savvy 0.9.4

A simple R extension interface
Documentation 
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// This protection mechanism is basically a simple Rust translation of the
// implementation of cpp11.
//
// https://github.com/r-lib/cpp11/blob/main/inst/include/cpp11/protect.hpp
//
// The more explanation on this can be found on the following links:
//
// - https://github.com/RcppCore/Rcpp/issues/1081
// - https://cpp11.r-lib.org/articles/internals.html#protection
//
// However, this implementation differs from these two in several points. First,
// cpp11 stores the anchor Robj in the global options. It says it's because
//
//     It is not constructed as a static variable directly since many
//     translation units may be compiled, resulting in unrelated instances of each
//     static variable.
//
// I'm not immediately sure when this actually happens, but I think I can skip
// the consideration.
//
// Note that, extendr uses a different mechanism of using HashMap to track the
// reference counts.
//
// https://github.com/extendr/extendr/blob/main/extendr-api/src/ownership.rs
//
// I'm not sure why they chose this design, but probably it is because
//
// - for parallel-proof implementation
// - `Robj` might be cloned
//
// But, my implementation doesn't implement `Clone` trait, so I don't need to
// worry that there still exists another instance on dropping it.

use savvy_ffi::{
    R_NilValue, R_PreserveObject, Rf_cons, Rf_protect, Rf_unprotect, CAR, CDR, SETCAR, SETCDR,
    SET_TAG, SEXP,
};
use std::sync::OnceLock;

// Protection mechanism by `Rf_protect()`. This struct is needed for
// auto-unprotect when returning from the scope.

pub(crate) struct LocalProtection {}

impl Drop for LocalProtection {
    fn drop(&mut self) {
        unsafe { Rf_unprotect(1) };
    }
}

/// Provide a protection that lasts within the function scope, i.e.,
/// automatically cleans up by `Rf_unprotect()`. This might not be very
/// efficient as this can execute `Rf_unprotect(1)` multiple times where it
/// could be `Rf_unprotect(n)` once. But, I found manual `Rf_unprotect()` is
/// almost impossible for human considering there are many early return by `?`,
/// so this should be better than failure.
pub(crate) fn local_protect(obj: SEXP) -> LocalProtection {
    unsafe { Rf_protect(obj) };
    LocalProtection {}
}

// Protection mechanism by a doubly-linked pairlist.
// cf. https://cpp11.r-lib.org/articles/internals.html#protection

pub(crate) struct PreservedList(SEXP);

// cf. https://doc.rust-lang.org/stable/nomicon/send-and-sync.html
unsafe impl Send for PreservedList {}
unsafe impl Sync for PreservedList {}

pub(crate) static PRESERVED_LIST: OnceLock<PreservedList> = OnceLock::new();

#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn insert_to_preserved_list(obj: SEXP) -> SEXP {
    unsafe {
        if obj == R_NilValue {
            return R_NilValue;
        }

        // Protect the object until the operation finishes
        let _obj_guard = local_protect(obj);

        let preserved = PRESERVED_LIST.get_or_init(|| {
            let r = Rf_cons(R_NilValue, R_NilValue);
            R_PreserveObject(r);
            PreservedList(r)
        });
        let token = Rf_cons(preserved.0, CDR(preserved.0));

        let _token_guard = local_protect(token);

        SET_TAG(token, obj);
        SETCDR(preserved.0, token);

        if CDR(token) != R_NilValue {
            SETCAR(CDR(token), token);
        }

        token
    }
}

#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn release_from_preserved_list(token: SEXP) {
    unsafe {
        if token == R_NilValue {
            return;
        }

        let before = CAR(token);
        let after = CDR(token);

        SETCDR(before, after);

        if after != R_NilValue {
            SETCAR(after, before);
        }
    }
}