From: "ioquatix (Samuel Williams)" <noreply@...>
Date: 2022-10-28T01:06:44+00:00
Subject: [ruby-core:110526] [Ruby master Feature#19078] Introduce `Fiber#storage` for inheritable fiber-scoped variables.

Issue #19078 has been updated by ioquatix (Samuel Williams).


> Often times, this is done to avoid holding a reference to the object in the thread, effectively making it immortal.

I think the correct way to make opaque per-instance keys is something like this:

```ruby
class ConnectionPool
  def initialize
    @connection_key = Object.new
    @count_key = Object.new
  end

  def checkout(options = {})
    if connection = ::Fiber[@connection_key]
      ::Fiber[@count_key] += 1
    else
      connection = ::Fiber[@connection_key] = @available.pop(options[:timeout] || @timeout)
      ::Fiber[@count_key] = 1
    end

    return connection
  end
end
```

I would say the above implementation is invalid, because inheriting the count and connection is likely to cause buggy behaviour, however the key point is, using `Object.new` for opaque keys should be totally fine.

----------------------------------------
Feature #19078: Introduce `Fiber#storage` for inheritable fiber-scoped variables.
https://bugs.ruby-lang.org/issues/19078#change-99851

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
----------------------------------------
Pull Request: https://github.com/ruby/ruby/pull/6612

This is an evolution of the previous ideas:

- https://bugs.ruby-lang.org/issues/19058
- https://bugs.ruby-lang.org/issues/19062

This PR introduces fiber scoped variables, and is a solution for problems like <https://github.com/ioquatix/ioquatix/discussions/17>.

The main interface is:

```ruby
Fiber[key] = value
Fiber[key] # => value
```

The variables are scoped (local to) a fiber and inherited into child fibers and threads.

```ruby
Fiber[:request_id] = SecureRandom.hex(16)

Fiber.new do
  p Fiber[:request_id] # prints the above request id
end
```

The fiber scoped variables are stored and can be accessed:

```ruby
Fiber.current.storage # => returns a Hash (copy) of the internal storage.
Fiber.current.storage= # => assigns a Hash (copy) to the internal storage.
```

Fiber itself has one new keyword argument:

```
Fiber.new(..., storage: hash, false, undef, nil)
```

This can control how the fiber variables are setup in a child context.

To minimise the performance overhead of some of the implementation choices, we are also simultaneously implementing <https://bugs.ruby-lang.org/issues/19077>.

## Examples

### Request loop

```ruby
Thread.new do
  while request = queue.pop
    Fiber.new(storage: {id: SecureRandom.hex(16)}) do
      handle_request.call(request)
    end
  end
end
```

OR

```ruby
Thread.new do
  while request = queue.pop
    Fiber.current.storage = {id: SecureRandom.hex(16)}
    handle_request.call(request)
  end
end
```




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