Feature #17274
closed
Ractor.make_shareable(obj)
Description
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_SHAREABLEis 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) + ifRTYPEDDATA_P(obj)is true andrb_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
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