[Rd] segfault during build of 2.1.0 on RH9; print.POSIXct implicated (PR#7827)

[Peter Dalgaard]

Jskud at jskud.com writes:
>     > unusual_but_ok <- c.POSIXlt(character(0))
>     > unusual_but_ok
>     character(0)
>     > unusual_and_faults <- c.POSIXct(character(0))
>     > unusual_and_faults
>     Segmentation fault
> 
> Running this test program under gdb, we find that we're running off the
> end of the stack, with 4222 stack frames showing -- apparently in an
> infinite recursion -- "as.character" shows up every 69 function calls:

This gives a protection stack overflow on FC3 and RH8. Is the stack
particularly small on RH9? I have 8MB on RH8 and 10MB on FC3. (The R
limits for expression depth and pointer protection were increased in
2.1.0). I can force a segfault, but only after "ulimit -s 1024" or so.
 
> #64 0x080ea1ef in Rf_usemethod (generic=0x81c8b12 "as.character", obj=0x0, call=0x85a04f4, 
...
> So it would seem that *printing* the unusual POSIXct value is suspect.

Pretty obviously, yes.

> Looking at a R-1.8.1 install, we find these definitions in base/R/base:
> 
>     print.POSIXct <- function(x, ...)
>     {
> 	print(format(x, usetz=TRUE), ...)
> 	invisible(x)
>     }
> 
>     print.POSIXlt <- function(x, ...)
>     {
> 	print(format(x, usetz=TRUE), ...)
> 	invisible(x)
>     }
> 
> However, looking at the 2.1.0 src file
> R-2.1.0/src/library/base/R/datetime.R, we find
> 
>     print.POSIXct <- function(x, ...)
>     {
> 	print(format(x, usetz=TRUE, ...), ...)
> 	invisible(x)
>     }
> 
>     print.POSIXlt <- function(x, ...)
>     {
> 	print(format(x, usetz=TRUE), ...)
> 	invisible(x)
>     }
> 
> Note the suspicious definition of print.POSIXct using *two* sets of
> ellipses, and that the print.POSIXct and print.POSIXlt definitions no
> longer match.

Probably both should use the double ellipses, but "..." will be empty
on automatic printing, so that's not it. 

The issue is an infinite recursion inside as.POSIXlt(). Specifically,
strptime() internally calls as.character(x) inside the fromchar()
function in as.POSIXlt(), and as.character.POSIXt() invokes format()
which calls as.POSIXlt() again.

I think the fix is to unclass(x) inside fromchar(), but perhaps others
know better?
 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907