[ruby-core:119769] [Ruby master Feature#20875] Atomic initialization for Ractor local storage

Issue #20875 has been reported by ko1 (Koichi Sasada).

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Feature #20875: Atomic initialization for Ractor local storage
https://bugs.ruby-lang.org/issues/20875

* Author: ko1 (Koichi Sasada)
* Status: Open
----------------------------------------
## Motivation

Now there is no way to initialize Ractor local storage in multi-thread.

For example, if we want to introduce per-Ractor counter, which should be protected with a per-Ractor Mutex for multi-threading support.

```ruby

def init
  Ractor[:cnt] = 0
  Ractor[:mtx] = Mutex.new
end

def inc
  init unless Ractor[:cnt]
  Ractor[:mtx].synchronize do
    Ractor[:cnt] += 1
  end
end
```

In this code, if `inc` was called on multiple threads, `init` can be called with multiple threads and `cnt` can not be synchronized correctly.

## Proposal

Let's introduce `Ractor.local_storage_init(sym){ block }` to initialize values in Ractor local storage.

If there is no slot for `sym`, synchronize with per-Ractor mutex and call `block` and the slot will be filled with the evaluation with the block result. The return value of this method will be the filled value.
Otherwise, returning corresponding value will be returned.

The implementation is like that (in C):

```ruby
class Ractor
  def self.local_storage_init(sym)
    Ractor.per_ractor_mutex.synchronize do
      if Ractor.local_storage_has_key?(sym)
        Ractor[:sym]
      else
        Ractor[:sym] = yield
      end
    end
  end
end
```

The above examples will be rewritten with the following code:

```ruby
def inc
  Ractor.local_storage_init(:mtx) do
    Ractor[:cnt] = 0
    Mutex.new
  end.synchronize do
    Ractor[:cnt] += 1
  end
end
```

## Discussion


### Approach

There are another approach like `pthread_atfork`, maybe like `Ractor.atcreate{ init }`. A library registers a callback which will be called when a new ractor is created.
However, there are many Ractors which don't use the library, so that ``atcreate` can be huge overhead for Ractor creation.

### Naming

I propose `local_storage_init`, but not sure it matches.
I also proposed `Ractor.local_variable_init(sym)`, but Matz said he doesn't like this naming because it should not be a "variable".
(there is a `Thread#thread_variable_get` method, though).

On another aspect, `lcoal_storage_init` seems it clears all of ractor local storage slots.




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