Issue #17159 has been updated by Eregon (Benoit Daloze).


Copying captured variables seems a nice feature to have, also for optimizations.
In fact, this already exists in TruffleRuby internally:
https://github.com/oracle/truffleruby/blob/574d6bd2425caa856707ffd713fdb8ffc87be1e1/src/main/java/org/truffleruby/extra/TruffleGraalNodes.java#L113-L120
It seems known as `fixTemps` in Smalltalk.

So I'd suggest to add this as a keyword argument, not sure about the name but `ractorize` doesn't explain what it does.
`copy_captured_locals: true`, `copy_captured_variables: true`, `capture_scope: false` maybe?

This can also be useful on `Proc` in general, so it might be better to have a method on `Proc` for that.
Having it as a keyword argument for `define_method` as a shortcut seems good, since on the implementation side it's useful to combine both operations into one to avoid intermediate Procs, bytecode, etc.

For Ractor there is a big limitation though: only shareable objects can be copied that way.
Otherwise it would lead to shared state and segv.

----------------------------------------
Bug #17159: extend `define_method` for Ractor
https://bugs.ruby-lang.org/issues/17159#change-87549

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Ractor prohibits to use non-isolated Procs.

Non-isolated example is here:

```
s = "foo"
pr = Proc.new{ p s }
```

This Proc pr can not be shared with multi-ractors because outer variable `s` can contain an unshareable object. Also outer binding is mutable object so it can lead race condition.

Because of these reasons, `define_method` are also problem on multi-Ractor program.
(current implementation allows it just because check is not implemented, and it leads BUG).


I think there are several patterns `define_method` are used.

(1) want to choose variable method names

```ruby
name = ...
define_method(name){ nil }
```

(2) want to embed variables to the code

```ruby
10.times{|i|
  define_method("foo{i}"){ i }
}
```

(3) want to use global state by local variables

```ruby
cnt = 0
define_method("inc"){ cnt += 1 }
```

(4) others I can't imagine

----

(1) is easy. `define_method(name, &Proc{nil}.isoplate)` will be allowed on multi-ractors.

(3) is not allowed because it introduces data races/race conditions. For example, we need to use shared hash.

```ruby
STATE = SharedHash.new(cnt: 0)
define_method("inc"){ STATE.transaction{ STATE[:cnt] += 1 }}
```

I think there are many (2) patterns and it should be saved.
To help (2) pattern, the easiest way is to use eval.

```ruby
10.times{|i|
  eval("def foo#{i} #{i}; end")
}

```

However, eval has several issues (it has huge freedom to explode the program, editor's syntax highlighting and so on).

Another approach is embed the current value to the code, like that:


```ruby
i = 0
define_method("foo", ractorise: true){ i }
#=> equivalent to:
#   define_method("foo"){ 0 }
# so that if outer scope's i changed, not affected.
i = 1
foo #=> 0

s = ""
define_method("bar", ractorise: true){ s }
#=> equivalent to:
#   define_method("bar"){ "" }
# so that if outer scope's s or s's value, it doesn't affect
s << "x"
bar #=> ""
```

However, it is very difference between current Proc semantics.
Another idea is to specify embedding value like that.

```ruby
i = 0
define_method("foo", i: i){ i }
#=> equivalent to:
#   define_method("foo"){ 0 }
# so that if outer scope's i changed, not affected.
i = 1
foo #=> 0

s = ""
define_method("bar", s: s){ s }
#=> equivalent to:
#   define_method("bar"){ "" }
# so that if outer scope's s or s's value, it doesn't affect
s << "x"
bar #=> ""
```

`i: i` and `s: s` are redundant. however, if there are no outer variable `i` or `s`, the `i` and `s` in blocks are compiled to `send(:i)` or `send(:s)`. But I agree these method invocation should be replaced is another idea.


Thoughts?

Thanks,
Koichi



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