[Rd] Segfault: .Call and classes with logical slots

[Luke Tierney]

On 27 Apr 2004, Douglas Bates wrote:

> torsten at hothorn.de writes:
> 
> > On Mon, 26 Apr 2004, John Chambers wrote:
> > 
> > > I think you need to PROTECT the vector you're putting in the slot as
> > > well as the overall object.  At any rate, the problem goes away for me
> > > with the revised version of dummy.c below.
> > 
> > yes, and it seems that PROTECT'ing the logical variable is sufficient
> > while PROTECT'ing the class but not the logical variable causes a
> > segfault again. I tried with numeric slots too: No problems.
> > 
> > > (Empirically, PROTECT'ing
> > > the class definition didn't seem to be needed, but experience suggests
> > > that too much protection  is better than too little.)
> > 
> > I tried to save (UN)PROTECT calls because of efficiency reasons. Anyway,
> > this helps me a lot, thanks!
> 
> Perhaps this example is an indication that gctorture is too
> aggressive.  I use constructions like

It can't be--it only forces gc in places that _could_ result in a gc
withut gctorture; it will not result in a gc in places that otherwise
could not.  The result may be a gc in places that otherwise would be
very unlikely to cause one but not in places that could not.

>    PROTECT(ans = ...);
> 
>    SET_SLOT(ans, install("lgl"), allocVector(LGLSXP,1));
>    LOGICAL(GET_SLOT(ans, install("lgl")))[0] = TRUE;
> 
> in many places in my code, having been assured by a usually reliable
> source (Luke) that SET_SLOT applied to a freshly allocated vector
> would be atomic with respect to garbage collection.  That is, under
> the usual conditions there would be no chance of a garbage
> collection being triggered between the allocVector and SET_SLOT
> operations.  It may be that gctorture is causing a garbage collection
> at a place where it otherwise could not occur and the additional
> (UN)PROTECT are redundant except when gctorture is active.

There are two different scenarios.  Some things are guaranteed not to
allocate, for example low level operations like SET_VECTOR_ELT.
Others, like setAttrib do allocate, but when they do they protect
(some of) their arguments.  So code that uses setAttrib does not need
to protect the arguments to the call (at least the value one--I'd have
to double check the others).  Other variables that are alive before
and after the setAttrib call will need to be protected.

Since slots are stored in attributes we can at most hope for the
second behavior.  But we do not have it.  SET_SLOT is a macro that
expands to R_do_slot_assign, which starts out as

SEXP R_do_slot_assign(SEXP obj, SEXP name, SEXP value) {
    SEXP input = name; int nprotect = 0;
    if(isSymbol(name) ) {
	input = PROTECT(allocVector(STRSXP, 1)); nprotect++; /******/
	SET_STRING_ELT(input, 0, PRINTNAME(name));
    }
    else if(!(isString(name) && LENGTH(name) == 1))
	error("invalid type or length for slot name");
    ...

The actual assignment uses setAttrib, which does operate in a way that
protects the value being assigned, but the unprotected allocation at
/******/ happens before we get there.

So unless we modify SET_SLOT to protect the value argument (and the
others as well to be safe), the value needs to be protected (as do any
other objects that might be needed after the call).

Best,

luke

-- 
Luke Tierney
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
   Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu