Add upb_Message_ClearOneof (and retain upb_Message_WhichOneofFieldNumber)

upb users currently need to manually fetch a oneof field in order to clear it.
In this CL, we add a convenience method to do that in one fell swoop.

PiperOrigin-RevId: 631454136

upb/message/accessors.h

@@ -54,6 +54,10 @@ UPB_API_INLINE void upb_Message_ClearBaseField(upb_Message* msg,
 UPB_API_INLINE void upb_Message_ClearExtension(upb_Message* msg,
                                                const upb_MiniTableExtension* e);
 
+UPB_API_INLINE void upb_Message_ClearOneof(upb_Message* msg,
+                                           const upb_MiniTable* m,
+                                           const upb_MiniTableField* f);
+
 UPB_API_INLINE bool upb_Message_HasBaseField(const upb_Message* msg,
                                              const upb_MiniTableField* f);
 

upb/message/accessors_test.cc

@@ -478,4 +478,28 @@ TEST(GeneratedCode, EnumClosedCheck) {
   upb_Arena_Free(arena);
 }
 
+TEST(GeneratedCode, OneofClear) {
+  upb_Arena* arena = upb_Arena_New();
+
+  protobuf_test_messages_proto2_TestAllTypesProto2* msg =
+      protobuf_test_messages_proto2_TestAllTypesProto2_new(arena);
+
+  const upb_MiniTable* table =
+      &protobuf_0test_0messages__proto2__TestAllTypesProto2_msg_init;
+
+  // oneof_uint32
+  const upb_MiniTableField* oneofField =
+      upb_MiniTable_FindFieldByNumber(table, 111);
+  EXPECT_TRUE(upb_MiniTableField_IsInOneof(oneofField));
+  protobuf_test_messages_proto2_TestAllTypesProto2_set_oneof_uint32(msg, 522);
+  EXPECT_TRUE(
+      protobuf_test_messages_proto2_TestAllTypesProto2_has_oneof_uint32(msg));
+
+  upb_Message_ClearOneof((upb_Message*)msg, table, oneofField);
+  EXPECT_FALSE(
+      protobuf_test_messages_proto2_TestAllTypesProto2_has_oneof_uint32(msg));
+
+  upb_Arena_Free(arena);
+}
+
 }  // namespace

upb/message/internal/accessors.h

@@ -693,6 +693,21 @@ UPB_API_INLINE void upb_Message_ClearExtension(
   }
 }
 
+UPB_API_INLINE void upb_Message_ClearOneof(struct upb_Message* msg,
+                                           const upb_MiniTable* m,
+                                           const upb_MiniTableField* f) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
+  uint32_t field_number = upb_Message_WhichOneofFieldNumber(msg, f);
+  if (field_number == 0) {
+    // No field in the oneof is set.
+    return;
+  }
+
+  const upb_MiniTableField* field =
+      upb_MiniTable_FindFieldByNumber(m, field_number);
+  upb_Message_ClearBaseField(msg, field);
+}
+
 UPB_API_INLINE void* upb_Message_ResizeArrayUninitialized(
     struct upb_Message* msg, const upb_MiniTableField* f, size_t size,
     upb_Arena* arena) {