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Share array entities if possible with ary.replace(frozen_ary) #6576

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Share array entities if possible with ary.replace(frozen_ary) #6576

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Share array entities if possible with ary.replace(frozen_ary) 
Currently mruby is limited to always placing array entities on the rewritable heap.
Therefore, if the original array was frozen and only rewritable objects survived the subsequent process, the array entity can be changed.
If an array object that actually shared the array entity is frozen with `ary.freeze`, there should be no problem, since the shared state is still kept.
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dearblue committed Jul 18, 2025
commit 3c653fc0d481a568aa8ff8f32c6c1dcc5fca418b
2 changes: 1 addition & 1 deletion src/array.c
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Expand Up @@ -511,7 +511,7 @@ ary_replace(mrb_state *mrb, struct RArray *a, struct RArray *b)
mrb_write_barrier(mrb, (struct RBasic*)a);
return;
}
if (!mrb_frozen_p(b) && len > ARY_REPLACE_SHARED_MIN) {
if (len > ARY_REPLACE_SHARED_MIN) {
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@gemini-code-assist gemini-code-assist Bot Jul 18, 2025

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critical

This change removes the !mrb_frozen_p(b) check, which allows ary_make_shared(mrb, b) to be called on a frozen array b.

The ary_make_shared() function modifies the internal state of the array passed to it. Specifically, it updates the aux union member to point to a mrb_shared_array struct and sets the MRB_ARY_SHARED flag:

// in ary_make_shared(mrb, a) where a is b
a->as.heap.aux.shared = shared;
ARY_SET_SHARED_FLAG(a);

Modifying a frozen object, even its internal metadata, violates the immutability guarantee of frozen objects in Ruby. This could lead to unexpected behavior and subtle bugs if other parts of the system rely on frozen objects being truly immutable.

While mrb_obj_freeze() for arrays does shrink the capacity to match the length (avoiding a realloc inside ary_make_shared()), the modification of the RArray struct itself remains a serious concern.

Is this change in the semantics of frozen arrays intentional? If so, this is a significant change that should be carefully documented. Otherwise, this change appears to introduce a critical bug.

An alternative to achieve sharing from a frozen array would require a mechanism that does not modify the source frozen array, which would likely be a more significant change involving array and GC management.

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@dearblue dearblue Jul 18, 2025

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Is this change in the semantics of frozen arrays intentional? If so, this is a significant change that should be carefully documented. Otherwise, this change appears to introduce a critical bug.

If this is to be a future concern, I think an equivalent to RSTR_NOFREE_P() needs to be introduced at this time.

ary_make_shared(mrb, b);
goto shared_b;
}
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