fix(multitude): memory leaks

[geeknoid]

• Arcs are now reference counted individually in order to avoid
  circular reference problem leading to memory leaks. This increases
  the amount of space used by the arc's payload by 4 bytes, and makes
  it so arcs are eagerly disposed (like boxes were) which is a
  semantic improvement.

• Arena::reset was confused and leading to memory leaks of shared chunks

[Copilot]

Pull request overview

This PR addresses an over-allocation bug in multitude::Arena::reset by ensuring reset only affects local-chunk state (instead of also retiring/clearing shared-chunk state), and it updates allocation routing helpers and regression coverage accordingly.

Changes:

• Adjust Arena::reset to stop detaching/emptying the current shared chunk (preventing “one new shared chunk per reset cycle” growth patterns).
• Consolidate oversized-allocation threshold checks into a single is_oversized helper and update call sites.
• Expand regression tests (including loom scenarios) and add new benches to measure/track the affected workload patterns.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
crates/multitude/src/arena/mod.rs	Changes reset behavior; unifies oversized routing helper documentation and usage.
crates/multitude/src/vec/mod.rs	Updates vec growth path to use the unified is_oversized.
crates/multitude/src/arena/alloc_value.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_utf16.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_unsized.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_str.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_slice_ref.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_slice_box.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_slice_arc.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/arena/alloc_prefixed.rs	Updates oversized routing checks to is_oversized.
crates/multitude/src/allocator_impl.rs	Updates allocator impl oversized routing to is_oversized.
crates/multitude/tests/arena.rs	Adds regression tests validating shared-chunk stability and nested-Arc validity across reset cycles.
crates/multitude/tests/alloc_ref.rs	Refines wasted-tail invariants to reflect local-vs-shared reset semantics.
crates/multitude/tests/loom.rs	Renames/retargets loom tests to reflect reset-vs-drop semantics around shared chunks.
crates/multitude/tests/mutant_kills_post_fix.rs	Updates post-fix mutant test commentary for the renamed oversized helper.
crates/multitude/tests/zst_uninit_arc_fix.rs	Updates regression comment to the unified is_oversized naming.
crates/multitude/Cargo.toml	Adds dev-dep + bench targets for the new arc-array benchmarks.
crates/multitude/benches/criterion_arc_array.rs	Introduces a criterion benchmark intended to compare global vs arena-backed arc-array builds.
crates/multitude/benches/gungraun_arc_array/main.rs	Adds a gungraun benchmark entrypoint for instruction-precise arc-array builds.
crates/multitude/benches/gungraun_arc_array/linux.rs	Implements the Linux-only gungraun arc-array benchmark bodies and setup.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (b9f7583) to head (9c93baf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #508   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         345      345           
  Lines       26248    26159   -89     
=======================================
- Hits        26248    26159   -89     

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[Copilot]

Pull request overview

Copilot reviewed 21 out of 22 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

crates/multitude/src/arena/alloc_utf16.rs:189

• This loop reserves total bytes with elem_align, but routes oversized allocations and refills based on total alone. Because try_alloc may need up to elem_align - 1 bytes of alignment padding at the front of the reservation, using total for is_oversized/refill_shared can miss the max-normal boundary and lead to repeated refills. Using a refill_hint = total + elem_align for routing/refill (while still reserving total) avoids the total vs total + align mismatch.

            if self.is_oversized(total) {
                return self.alloc_oversized_shared_with(total, |mutator, chunk_ptr| {
                    let (base, _chunk_unused) = mutator
                        .try_alloc_with_chunk(total, elem_align)
                        .expect("dedicated oversized chunk sized to fit utf-16 payload");

[Copilot]

Pull request overview

Copilot reviewed 23 out of 24 changed files in this pull request and generated 1 comment.

[martintmk]

I tried to rebase my example:
#509

on top of this change and I was hoping the amount of allocations will drop significantly, but that was not the case.

[Copilot]

Pull request overview

Copilot reviewed 21 out of 21 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 40 out of 41 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 56 out of 57 changed files in this pull request and generated 1 comment.