Feature #19062
Updated by ioquatix (Samuel Williams) about 2 years ago
After exploring <https://bugs.ruby-lang.org/issues/19058>, I felt uncomfortable about the performance issue of 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: nil)
@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.