feat(go): register exported properties as callbacks (#4104)

When passing arbitrary structs through an `interface{}` type, the JS value had no properties or methods, leading to it having an empty object JSON representation.

In order to improve the situation, discover exported properties of such structs, and register them as property callbacks, more closely matching the behavior of JS in the same configuration.

The callbacks use the same naming transformation as the standard JSON serialization mechanism in go: using the `json` tag if present with the same semantics as `encoding/json`, and fall back to the raw field name otherwise (with no case conversion applied).

Related: cdk8s-team/cdk8s#1326



---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/@jsii/go-runtime-test/project/callbacks_test.go

@@ -3,6 +3,7 @@ package tests
 import (
 	"testing"
 
+	"github.com/aws/jsii-runtime-go"
 	calc "github.com/aws/jsii/jsii-calc/go/jsiicalc/v3"
 )
 
@@ -18,10 +19,27 @@ func TestPureInterfacesCanBeUsedTransparently(t *testing.T) {
 	}
 }
 
+func TestPropertyAccessThroughAny(t *testing.T) {
+	any := &ABC{
+		PropA: "Hello",
+		ProbB: "World",
+	}
+	calc.AnyPropertyAccess_MutateProperties(any, jsii.String("a"), jsii.String("b"), jsii.String("result"))
+	if *any.PropC != "Hello+World" {
+		t.Errorf("Expected Hello+World; actual %v", any.PropC)
+	}
+}
+
 type StructReturningDelegate struct {
 	expected *calc.StructB
 }
 
 func (o *StructReturningDelegate) ReturnStruct() *calc.StructB {
 	return o.expected
 }
+
+type ABC struct {
+	PropA string  `json:"a"`
+	ProbB string  `json:"b"`
+	PropC *string `json:"result,omitempty"`
+}

packages/@jsii/go-runtime/jsii-runtime-go/internal/kernel/callbacks.go

@@ -3,6 +3,7 @@ package kernel
 import (
 	"fmt"
 	"reflect"
+	"strings"
 
 	"github.com/aws/jsii-runtime-go/internal/api"
 )
@@ -78,9 +79,45 @@ func (g *getCallback) handle(cookie string) (retval reflect.Value, err error) {
 	client := GetClient()
 
 	receiver := reflect.ValueOf(client.GetObject(g.ObjRef))
-	method := receiver.MethodByName(cookie)
 
-	return client.invoke(method, nil)
+	if strings.HasPrefix(cookie, ".") {
+		// Ready to catch an error if the access panics...
+		defer func() {
+			if r := recover(); r != nil {
+				if err == nil {
+					var ok bool
+					if err, ok = r.(error); !ok {
+						err = fmt.Errorf("%v", r)
+					}
+				} else {
+					// This is not expected - so we panic!
+					panic(r)
+				}
+			}
+		}()
+
+		// Need to access the underlying struct...
+		receiver = receiver.Elem()
+		retval = receiver.FieldByName(cookie[1:])
+
+		if retval.IsZero() {
+			// Omit zero-values if a json tag instructs so...
+			field, _ := receiver.Type().FieldByName(cookie[1:])
+			if tag := field.Tag.Get("json"); tag != "" {
+				for _, attr := range strings.Split(tag, ",")[1:] {
+					if attr == "omitempty" {
+						retval = reflect.ValueOf(nil)
+						break
+					}
+				}
+			}
+		}
+
+		return
+	} else {
+		method := receiver.MethodByName(cookie)
+		return client.invoke(method, nil)
+	}
 }
 
 type setCallback struct {
@@ -93,9 +130,70 @@ func (s *setCallback) handle(cookie string) (retval reflect.Value, err error) {
 	client := GetClient()
 
 	receiver := reflect.ValueOf(client.GetObject(s.ObjRef))
-	method := receiver.MethodByName(fmt.Sprintf("Set%v", cookie))
+	if strings.HasPrefix(cookie, ".") {
+		// Ready to catch an error if the access panics...
+		defer func() {
+			if r := recover(); r != nil {
+				if err == nil {
+					var ok bool
+					if err, ok = r.(error); !ok {
+						err = fmt.Errorf("%v", r)
+					}
+				} else {
+					// This is not expected - so we panic!
+					panic(r)
+				}
+			}
+		}()
+
+		// Need to access the underlying struct...
+		receiver = receiver.Elem()
+		field := receiver.FieldByName(cookie[1:])
+		meta, _ := receiver.Type().FieldByName(cookie[1:])
+
+		field.Set(convert(reflect.ValueOf(s.Value), meta.Type))
+		// Both retval & err are set to zero values here...
+		return
+	} else {
+		method := receiver.MethodByName(fmt.Sprintf("Set%v", cookie))
+		return client.invoke(method, []interface{}{s.Value})
+	}
+}
+
+func convert(value reflect.Value, typ reflect.Type) reflect.Value {
+retry:
+	vt := value.Type()
+
+	if vt.AssignableTo(typ) {
+		return value
+	}
+	if value.CanConvert(typ) {
+		return value.Convert(typ)
+	}
+
+	if typ.Kind() == reflect.Ptr {
+		switch value.Kind() {
+		case reflect.String:
+			str := value.String()
+			value = reflect.ValueOf(&str)
+		case reflect.Bool:
+			bool := value.Bool()
+			value = reflect.ValueOf(&bool)
+		case reflect.Int:
+			int := value.Int()
+			value = reflect.ValueOf(&int)
+		case reflect.Float64:
+			float := value.Float()
+			value = reflect.ValueOf(&float)
+		default:
+			iface := value.Interface()
+			value = reflect.ValueOf(&iface)
+		}
+		goto retry
+	}
 
-	return client.invoke(method, []interface{}{s.Value})
+	// Unsure what to do... let default behavior happen...
+	return value
 }
 
 func (c *Client) invoke(method reflect.Value, args []interface{}) (retval reflect.Value, err error) {

packages/@jsii/go-runtime/jsii-runtime-go/internal/kernel/manage-object.go

@@ -1,7 +1,9 @@
 package kernel
 
 import (
+	"fmt"
 	"reflect"
+	"strings"
 
 	"github.com/aws/jsii-runtime-go/internal/api"
 )
@@ -18,6 +20,14 @@ func (c *Client) ManageObject(v reflect.Value) (ref api.ObjectRef, err error) {
 	}
 	interfaces, overrides := c.Types().DiscoverImplementation(vt)
 
+	found := make(map[string]bool)
+	for _, override := range overrides {
+		if prop, ok := override.(*api.PropertyOverride); ok {
+			found[prop.JsiiProperty] = true
+		}
+	}
+	overrides = appendExportedProperties(vt, overrides, found)
+
 	var resp CreateResponse
 	resp, err = c.Create(CreateProps{
 		FQN:        objectFQN,
@@ -33,3 +43,49 @@ func (c *Client) ManageObject(v reflect.Value) (ref api.ObjectRef, err error) {
 
 	return
 }
+
+func appendExportedProperties(vt reflect.Type, overrides []api.Override, found map[string]bool) []api.Override {
+	if vt.Kind() == reflect.Ptr {
+		vt = vt.Elem()
+	}
+
+	if vt.Kind() == reflect.Struct {
+		for idx := 0; idx < vt.NumField(); idx++ {
+			field := vt.Field(idx)
+			// Unexported fields are not relevant here...
+			if !field.IsExported() {
+				continue
+			}
+
+			// Anonymous fields are embed, we traverse them for fields, too...
+			if field.Anonymous {
+				overrides = appendExportedProperties(field.Type, overrides, found)
+				continue
+			}
+
+			jsonName := field.Tag.Get("json")
+			if jsonName == "-" {
+				// Explicit omit via `json:"-"`
+				continue
+			} else if jsonName != "" {
+				// There could be attributes after the field name (e.g. `json:"foo,omitempty"`)
+				jsonName = strings.Split(jsonName, ",")[0]
+			}
+			// The default behavior is to use the field name as-is in JSON.
+			if jsonName == "" {
+				jsonName = field.Name
+			}
+
+			if !found[jsonName] {
+				overrides = append(overrides, &api.PropertyOverride{
+					JsiiProperty: jsonName,
+					// Using the "." prefix to signify this isn't actually a getter, just raw field access.
+					GoGetter: fmt.Sprintf(".%s", field.Name),
+				})
+				found[jsonName] = true
+			}
+		}
+	}
+
+	return overrides
+}

packages/jsii-calc/lib/compliance.ts

@@ -3141,3 +3141,24 @@ export class PromiseNothing {
     return PromiseNothing.promiseIt();
   }
 }
+
+export class AnyPropertyAccess {
+  /**
+   * Sets obj[resultProp] to `${obj[propA]}+${obj[propB]}`.
+   *
+   * @param obj the receiver object.
+   * @param propA the first property to read.
+   * @param propB the second property to read.
+   * @param resultProp the property to write into.
+   */
+  public static mutateProperties(
+    obj: any,
+    propA: string,
+    propB: string,
+    resultProp: string,
+  ) {
+    obj[resultProp] = `${obj[propA]}+${obj[propB]}`;
+  }
+
+  private constructor() {}
+}

packages/jsii-calc/test/assembly.jsii

@@ -1538,6 +1538,72 @@
       "name": "AnonymousImplementationProvider",
       "symbolId": "lib/compliance:AnonymousImplementationProvider"
     },
+    "jsii-calc.AnyPropertyAccess": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "stable"
+      },
+      "fqn": "jsii-calc.AnyPropertyAccess",
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/compliance.ts",
+        "line": 3145
+      },
+      "methods": [
+        {
+          "docs": {
+            "stability": "stable",
+            "summary": "Sets obj[resultProp] to `${obj[propA]}+${obj[propB]}`."
+          },
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 3154
+          },
+          "name": "mutateProperties",
+          "parameters": [
+            {
+              "docs": {
+                "summary": "the receiver object."
+              },
+              "name": "obj",
+              "type": {
+                "primitive": "any"
+              }
+            },
+            {
+              "docs": {
+                "summary": "the first property to read."
+              },
+              "name": "propA",
+              "type": {
+                "primitive": "string"
+              }
+            },
+            {
+              "docs": {
+                "summary": "the second property to read."
+              },
+              "name": "propB",
+              "type": {
+                "primitive": "string"
+              }
+            },
+            {
+              "docs": {
+                "summary": "the property to write into."
+              },
+              "name": "resultProp",
+              "type": {
+                "primitive": "string"
+              }
+            }
+          ],
+          "static": true
+        }
+      ],
+      "name": "AnyPropertyAccess",
+      "symbolId": "lib/compliance:AnyPropertyAccess"
+    },
     "jsii-calc.AsyncVirtualMethods": {
       "assembly": "jsii-calc",
       "docs": {
@@ -18843,5 +18909,5 @@
     }
   },
   "version": "3.20.120",
-  "fingerprint": "EH7xszNdCh9PCFUZ8Foi7g2CPhdrKeZm8CQaUCNv4GQ="
+  "fingerprint": "kOCIHox3N0mzJsbC3zUF0dELGRsdq5jP57dDIOu3fDE="
 }