Feature #17274: Ractor.make_shareable(obj)

• Status: Closed
• Assignee: -
• Target version: -

[ko1 (Koichi Sasada)]

This ticket describes the semantics of "shareable" and proposes a new method Ractor.make_shareable(obj).
With this method, obj becomes a shareable object by freezing it and reachable objects if it is necessary and it is possible.

Background

"Shareable object" is new term used by Ractors.

• (1) We can send a reference to send a shareable object instead of doing deep copy.
• (2) We can access to a constant which contains a shareable object from non-main ractors.

(1) is (mainly) performance and (2) is programmability (how to rewrite the libraries and so on). See [Feature #1727] for the examples of (2).

The definition of shareable object is thread-safe, ractor-safe object, they are safe to access from multiple ractors simultaneously.

The following conditions are definition of "shareable object" (obj is shareable object if ...).

• SPECIAL_CONST objects are shareable (also be frozen).
• if RBASIC(obj)->flags | FL_SHAREABLE is true, it is shareable.
• T_OBJECT: if all instance variables only refer to shareable objects (def1) and itself is frozen (def2)
• T_ARRAY: (def1) + (def2) + if all elements are shareable objects
• T_HASH: (def1) + (def2) + if all keys and values are sharable objects and default_proc/value (IFNONE, in C-level) is a sharable object
• T_STRUCT: (def1) + (def2) + if all members are shareable objects
• T_RATIONAL: (def1) + (def2) + if num/den are shareaable
• T_COMPLEXL: (def1) + (def2) + if imag/real are shareable
• T_STRING, T_FILE, T_MATCH, T_REGEXP: (def1) + (def2)

T_DATA (user customizable data structure) is difficult problem because if it is frozen, it can modify a state (== we can use (def2)), for example current Queue implementation ignores frozen flag. So we define the semantics like:

• T_DATA: (def1) + if RTYPEDDATA_P(obj) is true and rb_data_type_t::flags | RUBY_TYPED_FROZEN_SHAREABLE, we rely on (def2). Otherwize, this T_DATA object can not become a shareable object. Also we need to check reachable objects are shareable.

Ractor.shareable?(obj) checks this definitions.

Note that you can add FL_SHAREABLE flag to any objects, so if you know there is no mutation or enough protected, you can set the flag and it will be a shareable object. For example, [Feature #17261] use this flag and TVars are shareable objects.

Proposal

As you can see, most of objects are shareable if they are frozen and they are only refers shareable/frozen objects.
Ractor.make_shareable(obj) tries to freeze objects recursively if it is non-shareable objects.

# puseudo-code

def Ractor.make_shareable(obj)
  return obj if Ractor.shareable(obj)

  obj.freeze

  if obj.is T_DATA and (obj.type.flags | RUBY_TYPED_FROZEN_SHAREABLE) == 0
    raise "can not make shareable object for ..."
  end

  obj.reachable_objects{|o|
    Ractor.make_shareable(o)
  }

  # only refer to the shareable objects, so it can be a shareable.
  obj.set! FL_SHAREBLE
end

If it raises an error in the middle of the process, half-baked state are remained.

begin
  Ractor.make_shareable [ a1 = [1, 2],
                          Thread.new{},
                          a2 = [3, 4]]
rescue Ractor::Error
end

p Ractor.shareable?(a1) #=> true
p Ractor.shareable?(a2) #=> false

Implementation

https://github.com/ruby/ruby/pull/3678
and it was already merged to propose https://bugs.ruby-lang.org/issues/17273

[ko1 (Koichi Sasada)]

• Description updated (diff)

[Eregon (Benoit Daloze)]

• Related to Feature #17145: Ractor-aware `Object#deep_freeze` added

[Eregon (Benoit Daloze)]

@ko1 (Koichi Sasada) What's the difference with Object#deep_freeze(skip_shareable: false) from #17145 ?

[ko1 (Koichi Sasada)]

Eregon (Benoit Daloze) wrote in #note-3:

@ko1 (Koichi Sasada) What's the difference with Object#deep_freeze(skip_shareable: false) from #17145 ?

Almost same.

• the ability is to focus skip_shareable: true.
• this ticket describes the detailed semantics of making sharable protocol, including T_DATA. We don't need to describe T_CLASS/T_MODULE/T_ICLASS because they are sharable by birth.

[Eregon (Benoit Daloze)]

• Related to Feature #17273: shareable_constant_value pragma added

[marcandre (Marc-Andre Lafortune)]

• Status changed from Open to Closed