compiler/prelude/prelude.go b/compiler/prelude/prelude.go
@@ -296,6 +296,14 @@ var $clone = function(src, type) {
   return clone;
 };
 
+var $copyInterfaceVal = function(src) {
+	if (src.constructor.copy) {
+		return new src.constructor($clone(src.$val, src.constructor));
+	}
+
+	return src;
+};
+
 var $pointerOfStructConversion = function(obj, type) {
   if(obj.$proxies === undefined) {
     obj.$proxies = {};

diff --git a/compiler/prelude/prelude_min.go b/compiler/prelude/prelude_min.go
diff --git a/tests/interface_test.go b/tests/interface_test.go
@@ -0,0 +1,88 @@
+package tests
+
+import (
+	"fmt"
+	"testing"
+)
+
+type Struct struct {
+	Name string
+}
+
+func (s Struct) SetName(n string) {
+	s.Name = n
+}
+
+type SetName interface {
+	SetName(n string)
+}
+
+func TestAssignStructValInterface(t *testing.T) {
+	s := Struct{
+		Name: "Rob",
+	}
+
+	var i1 interface{} = s
+	var i2 interface{} = i1
+
+	s.Name = "Pike"
+
+	ss := fmt.Sprintf("%#v", s)
+	i1s := fmt.Sprintf("%#v", i1)
+	i2s := fmt.Sprintf("%#v", i2)
+
+	if exp := "tests.Struct{Name:\"Pike\"}"; ss != exp {
+		t.Fatalf("ss should have been %q; got %q", exp, ss)
+	}
+
+	iexp := "tests.Struct{Name:\"Rob\"}"
+
+	if i1s != iexp {
+		t.Fatalf("is should have been %q; got %q", iexp, i1s)
+	}
+
+	if i2s != iexp {
+		t.Fatalf("is should have been %q; got %q", iexp, i2s)
+	}
+}
+
+func TestStructValInterfaceMethodCall(t *testing.T) {
+	var i SetName = Struct{
+		Name: "Rob",
+	}
+
+	i.SetName("Pike")
+
+	is := fmt.Sprintf("%#v", i)
+
+	if exp := "tests.Struct{Name:\"Rob\"}"; is != exp {
+		t.Fatalf("is should have been %q; got %q", exp, is)
+	}
+}
+
+func TestAssignArrayInterface(t *testing.T) {
+	a := [2]int{1, 2}
+
+	var i1 interface{} = a
+	var i2 interface{} = i1
+
+	a[0] = 0
+
+	as := fmt.Sprintf("%#v", a)
+	i1s := fmt.Sprintf("%#v", i1)
+	i2s := fmt.Sprintf("%#v", i2)
+
+	if exp := "[2]int{0, 2}"; as != exp {
+		t.Fatalf("ss should have been %q; got %q", exp, as)
+	}
+
+	iexp := "[2]int{1, 2}"
+
+	if i1s != iexp {
+		t.Fatalf("is should have been %q; got %q", iexp, i1s)
+	}
+
+	if i2s != iexp {
+		t.Fatalf("is should have been %q; got %q", iexp, i2s)
+	}
+}