[Rd] Garbage collection of seemingly PROTECTed pairlist

[Rory Nolan]

I want to write an R function using R's C interface that takes a 2-column
matrix of increasing, non-overlapping integer intervals and returns a list
with those intervals plus some added intervals, such that there are no
gaps. For example, it should take the matrix rbind(c(5L, 6L), c(7L, 10L),
c(20L, 30L)) and return list(c(5L, 6L), c(7L, 10L), c(11L, 19L), c(20L,
30L)). Because the output is of variable length, I use a pairlist (because
it is growable) and then I call Rf_PairToVectorList() at the end to make it
into a regular list.

I'm getting a strange garbage collection error. My PROTECTed pairlist prlst
gets garbage collected away and causes a memory leak error when I try to
access it.

Here's my code.

#include <Rinternals.h>


SEXP C_int_mat_nth_row_nrnc(int *int_mat_int, int nr, int nc, int n) {
  SEXP out = PROTECT(Rf_allocVector(INTSXP, nc));
  int *out_int = INTEGER(out);
  if (n <= 0 | n > nr) {
    for (int i = 0; i != nc; ++i) {
      out_int[i] = NA_INTEGER;
    }
  } else {
    for (int i = 0; i != nr; ++i) {
      out_int[i] = int_mat_int[n - 1 + i * nr];
    }
  }
  UNPROTECT(1);
  return out;}

SEXP C_make_len2_int_vec(int first, int second) {
  SEXP out = PROTECT(Rf_allocVector(INTSXP, 2));
  int *out_int = INTEGER(out);
  out_int[0] = first;
  out_int[1] = second;
  UNPROTECT(1);
  return out;}

SEXP C_fullocate(SEXP int_mat) {
  int nr = Rf_nrows(int_mat), *int_mat_int = INTEGER(int_mat);
  int last, row_num;  // row_num will be 1-indexed
  SEXP prlst0cdr = PROTECT(C_int_mat_nth_row_nrnc(int_mat_int, nr, 2, 1));
  SEXP prlst = PROTECT(Rf_list1(prlst0cdr));
  SEXP prlst_tail = prlst;
  last = INTEGER(prlst0cdr)[1];
  row_num = 2;
  while (row_num <= nr) {
    Rprintf("row_num: %i\n", row_num);
    SEXP row = PROTECT(C_int_mat_nth_row_nrnc(int_mat_int, nr, 2, row_num));
    Rf_PrintValue(prlst);  // This is where the error occurs
    int *row_int = INTEGER(row);
    if (row_int[0] == last + 1) {
      Rprintf("here1");
      SEXP next = PROTECT(Rf_list1(row));
      prlst_tail = SETCDR(prlst_tail, next);
      last = row_int[1];
      UNPROTECT(1);
      ++row_num;
    } else {
      Rprintf("here2");
      SEXP next_car = PROTECT(C_make_len2_int_vec(last + 1, row_int[0] - 1));
      SEXP next = PROTECT(Rf_list1(next_car));
      prlst_tail = SETCDR(prlst_tail, next);
      last = row_int[0] - 1;
      UNPROTECT(2);
    }
    UNPROTECT(1);
  }
  SEXP out = PROTECT(Rf_PairToVectorList(prlst));
  UNPROTECT(3);
  return out;}

As you can see, I have some diagnostic print statements in there. The
offending line is line 40, which I have marked with a comment of // This is
where the error occurs. I have a minimal reproducible package at
https://github.com/rorynolan/testpkg and I have run R CMD CHECK with
valgrind using GitHub actions, the results of which are at
https://github.com/rorynolan/testpkg/runs/1076595757?check_suite_focus=true.
That's where I found out which line is causing the error. This function
works as expected sometimes, and then sometimes this issue appears. This
lends weight to the suspicion that it's a garbage collection issue.

I really want to know what my mistake is. I'm not that interested in
alternative implementations; I want to understand the mistake that I'm
making so that I can avoid making it in future.

I have asked the question on stackoverflow to little avail, but the
discussion there may prove helpful.
https://stackoverflow.com/questions/63759604/garbage-collection-of-seemingly-protected-pairlist.



Thanks,

Rory

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