[ruby-core:106790] [Ruby master Bug#18428] Calling Object#singleton_class mutates object's class from the C API's perspective

Issue #18428 has been updated by antstorm (Anthony Dmitriyev).


Ah, I see, that makes sense. Thank you for the quick response!

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Bug #18428: Calling Object#singleton_class mutates object's class from the C API's perspective
https://bugs.ruby-lang.org/issues/18428#change-95494

* Author: antstorm (Anthony Dmitriyev)
* Status: Rejected
* Priority: Normal
* ruby -v: ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [arm64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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Hi,

I'm not 100% sure if this is an actual bug, but it feels like it. When a `Object#singleton_class` method is called on an object it swaps the object's class with a singleton's class (as far as `CLASS_OF` C call is concerned).

Here's a simple example to reproduce the issue:

1. The C extension that adds a `#c_class` method to the Object, returning back the value of `CLASS_OF` call

``` c
#include "ruby.h"
#include "extconf.h"

static VALUE c_class(VALUE self) {
  return CLASS_OF(self);
}

void Init_c_class(void) {
  rb_define_method(rb_cObject, "c_class", c_class, 0);
}
```

2. With that extension required, we can expose the issue from the console:

```ruby
>> a = 'test'
=> "test"
>> a.class
=> String
>> a.c_class
=> String
>> a.singleton_class
=> #<Class:#<String:0x00000001168372e8>>
>> a.class
=> String
>> a.c_class
=> #<Class:#<String:0x00000001168372e8>>
```

I would expect the last call to `c_class` to return the same value as previously — `String`, instead it returns a singleton class.

Originally noticed this behaviour when passing a string to Google protobuf generated files, which raises this error — https://github.com/protocolbuffers/protobuf/blob/master/ruby/ext/google/protobuf_c/convert.c#L161. If the string had `singleton_class` called on it this would raise.



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