fix: type-aware null markers for opaque fields in JavaWireFormat codegen

java_extractor now emits specific write methods (bundle, typed_object,
byte_array, blob, binder) instead of generic 'opaque'. The codegen
uses these to write correct null markers:
  - bundle: int32(-1) (writeBundle null protocol)
  - typed_object: int32(0) (writeTypedObject null protocol)
  - others: int32(0) with length-based skip on unmarshal

Author: xaionaro@dx.center

tools/cmd/spec2go/main.go

@@ -320,14 +320,15 @@ func convertParcelableToAST(
 			validName := isValidJavaWireFieldName(jwf.Name)
 			resolvableCond := isResolvableJavaWireCondition(jwf.Condition)
 			_, knownType := javaWireTypeToAIDL[jwf.WriteMethod]
-			isOpaque := jwf.WriteMethod == "opaque"
+			isOpaque := !knownType
 
 			// Treat fields with invalid names, unresolvable conditions,
 			// or unknown types as opaque in the wire format.
-			if !validName || !resolvableCond || (!knownType && !isOpaque) {
+			if !validName || !resolvableCond || isOpaque {
 				wireFields = append(wireFields, parser.JavaWireField{
 					Name:        jwf.Name,
-					WriteMethod: "opaque",
+					WriteMethod: jwf.WriteMethod,
+					Condition:   jwf.Condition,
 				})
 				continue
 			}

tools/pkg/codegen/parcelable_gen.go

@@ -808,9 +808,12 @@ func writeJavaWireMarshalParcel(
 	for _, wf := range wireFields {
 		info, ok := javaWireMethodMap[wf.WriteMethod]
 		if !ok {
-			// Opaque field (Bundle, Parcelable, etc.) — write null marker.
-			// Android's writeBundle(null) writes int32(-1).
-			f.P("\tp.WriteInt32(-1) // null %s", wf.Name)
+			switch wf.WriteMethod {
+			case "bundle":
+				f.P("\tp.WriteInt32(-1) // null %s (Bundle)", wf.Name)
+			default:
+				f.P("\tp.WriteInt32(0) // null %s", wf.Name)
+			}
 			continue
 		}
 
@@ -860,18 +863,38 @@ func writeJavaWireUnmarshalParcel(
 		info, ok := javaWireMethodMap[wf.WriteMethod]
 
 		if !ok {
-			// Opaque field: skip by reading length-prefixed data.
-			// Android's writeBundle/writeParcelable writes int32(length)
-			// followed by <length> bytes.
-			f.P("\t{")
-			f.P("\t\t_opaqueLen, _opaqueErr := p.ReadInt32()")
-			f.P("\t\tif _opaqueErr != nil {")
-			f.P("\t\t\treturn _opaqueErr")
-			f.P("\t\t}")
-			f.P("\t\tif _opaqueLen > 0 {")
-			f.P("\t\t\tp.SetPosition(p.Position() + int(_opaqueLen))")
-			f.P("\t\t}")
-			f.P("\t}")
+			switch wf.WriteMethod {
+			case "bundle":
+				f.P("\t{")
+				f.P("\t\t_opaqueLen, _opaqueErr := p.ReadInt32()")
+				f.P("\t\tif _opaqueErr != nil {")
+				f.P("\t\t\treturn _opaqueErr")
+				f.P("\t\t}")
+				f.P("\t\tif _opaqueLen > 0 {")
+				f.P("\t\t\tp.SetPosition(p.Position() + int(_opaqueLen))")
+				f.P("\t\t}")
+				f.P("\t}")
+			case "typed_object":
+				f.P("\t{")
+				f.P("\t\t_opaqueFlag, _opaqueErr := p.ReadInt32()")
+				f.P("\t\tif _opaqueErr != nil {")
+				f.P("\t\t\treturn _opaqueErr")
+				f.P("\t\t}")
+				f.P("\t\tif _opaqueFlag != 0 {")
+				f.P("\t\t\treturn nil // non-null %s: cannot skip unknown-size typed object", wf.Name)
+				f.P("\t\t}")
+				f.P("\t}")
+			default:
+				f.P("\t{")
+				f.P("\t\t_opaqueLen, _opaqueErr := p.ReadInt32()")
+				f.P("\t\tif _opaqueErr != nil {")
+				f.P("\t\t\treturn _opaqueErr")
+				f.P("\t\t}")
+				f.P("\t\tif _opaqueLen > 0 {")
+				f.P("\t\t\tp.SetPosition(p.Position() + int(_opaqueLen))")
+				f.P("\t\t}")
+				f.P("\t}")
+			}
 			continue
 		}
 

tools/pkg/parcelspec/java_extractor.go

@@ -73,13 +73,13 @@ var javaWriteMethodToSpecType = map[string]string{
 	"writeFloat":        "float32",
 	"writeDouble":       "float64",
 	"writeBoolean":      "bool",
-	"writeBundle":       "opaque",
-	"writeParcelable":   "opaque",
-	"writeTypedObject":  "opaque",
+	"writeBundle":       "bundle",
+	"writeParcelable":   "typed_object",
+	"writeTypedObject":  "typed_object",
 	"writeByte":         "int32",
-	"writeByteArray":    "opaque",
-	"writeBlob":         "opaque",
-	"writeStrongBinder": "opaque",
+	"writeByteArray":    "byte_array",
+	"writeBlob":         "blob",
+	"writeStrongBinder": "binder",
 }
 
 // deriveFieldName converts a Java field name to a spec field name

tools/pkg/parcelspec/java_extractor_test.go

@@ -76,7 +76,7 @@ public class AllTypes implements Parcelable {
 		{"Latitude", "float64"},
 		{"Label", "string8"},
 		{"Description", "string16"},
-		{"Extras", "opaque"},
+		{"Extras", "bundle"},
 	}
 
 	for i, exp := range expected {
@@ -246,7 +246,7 @@ func TestExtractSpec_Location(t *testing.T) {
 	// The last field should be Extras (opaque, unconditional).
 	lastField := spec.Fields[len(spec.Fields)-1]
 	require.Equal(t, "Extras", lastField.Name)
-	require.Equal(t, "opaque", lastField.Type)
+	require.Equal(t, "bundle", lastField.Type)
 	require.Empty(t, lastField.Condition)
 }