(fix): fallback type memory leak in checkpoint/rewind cycle

[mgyoo86]

Problem

Fallback types (e.g., ForwardDiff.Dual) stored in pool.others leaked n_active on repeated @with_pool calls. After N iterations, n_active grew to N instead of returning to 0.

Root cause — two independent bugs in _touched_has_others tracking:

1. get_typed_pool!: When creating a new fallback type inside a checkpoint scope, _touched_has_others was not set to true. The rewind path (_typed_lazy_rewind!) skipped pool.others entirely.

2. checkpoint!(pool, types...): Always pushed has_others = false, regardless of whether any type was a fallback. On the 2nd+ call (type already exists, get_typed_pool! closure doesn't fire), the flag stayed false — causing the same skip in _typed_lazy_rewind!.

Fix

Location	Change
get_typed_pool!	Set _touched_has_others[depth] = true when auto-checkpointing a new fallback type
checkpoint!(pool, ::Type{T})	Push _fixed_slot_bit(T) == UInt16(0) (constant-folded at compile time)
checkpoint!(pool, types...)	Compute has_any_fallback at compile time in @generated body via _fixed_slot_bit
_checkpoint_typed_pool!	Add idempotency guard to prevent double-push from get_typed_pool! auto-checkpoint + explicit checkpoint
CUDA extension	Mirror all fixes for CuAdaptiveArrayPool parity

Performance impact: zero. All fallback detection is resolved at compile time (@generated body or constant folding). The idempotency guard adds one @inbounds array-end check (~0.3ns).

Tests

• Added 28-section test_fallback_reclamation.jl covering all fallback lifecycle scenarios
• Tests 29, 29b specifically reproduce the real-world ForwardDiff.gradient leak pattern with repeated typed checkpoints
• TDD-validated: tests fail without fix, pass with fix
• Full suite: 387 tests passing, 96% coverage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.47%. Comparing base (3645a32) to head (13a64de).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #19   +/-   ##
=======================================
  Coverage   96.46%   96.47%           
=======================================
  Files           9        9           
  Lines        1273     1276    +3     
=======================================
+ Hits         1228     1231    +3     
  Misses         45       45           

Files with missing lines	Coverage Δ
src/state.jl	98.71% <100.00%> (+0.01%)	⬆️
src/types.jl	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

Fixes a leak in fallback-type (pool.others) accounting during repeated checkpoint/rewind cycles—especially in typed / typed-lazy macro paths where _acquire_impl! bypasses _record_type_touch!—and adds regression tests (including a ForwardDiff.Dual-like scenario) plus CUDA extension parity.

Changes:

• Ensure fallback pools mark has_others during first-touch creation (get_typed_pool!) so lazy/typed-lazy rewind doesn’t skip pool.others.
• Correct checkpoint! (single- and multi-type) to push has_others=true when any tracked type is a fallback; add an idempotency guard to avoid double checkpoint pushes.
• Add an extensive fallback reclamation test suite and include it in runtests.jl; mirror fixes in the CUDA extension.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/types.jl	Marks _touched_has_others when creating a new fallback typed pool inside a checkpointed scope.
src/state.jl	Fixes has_others tracking for typed checkpoints; adds checkpoint idempotency guard.
ext/AdaptiveArrayPoolsCUDAExt/dispatch.jl	Mirrors the get_typed_pool! fallback-touch fix for CUDA pools.
ext/AdaptiveArrayPoolsCUDAExt/state.jl	Mirrors has_others logic for CUDA typed checkpoints (single + varargs).
test/test_fallback_reclamation.jl	Adds comprehensive regression coverage for fallback lifecycle / leak scenarios.
test/runtests.jl	Runs the new fallback reclamation test file in the suite.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.