[ruby-core:110352] [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).


> Can you give an example of the problem caused by mutability?

Sure, you can look at how `ActiveSupport::IsolatedExecutionState` is used today. For instance [`ActiveSupport::CurrentAttributes`](https://github.com/rails/rails/blob/caf94136045f79b1d9103ebf17fc74b7ff7f8727/activesupport/lib/active_support/current_attributes.rb#L164) (but applies to most mutable values in there).

If you simply inherit this hash, a child fiber changing `Something.current` would change it in its parent, sibling and child fibers too, which is super unlikely to be what you want.

Of course the fix is to always copy that registry hash before mutating it, but that's the common question of wether we should expose common traps like this or not.

It's interesting to note that the [JEP 429](https://openjdk.org/jeps/429) you mention in the other ticket very explicitly only allow immutable objects.

As always, just because you can shoot yourself in the foot with it doesn't mean it shouldn't be provided, but I think it's interesting to point it out.

>  Please review the linked issue 

Apologies, I initially missed it.

Again, I think this is definitely a common need, just like POSIX environment variables. And if this was to go through I'd like the same thing for thread locals.

I just think we need to be careful with the semantic. I wonder if applying something akin to `Ractor.make_shareable` on assignation could make sense (just a quick thought).





----------------------------------------
Bug #19062: Introduce `Fiber#locals` for shared inheritable state.
https://bugs.ruby-lang.org/issues/19062#change-99645

* 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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