napi-build

Build support for napi-rs

Setup N-API build in your build.rs:

extern crate napi_build;

fn main() {
    napi_build::setup();
}

wasm32-wasip1-threads allocator lock

On wasm32-wasip1-threads, V8 keeps a stale memory size on a thread that did not grow the shared memory itself, so memory.fill / memory.copy (and, on hosts without V8's wasm trap handler, any access) on heap pages another thread grew can trap with "memory access out of bounds". napi works around it in the allocator: one lock around every call into wasi-libc's allocator, and a refresh of the thread's memory size under that lock.

setup() does the link side. With the wasi-heap-sync feature, and only for the exact wasm32-wasip1-threads target, it passes --wrap for malloc, free, calloc, realloc, posix_memalign, aligned_alloc, malloc_usable_size, __libc_malloc, __libc_free, __libc_calloc and sbrk, so every caller (Rust's System, wasi-libc, emnapi) reaches the __wrap_* functions napi defines. It also links a small bundled wasm object that keeps the malloc / free exports @emnapi/core calls pointed at the wrapped entries (source: src/wasi_heap_sync_exports.c).

• The feature is off by default. napi turns it on through its build-dependency; an addon does not enable it by hand.
• The addon's napi-build must be the same package as napi's build-dependency: the same major version from the same source (a path or git napi needs napi-build from the same checkout). Otherwise Cargo builds two copies, the addon's setup() runs the one without the feature, and the link fails with an undefined symbol, napi_wasi_heap_sync_needs_napi_build_setup_with_wasi_heap_sync.
• Opt out with --cfg napi_wasi_no_heap_sync in the target rustflags, e.g. RUSTFLAGS="--cfg napi_wasi_no_heap_sync". napi-build and napi both read it, and setup() declares it, so addon code can test it too.
• A #[global_allocator] is locked only when it ends in libc malloc, as std's System does. An allocator that calls memory.grow itself (the mimalloc WASI build, talc, lol_alloc, the dlmalloc crate) bypasses the lock.

The cost, the napi_wasm_heap_sync_stat test counters and the other gaps are in the WASI docs.