[ruby-core:101271] [Ruby master Feature#17323] Ractor::LVar to provide ractor-local storage

Issue #17323 has been updated by marcandre (Marc-Andre Lafortune).


I'm having difficulties because we don't have Ractor-local storage...

I'd like to implement a Ractor compatible `SharedQueue`.

I'd like to do it without going through a bridge Ractor, so I'm trying to refine `Ractor` to add `channel`s as in #17365.

To do that, I need a Ractor-local "saved messages queue", but there is currently no API for that.

I guess I'll have to roll my own using `Thread.main.thread_variable_get` and wrap the `set` inside a global Mutex...

----------------------------------------
Feature #17323: Ractor::LVar to provide ractor-local storage
https://bugs.ruby-lang.org/issues/17323#change-88951

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby supports thread and fiber local storage:

* `Thread#[sym]` provides *Fiber* local storage
* `Thread#thread_variable_get(sym)`

These APIs can access other threads/fibers like that:

```ruby
th = Thread.new{
  Thread.current.thread_variable_set(:a, 10)
}
th.join
# access from main thread to child thread
p th.thread_variable_get(:a)
```

To make Ractor local storage, this kind of feature should not be allowed to protect isolation.

This ticket propose alternative API `Ractor::LVar` that allows to provide Ractor local variable.

```ruby
LV1 = Ractor::LVar.new

p LV1.value #=> nil # default value
LV1.value = 'hello' # can set unshareable objects because LVar is ractor local.

Ractor.new do
  LV1.value = 'world' # set Ractor local variable
end.take

p LV1.value #=> 'hello'


# Lvar.new can accept default_proc which should be isolated Proc.

LV2 = Ractor::LVar.new{ "x" * 4 }
p LV2.value #=> "xxxx"
LV2.value = 'yyy'

Ractor.new do
  p LV2.value #=> 'xxx'
end

p LV2.value #=> 'yyy'
```

This API doesn't support accessing from other ractors.

`Ractor::LVar` is from `Ractor::TVar`, but I have no strong opinion about it.
For example, `Ractor::LocalVariable` is longer and clearer.

Implementation: https://github.com/ruby/ruby/pull/3762




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