This fixes a lock-free data race in libpas (bmalloc) that could return a stale/freed medium-directory pointer and drive allocator confusion. pas_segregated_heap_medium_directory_tuple_for_index supports a fast, lock-free lookup guarded by a mutation-count ‘dependency’ scheme: it reads directory metadata without holding the heap lock and validates afterward that the mutation count did not change. The pre-patch impl returned a raw pas_segregated_heap_medium_directory_tuple* (result.tuple) that callers then DEREFERENCED after the lock-free search (reading directory->allocator_index and pas_compact_atomic_segregated_size_directory_ptr_load(&directory->directory)). Because a concurrent mutation (e.g. the scavenger decommitting/rematerializing expendable memory, or directory reallocation) can free or reuse that tuple’s memory between the read and the dereference, callers could dereference stale/freed metadata and obtain a wrong size directory or allocator_index.

The fix introduces pas_segregated_heap_medium_directory_result, which captures the needed values (directory pointer and allocator_index) DURING the guarded read and folds them into the mutation-count dependency chain, returning them BY VALUE; the raw tuple pointer is kept only as tuple_unsafe_without_lock, explicitly documented as valid only while holding the heap lock. Callers (medium_allocator_index_for_index, medium_size_directory_for_index) now use result.allocator_index / result.directory directly instead of dereferencing the tuple, and lock-holding callers use tuple_unsafe_without_lock. Added PAS_TESTING_ASSERTs (size <= allocator->object_size) in pas_local_allocator_try_allocate guard the downstream consequence — using a wrong allocator index to allocate into a size class that is too small.

The restored invariant is that values obtained on the lock-free path are snapshotted within the guarded read rather than fetched by dereferencing a pointer that a concurrent mutation can invalidate.

A race in the core allocator confined to the process using it (WebContent), where a lock-free lookup could dereference freed medium-directory metadata and yield a wrong size class / allocator index — a path to heap metadata corruption. Allocator-internal races are difficult to weaponize and timing-dependent; the advisory rates it a crash, but allocator confusion is a serious corruption class if made reliable.