[ruby-core:110349] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state.

["byroot (Jean Boussier)" <noreply@ruby-lang.org>]

Issue #19062 has been updated by byroot (Jean Boussier).


So that is indeed a need that arise when you start using fibers or even threads. Existing Ruby code rely a lot on Fiber / Thread global state.

The problem however is the semantic. If all the values in `Fiber.locals` are immutable, it's quite painless, but if they are mutable what do you do?

Do you share that mutable state? Do you copy or deep copy that state?

Then what if I want to spawn a new Fiber with a clean state? All this to say, I'm not sure whether this is something the programming language should offer. It sounds a bit like a library / framework construct to me.

Do we have any precedent of a similar feature in another language? I suppose UNIX environment variables work a bit like this, but they are limited to `Hash[String, String]` which simplify the mutability concerns.

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Bug #19062: Introduce `Fiber#locals` for shared inheritable state.
https://bugs.ruby-lang.org/issues/19062#change-99642

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
After exploring <https://bugs.ruby-lang.org/issues/19058>, I felt uncomfortable about the performance of copying lots of inheritable attributes. Please review that issue for the background and summary of the problem.

## Proposal

Introduce `Fiber#locals` which is a hash table of local attributes which are inherited by child fibers.

```ruby
Fiber.current.locals[:x] = 10

Fiber.new do
  pp Fiber.current.locals[:x] # => 10
end
```

It's possible to reset `Fiber.current.locals`, e.g.

```ruby
def accept_connection(peer)
  Fiber.new(locals: nil) do # This causes a new hash table to be allocated.
    # Generate a new request id for all fibers nested in this one:
    Fiber[:request_id] = SecureRandom.hex(32)
    @app.call(env)
  end.resume
end
```

A high level overview of the proposed changes:

```ruby
class Fiber
  def initialize(..., locals: Fiber.current.locals)
    @locals = locals || Hash.new
  end

  attr_accessor :locals

  def self.[] key
    self.current.locals[key]
  end

  def self.[]= key, value
    self.current.locals[key] = value
  end
end
```

See the pull request <https://github.com/ruby/ruby/pull/6566> for the full proposed implementation.

## Expected Usage

Currently, a lot of libraries use `Thread.current[:x]` which is unexpectedly "fiber local". A common bug shows up when lazy enumerators are used, because it may create an internal fiber. Because `locals` are inherited, code which uses `Fiber[:x]` will not suffer from this problem.

Any program that uses true thread locals for per-request state, can adopt the proposed `Fiber#locals` and get similar behaviour, without breaking on per-fiber servers like Falcon, because Falcon can "reset" `Fiber.current.locals` for each request fiber, while servers like Puma won't have to do that and will retain thread-local behaviour.

Libraries like ActiveRecord can adopt `Fiber#locals` to avoid the need for users to opt into different "IsolatedExecutionState" models, since it can be transparently handled by the web server (see <https://github.com/rails/rails/pull/43596> for more details).

We hope by introducing `Fiber#locals`, we can avoid all the confusion and bugs of the past designs.



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