Module refcount_protect 

Reference-counted GC protection using a BTreeMap + VECSXP backing.

This module provides an alternative to gc_protect that uses reference counting instead of R’s LIFO protect stack. This allows releasing protections in any order and avoids the --max-ppsize limit.

Architecture

┌─────────────────────────────────────────────────────────────────────┐
│  RefCountedArena / ThreadLocalArena                                 │
│  ┌─────────────────────────┐   ┌───────────────────────────────────┐│
│  │  BTreeMap<usize, Entry> │   │  VECSXP (R_PreserveObject'd)      ││
│  │  ────────────────────── │   │  ─────────────────────────────    ││
│  │  sexp_a → {count:2, i:0}│◄──┤  [0]: sexp_a                      ││
│  │  sexp_b → {count:1, i:1}│◄──┤  [1]: sexp_b                      ││
│  │  sexp_c → {count:1, i:2}│◄──┤  [2]: sexp_c                      ││
│  └─────────────────────────┘   │  [3]: <free>                      ││
│                                └───────────────────────────────────┘│
└─────────────────────────────────────────────────────────────────────┘

Available Types

Type	Storage	Use Case
RefCountedArena	RefCell	General purpose, ordered, ppsize-scale workloads
ThreadLocalArena	thread_local	Lowest overhead — no RefCell borrow checking

Only these two flavors are instantiated anywhere in the tree today. This module previously also shipped HashMap- and ahash-backed variants (HashMapArena, ThreadLocalHashArena, FastHashMapArena, ThreadLocalFastHashArena) behind a generic MapStorage abstraction; they were removed for having zero production or test consumers (see https://github.com/a2-ai/miniextendr/issues/ISSUE_NUMBER_PLACEHOLDER). Re-add a flavor (and the generic MapStorage plumbing needed to support more than one map type again) when a real consumer appears.

Structs

ArenaGuard

An RAII guard that unprotects a SEXP when dropped.

ArenaState 🔒

Core arena state without interior mutability.

Entry 🔒

Entry in the reference count map.

RefCountedArena

A reference-counted arena for GC protection, backed by a BTreeMap.

ThreadLocalArena

Thread-local, BTreeMap-backed reference-counted GC protection arena.

ThreadLocalState 🔒

State wrapper for the thread-local arena.

Constants

THREAD_LOCAL_STATE 🔒

Type Aliases

NoSendSync 🔒

Enforces !Send + !Sync (R API is not thread-safe).