Feature #19058
Updated by ioquatix (Samuel Williams) about 2 years ago
There are many instances of programs using globals, thread locals and fiber locals for shared state. Unfortunately these programs often have bugs or unusual behaviour when objects are used on different threads or fibers.
Here is a simple example of the kind of problem that can occur:
```ruby
class Users
def each
return to_enum unless block_given?
p each: Fiber.current
yield "A"
yield "B"
yield "C"
end
end
p Users.new.each.zip(Users.new.each)
```
When the enumeration depends on a fiber-local connection instance (e.g. `Thread.current[:connection]`) it could unexpectedly break the operation because within the enumerate, the fiber is different leading to missing connection.
In all these cases, the problem can be solved by not relying on implicit/invisible state. Unfortunately, many programs take advantage of process (global), thread or fiber local variables to create more ergonomic interfaces, e.g.
```ruby
DB.connect(username, password, host)
DB.query("SELECT * FROM BUGS"); # implicit dependency on connection (ideally from shared pool). connection.
```
Ruby provides several, somewhat confusing options.
```ruby
Thread.current[:x] # Fiber local.
Thread.current.thread_variable_get(:x) # Thread local.
class Thread
attr :x
end
Thread.current.x # thread local
class Fiber
attr :x
end
Fiber.current.x # fiber local
```
Over the years there have been multiple issues about the above behaviour:
- https://bugs.ruby-lang.org/issues/1717
- https://bugs.ruby-lang.org/issues/7097
- https://bugs.ruby-lang.org/issues/8215
- https://bugs.ruby-lang.org/issues/13893
- Rails is also working around this issue: https://github.com/rails/rails/pull/43596
The heart of the issue is: there should be a consistent and convenient way to define state attached to the current execution context, which gets inherited by child execution contexts. Essentially:
```ruby
let(x: 10) do
# In every execution context created within this block, unless otherwise changed, `x` is bound to the value 10. That means, threads, fibers, etc.
end
```
This is sometimes referred to as dynamic scope. This proposal is to introduce a similar dynamic scope for fiber inheritable attributes.
## Proposal
We have several units of execution in Ruby, implicitly a process, which has threads which has fibers. Internally, Ruby has an "execution context". Each fiber has an execution context. Each thread has a main fiber and execution context. I propose to introduce an interface for defining a per-fiber context which is semantically very similar to dynamic variables. True dynamic variables are stack frame scoped, but my proposal doesn't go that far. This proposal is similar to Kotlin's Coroutine Contexts <https://kotlinlang.org/docs/coroutine-context-and-dispatchers.html> and JEP 429: Extent-Local Variables <https://openjdk.org/jeps/429>.
```ruby
require 'fiber'
# Compatible shim to introduce proposed behaviour:
class Fiber
module Inheritable
def initialize(...)
super(...)
self.inherit_attributes_from(Fiber.current)
end
def self.prepended(klass)
klass.extend(Singleton)
klass.instance_variable_set(:@inheritable_attributes, Hash.new)
end
module Singleton
def inheritable(key, default: nil)
@inheritable_attributes[:"@#{key}"] = default
end
def inheritable_attributes
@inheritable_attributes
end
end
def inherit_attributes_from(fiber)
self.class.inheritable_attributes.each do |name, default|
value = fiber.instance_variable_get(name) || default
self.instance_variable_set(name, value)
end
end
end
end
unless Fiber.respond_to?(:inheritable)
Fiber.prepend(Fiber::Inheritable)
end
```
This allows you to write the following code:
```ruby
class Fiber
inheritable attr_accessor :connection
end
# When lazy enumerator creates internal fiber, the connection and related state will be inherited correctly:
p User.first(100).find_each.zip(Post.first(100).find_each)
```
This proposed implementation was discussed here too: <https://github.com/socketry/fiber-local/pull/1>.
Some open questions:
- Should we introduce the same interface for Thread?
- (or) Should Thread.new's main fiber inherit from the current Fiber?
- Kotlin's coroutine context can be shared but is also immutable. This makes it a little bit harder to use. Should we consider their design more closely?
- Should we have options to bypass inheriting attributes? e.g. `Fiber.new(inherit_attributes_from: ...)`.