terminal/starbind: allow modification of structs returned by API (#3872)

Structs returned to starlark scripts by API calls were immutable, this
made amend_breakpoint nearly impossible to use since its argument must
be a api.Breakpoint struct which the caller has received from
get_breakpoint and modified.

Author: aarzilli

_fixtures/amend_breakpoint.star

@@ -0,0 +1,6 @@
+bp = get_breakpoint(0, "afuncbreak").Breakpoint
+bp.Stacktrace = 2
+bp.HitCond = "== 2"
+amend_breakpoint(bp)
+bp2 = get_breakpoint(0, "afuncbreak").Breakpoint
+print(bp)

_scripts/gen-starlark-bindings.go

@@ -226,7 +226,7 @@ func genMapping(bindings []binding) []byte {
 
 		fmt.Fprintf(buf, "err := env.ctx.Client().CallAPI(%q, &rpcArgs, &rpcRet)\n", binding.fn.Name())
 		fmt.Fprintf(buf, "if err != nil { return starlark.None, err }\n")
-		fmt.Fprintf(buf, "return env.interfaceToStarlarkValue(rpcRet), nil\n")
+		fmt.Fprintf(buf, "return env.interfaceToStarlarkValue(&rpcRet), nil\n")
 
 		fmt.Fprintf(buf, "})\n")
 

pkg/terminal/starbind/conv.go

@@ -57,6 +57,11 @@ func (env *Env) interfaceToStarlarkValue(v interface{}) starlark.Value {
 		return starlark.None
 	case error:
 		return starlark.String(v.Error())
+	case reflect.Value:
+		if v.Type().Kind() == reflect.Struct {
+			return structAsStarlarkValue{v, env}
+		}
+		return env.interfaceToStarlarkValue(v.Interface())
 	default:
 		vval := reflect.ValueOf(v)
 		switch vval.Type().Kind() {
@@ -185,9 +190,48 @@ func (v structAsStarlarkValue) Attr(name string) (starlark.Value, error) {
 	}
 	r := v.v.FieldByName(name)
 	if !r.IsValid() {
-		return starlark.None, fmt.Errorf("no field named %q in %T", name, v.v.Interface())
+		return starlark.None, starlark.NoSuchAttrError(fmt.Sprintf("no field named %q in %T", name, v.v.Interface()))
 	}
-	return v.env.interfaceToStarlarkValue(r.Interface()), nil
+	return v.env.interfaceToStarlarkValue(r), nil
+}
+
+func (v structAsStarlarkValue) SetField(name string, value starlark.Value) (err error) {
+	defer func() {
+		// reflect.Value.SetInt, SetFloat, etc panic
+		ierr := recover()
+		if ierr == nil {
+			return
+		}
+		err, _ = ierr.(error)
+		if err == nil {
+			panic(ierr)
+		}
+		err = fmt.Errorf("can not assign to %T.%q: %v", v.v.Interface(), name, err)
+	}()
+	if r, err := v.valueAttr(name); err != nil || r != nil {
+		return starlark.NoSuchAttrError(fmt.Sprintf("no field named %s in %T", name, v.v.Interface()))
+	}
+	r := v.v.FieldByName(name)
+	if !r.IsValid() {
+		return starlark.NoSuchAttrError(fmt.Sprintf("no field named %q in %T", name, v.v.Interface()))
+	}
+	switch value := value.(type) {
+	case starlark.Int:
+		n, ok := value.Int64()
+		if !ok {
+			return fmt.Errorf("can not assign big integer to %T.%q", v.v.Interface(), name)
+		}
+		r.SetInt(n)
+	case starlark.Float:
+		r.SetFloat(float64(value))
+	case starlark.String:
+		r.SetString(value.GoString())
+	case starlark.Bool:
+		r.SetBool(bool(value))
+	default:
+		return fmt.Errorf("can not assign value of type %T to %T.%q", value, v.v.Interface(), name)
+	}
+	return nil
 }
 
 func (v structAsStarlarkValue) valueAttr(name string) (starlark.Value, error) {