[Rd] Garbage collection of seemingly PROTECTed pairlist

[Iñaki Ucar]

Hi,

In line 5, you are allocating a vector of length nc. Then, in line 12, you
are using nr as a limit, so if nr goes beyond nc, which is happening in
line 39, you are in trouble.

Iñaki

On Sat, 12 Sep 2020 at 03:30, Rory Nolan <rorynoolan using gmail.com> wrote:

> 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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>
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>


-- 
Iñaki Úcar

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