[ruby-core:100479] [Ruby master Feature#17278] On-demand sharing of constants for Ractor

[daniel@dan42.com]

Issue #17278 has been reported by Dan0042 (Daniel DeLorme).

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Feature #17278: On-demand sharing of constants for Ractor
https://bugs.ruby-lang.org/issues/17278

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* Priority: Normal
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### Description 

This proposal aims to reduce (but not eliminate) the need for freezing/sharing boilerplate code needed by ractors.

```ruby
A = [1, [2, [3, 4]]]
H = {a: "a"}
Ractor.new do
  p A  #A is not actually modified anywhere, so ok
end.take
H[:b] = "b"  #H was never touched by ractor, so ok
```

## Background

Ractors require objects to be preemptively deep-frozen in order to be shared between ractors. This has an especially visible and restrictive effect on globals and constants. I tried thinking of a different way, and maybe I found one. So please allow me to humbly present this possibility.

## Proposal

A constant would be by default in a "auto-shareable" state (A) which can change atomically to either
(B) "non-shareable" if it is modified by the main ractor
(C) "shareable" (and frozen) if it is accessed by a non-main ractor

In detail:
1. When an object is assigned to a constant, it is added to a list of ractor-reachable objects
2. When the first ractor is created, the objects in that list are recursively marked with FL_AUTOSHARE
   * after this point, constant assignments result directly in FL_AUTOSHARE
3. In the main ractor, a call to `rb_check_frozen` (meaning the object is being modified) will
   1. if FL_AUTOSHARE is set (state A)
      * [with ractor lock]
         * unless object is shareable
             * unset FL_AUTOSHARE (state B)
   2. raise error if frozen
      * ideally with different message if object has FL_SHAREABLE
4. When a non-main ractor accesses a non-shareable constant
   1. if object referenced by constant has FL_AUTOSHARE set (state A)
      * [with ractor lock]
         * if all objects recursively are still marked with FL_AUTOSHARE
             * make_shareable (state C)
         * else
             * unset top objects's FL_AUTOSHARE (state B)
   2. raise error if not shareable 

## Result

So in the case that these 2 things happen in parallel:
1) main ractor modifies content of constant X
2) non-main ractor accesses constant X

There are 2 possible outcomes:
a) main ractor error "can't modify frozen/shared object"
b) non-main ractor error "can not access non-shareable objects in constant X"

## Benefits

In the normal case where non-frozen constants are left untouched after being assigned, this allows to skip a lot of `.freeze` or `Ractor.make_shareable` or `# shareable_constant_value: true` boilerplate.

When you get the error "can not access non-sharable objects in constant X by non-main Ractor", first you have to make that constant X shareable. Then this can trigger a secondary error that X is frozen, that you also have to debug. This way cuts the debugging in half by skipping directly to the FrozenError.

## Downsides

When you get the error "can not access non-sharable objects in constant X by non-main Ractor" you may want to solve the issue by e.g. copying the constant X rather than freezing it. This way makes it slightly harder  to find where X is being accessed in the non-main ractor.

In the case of conflict, whether the error occurs in the main ractor or the non-main ractor can be non-deterministic.

## Applicability

This probably applies as well to global variables, class variables, and class instance variables.



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