[ruby-core:73439] [CommonRuby - Feature #8259] Atomic attributes accessors

Issue #8259 has been updated by Petr Chalupa.


I would like to revive this issue again and link it to related efforts to provide complete set of low-level tools for writing concurrent libraries. The aggregating issue of this effort can be found [here](https://bugs.ruby-lang.org/issues/12019).

Summary can be found in this document: https://docs.google.com/document/d/1c07qfDArx0bhK9sMr24elaIUdOGudiqBhTIRALEbrYY/edit#. It suggests to use following syntax `attr :name, atomic: true`, which makes the variable `name` volatile but it also generates atomic helpers: `get_and_set_name`, `compare_and_set_name`, `compare_and_exchange_name`, `update_name`. They use referential equality for comparison.

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Feature #8259: Atomic attributes accessors
https://bugs.ruby-lang.org/issues/8259#change-56656

* Author: Yura Sokolov
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Motivated by this gist ((<URL:https://gist.github.com/jstorimer/5298581>)) and atomic gem

I propose Class.attr_atomic which will add methods for atomic swap and CAS:

```ruby
  class MyNode
    attr_accessor :item
    attr_atomic :successor

    def initialize(item, successor)
      @item = item
      @successor = successor
    end
  end
  node = MyNode.new(i, other_node)

  # attr_atomic ensures at least #{attr} reader method exists. May be, it should
  # be sure it does volatile access.
  node.successor

  # #{attr}_cas(old_value, new_value) do CAS: atomic compare and swap
  if node.successor_cas(other_node, new_node)
    print "there were no interleaving with other threads"
  end

  # #{attr}_swap atomically swaps value and returns old value.
  # It ensures that no other thread interleaves getting old value and setting
  # new one by cas (or other primitive if exists, like in Java 8)
  node.successor_swap(new_node)
```

It will be very simple for MRI cause of GIL, and it will use atomic primitives for
other implementations.

Note: both (({#{attr}_swap})) and (({#{attr}_cas})) should raise an error if instance variable were not explicitly set before.

Example for nonblocking queue: ((<URL:https://gist.github.com/funny-falcon/5370416>))

Something similar should be proposed for Structs. May be override same method as (({Struct.attr_atomic}))

Open question for reader:
should (({attr_atomic :my_attr})) ensure that #my_attr reader method exists?
Should it guarantee that (({#my_attr})) provides 'volatile' access?
May be, (({attr_reader :my_attr})) already ought to provide 'volatile' semantic?
May be, semantic of (({@my_attr})) should have volatile semantic (i doubt for that)?




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