[ruby-core:102114] [Ruby master Bug#17543] Ractor isolation broken by `self` in shareable proc

Issue #17543 has been updated by marcandre (Marc-Andre Lafortune).


Eregon (Benoit Daloze) wrote in #note-3:
> Not preserving `self` would be confusing, e.g., when calling other methods like:
> ```ruby
> class Foo
>   def bar
>     ...
>   end
> 
>   def foo
>     test { bar }
>   end
> end
> ```

In my proposal, calling that block would result in a `NoMethodError` on `<#DetachedSelf>`. Is it that confusing? The detached self could be created dynamically with the location of the call to `make_shareable` to help debugging.

> I think the only case where it's acceptable to change the self of a proc implicitly

I am not sure why you state "implicitly". It is explicit that `make_shareable` changes the block in more ways than just freezing, in particular by detaching the local variables and binding.

> is `Ractor.new {}` because that already cannot capture any local variables:
> ```
> ruby -e 'o=Object.new; Ractor.new { o }'                            
> <internal:ractor>:267:in `new': can not isolate a Proc because it accesses outer variables (o). (ArgumentError)
> ```
> 
> Capturing variables or self is very similar IMHO, so I think they should be treated the same.
> ```
> ruby -e 'o=Object.new; Ractor.make_shareable(-> { o }); p o.frozen?'
> <internal:ractor>:816:in `make_shareable': can not make shareable Proc because it can refer unshareable object #<Object:0x0000000000d5ca28> from variable `o' (Ractor::IsolationError)
> ```

Note that they are *not* treated the same way. `Ractor.new{}` forbids any reference to outside variables, while `make_shareable(proc)` allows it as long as the values are themselves shareable.

```
o=Object.new.freeze
Ractor.new { o } # => ArgumentError
Ractor.make_shareable(-> { o }) # => no error
```

> The most obvious fix is therefore to raise the same error if `self` is not shareable.

I agree that is the most obvious, but that makes `Ractor.make_shareable(-> { puts "hello" })` fail, which is far from being useful.


----------------------------------------
Bug #17543: Ractor isolation broken by `self` in shareable proc
https://bugs.ruby-lang.org/issues/17543#change-89970

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: 3.0.0p0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Discussing with @MaxLap we realized that the `self` in a shareable proc is not properly isolated:

```
class Foo
  attr_accessor :x

  def pr
    Ractor.make_shareable(Proc.new { self })
  end
end

f = Foo.new
f.x = [1, 2, 3]
Ractor.new(f.pr) { |pr| pr.call.x << :oops }
p f.x # => [1, 2, 3, :oops]
```

If the `self` refers to a shareable object then it's fine, but for non-shareable objects it has to be reset to `nil` or to a global shareable object that would have an instructive `inspect`.

```ruby
Ractor::DETACHED_SELF = Object.new
def << Ractor::DETACHED_SELF
  def inspect
    '<#detached self>'
  end
  alias to_s inspect
end
Ractor::DETACHED_SELF.freeze
```




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