Module worker 

Worker thread infrastructure for safe Rust-R FFI.

Why a worker thread at all?

R’s error handling uses longjmp, which skips Rust destructors and leaks resources. To contain that, #[miniextendr]-generated function bodies run on a separate Rust thread (the “worker”). R only longjmps on its own main thread, so Rust frames on the worker are unwind-safe.

That split means user code is off the R main thread whenever the worker-thread cargo feature is enabled. Anything that calls R’s C API (allocating SEXPs, walking attributes, accessing INTEGER(x)) must cross back to main via with_r_thread.

Public API

• with_r_thread — Execute a closure on R’s main thread. This is the bridge: call it from inside a #[miniextendr] body whenever you need to touch the R FFI.
• is_r_main_thread — Check if the current thread is R’s main thread.
• Sendable — #[doc(hidden)] wrapper used by the macros to ferry SEXP (and other !Send types) across the worker channel. The author asserts the value is only consumed on the main thread.

Tradeoffs

• Default to checked FFI variants (Rf_allocVector, INTEGER, …) so the debug-assertion catches accidental off-thread calls.
• Inside a with_r_thread body, the assertion is redundant — the *_unchecked variants in crate::sys are safe to call there (recognised by the lint MXL301, alongside ALTREP callbacks and crate::unwind_protect::with_r_unwind_protect bodies).
• Don’t raise R errors directly from worker-thread code. Rf_error would longjmp through Rust frames on the wrong thread. Panic instead; the framework converts the panic into a structured R condition (see crate::error_value). The lint MXL300 enforces this.

Feature gate: worker-thread

Without the worker-thread cargo feature, all calls execute inline on R’s main thread:

• with_r_thread(f) runs f() directly (panics if not on main thread)
• run_on_worker(f) runs f() directly, returns Ok(f())

With the feature enabled, a dedicated worker thread is spawned at init time. with_r_thread routes calls from the worker back to main, and run_on_worker dispatches closures to the worker with bidirectional communication. The worker has a 16 MB stack — keep proptest! invocations on it modest (see the project CLAUDE.md).

Initialization

miniextendr_runtime_init must be called from R’s main thread before any R FFI APIs. Typically done in R_init_<pkgname>().

Cross references

• crate::unwind_protect::with_r_unwind_protect — catch R errors with Rust cleanup; sibling to with_r_thread.
• crate::sys — checked vs *_unchecked FFI surface.
• crate::ffi_guard — guard taxonomy across boundaries.

Structs

Sendable 👻

Wrapper to mark values as Send for main-thread routing.

Statics

R_MAIN_THREAD_ID 🔒

Functions

assert_runtime_initialized 🔒

Panic if the runtime hasn’t been initialized.

has_worker_context 🔒

Check whether the current thread has a worker routing context.

is_r_main_thread

Check if the current thread is R’s main thread.

miniextendr_runtime_init 👻

Initialize the miniextendr runtime.

miniextendr_runtime_shutdown 👻

Shut down the miniextendr worker thread synchronously.

panic_message_to_r_error 👻

Raise an R error from a panic message. Does not return.

run_on_worker 👻

Run a closure on the worker thread with proper cleanup on panic.

with_r_thread

Execute a closure on R’s main thread, returning the result.