Reuse database object about duckdb-r HOT 2 CLOSED

cpp11::cpp_source(
  code = '
#define CPP11_USE_FMT
#include "cpp11/function.hpp"
#include "cpp11/list.hpp"
#include "cpp11/external_pointer.hpp"

struct Db {
  Db() { cpp11::message("Db(): {}", (void*)this); }
  ~Db() { cpp11::message("~Db(): {}", (void*)this); }
};

class Dual {
public:
  Dual(Db* db) : precious_(db) { cpp11::message("Dual(db): {}", (void*)this); }
  Dual(std::shared_ptr<Db> db) : precious_(db) { cpp11::message("Dual(dbp): {}", (void*)this); }
  Dual(Dual* dual) : precious_(dual->get()) {
    if (!precious_) {
      cpp11::stop("dual is already released");
    }
    cpp11::message("Dual(dual): {}", (void*)this);
  }
  ~Dual() { cpp11::message("~Dual(): {}", (void*)this); }

  std::shared_ptr<Db> get() const {
    if (precious_) {
      return precious_;
    } else {
      return disposable_.lock();
    }
  }

  std::shared_ptr<Db> operator->() const {
    return get();
  }

  bool has() const {
    return !!get();
  }

  void lock() {
    if (precious_) {
      cpp11::stop("already locked");
    }
    precious_ = disposable_.lock();
    disposable_.reset();
  }

  void unlock() {
    if (!precious_) {
      cpp11::stop("already unlocked");
    }
    disposable_ = precious_;
    precious_.reset();
  }

  void reset() {
    precious_.reset();
    disposable_.reset();
  }

private:
  std::shared_ptr<Db> precious_;
  std::weak_ptr<Db> disposable_;
};

[[cpp11::register]]
SEXP new_dual() {
  cpp11::external_pointer<Dual> x(new Dual(new Db()));
  return x;
}

[[cpp11::register]]
SEXP clone_dual(SEXP dual__) {
  cpp11::external_pointer<Dual> dual_(dual__);
  auto dual = dual_.get();
  if (!dual) {
    cpp11::stop("dual is already released");
  }
  cpp11::external_pointer<Dual> x(new Dual(dual));
  return x;
}

[[cpp11::register]]
bool has_db_from_dual(SEXP dual__) {
  cpp11::external_pointer<Dual> dual_(dual__);
  auto dual = dual_.get();
  if (!dual) {
    return false;
  }
  dual_.get()->has();
}

[[cpp11::register]]
void release_dual(SEXP dual__) {
  cpp11::external_pointer<Dual> dual_(dual__);
  dual_.reset();
}
  '
)

x <- new_dual()
#> Db(): 0x6000024fccb0
#> Dual(db): 0x6000026f2da0
db <- x
db
#> <pointer: 0x6000026f2da0>
has_db_from_dual(db)
#> [1] FALSE
gc()
#>           used (Mb) gc trigger (Mb) limit (Mb) max used (Mb)
#> Ncells  763838 40.8    1454181 77.7         NA  1454181 77.7
#> Vcells 1340195 10.3    8388608 64.0      24576  2048527 15.7
has_db_from_dual(db)
#> [1] FALSE
y <- clone_dual(x)
#> Dual(dual): 0x6000026f2e00

has_db_from_dual(db)
#> [1] FALSE
release_dual(x)
#> ~Dual(): 0x6000026f2da0
has_db_from_dual(db)
#> [1] FALSE
release_dual(y)
#> ~Dual(): 0x6000026f2e00
#> ~Db(): 0x6000024fccb0
has_db_from_dual(db)
#> [1] FALSE

clone_dual(db)
#> Error: dual is already released