[Rd] [External] Possible ALTREP bug

[Jim Hester]

>From reading the discussion on the Bioconductor issue tracker it seems like
the reason the GC is not suspended for the non-string ALTREP Elt methods is
primarily due to performance concerns.

If this is the case perhaps an additional flag could be added to the
`R_set_altrep_*()` functions so ALTREP authors could indicate if GC should
be halted when that particular method is called for that particular ALTREP
class.

This would avoid the performance hit (other than a boolean check) for the
standard case when no allocations are expected, but allow authors to
indicate that R should pause GC if needed for methods in their class.

On Fri, May 28, 2021 at 9:42 AM <luke-tierney using uiowa.edu> wrote:

> integer and real Elt methods are not expected to allocate. You would
> have to suspend GC to be able to do that. This currently can't be done
> from package code.
>
> Best,
>
> luke
>
> On Fri, 28 May 2021, Gábor Csárdi wrote:
>
> > I have found some weird SEXP corruption behavior with ALTREP, which
> > could be a bug. (Or I could be doing something wrong.)
> >
> > I have an integer ALTREP vector that calls back to R from the Elt
> > method. When this vector is indexed in a lapply(), its first element
> > gets corrupted. Sometimes it's just a type change to logical, but
> > sometimes the corruption causes a crash.
> >
> > I saw this on macOS from R 3.5.3 to 4.2.0. I created a small package
> > that demonstrates this: https://github.com/gaborcsardi/redfish
> >
> > The R callback in this package calls `loadNamespace("Matrix")`, but
> > the same crash happens for other packages as well, and sometimes it
> > also happens if I don't load any packages at all. (But that example
> > was much more complicated, so I went with the package loading.)
> >
> > It is somewhat random, and sometimes turning off the JIT avoids the
> > crash, but not always.
> >
> > Hopefully I am just doing something wrong in the ALTREP code (see
> > https://github.com/gaborcsardi/redfish/blob/main/src/test.c), and it
> > is not actually a bug.
> >
> > Thanks,
> > Gabor
> >
> > ______________________________________________
> > R-devel using r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
> --
> Luke Tierney
> Ralph E. Wareham Professor of Mathematical Sciences
> University of Iowa                  Phone:             319-335-3386
> Department of Statistics and        Fax:               319-335-3017
>     Actuarial Science
> 241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
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