sigsuspend gets interrupted by ptrace

Consider this simple program:

#include <unistd.h>

int main(void)
{
          pause();
          return 0;
}

$ ./a.out &
$ truss -p `pgrep a.out`
process exit, rval = 0
[1]   Done                    ./a.out
$

Surely this must be wrong. The program should have just continued running after 
truss was attached to it.

(PR Filed under kern/114155)

Cheers
Michiel