From: daniel@...42.com
Date: 2020-11-13T19:53:41+00:00
Subject: [ruby-core:100842] [Ruby master Feature#17323] Ractor::LVar to provide ractor-local storage

Issue #17323 has been updated by Dan0042 (Daniel DeLorme).


Would it be possible to somehow have ractor-local variables that are automatically dereferenced instead of having to append `.value` everywhere?

I'm thinking of cases like this where a class-level mutable constant (or classvar) makes it hard to make this code compatible with ractors.

```ruby
class X
  # original, not compatible with Ractor, so how do we fix?
  CACHE = {}

  # like this, except now we need to append .value to every CACHE access
  CACHE = Ractor::LVar.new{ {} }

  def initialize(value)
    @value = value
  end

  def analyzed
    # here, lookup of CACHE constant could automatically return the
    # ractor-local Hash inside the LVar instead of the LVar itself
    CACHE[@value] ||= analyze(@value)
  end
end
```

The problem with the example above is that it's too magical to have a variable or constant return an object different from what was assigned. So what I'm saying is that I'd like _something like_ this, that achieves the same effect.

I have the feeling that a different syntax would be needed, to differentiate it from assignment. Maybe `CONST: expr` could be used to lazily evaluate `expr` once per ractor; then it would make sense for `CONST` to return this value rather than the LVar used behind the scenes. The example above would become `CACHE: {}`. But of course introducing new syntax is not something to be done lightly.

----------------------------------------
Feature #17323: Ractor::LVar to provide ractor-local storage
https://bugs.ruby-lang.org/issues/17323#change-88484

* 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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