R-alpha: browser()

[Thomas Lumley]

Kurt pointed out that
R> browser() 
is now the shortest segfault.  The problem is in do_browser in main.c at
about line 420:
       if( !DEBUG(rho) ) {
                cptr=R_GlobalContext;
                while (cptr->callflag != CTXT_RETURN) 
                        cptr = cptr->nextcontext;
                Rprintf("Called from: ");
                PrintValueRec(cptr->call);
        }
When browser() is called from the top level rather than from a function we
have cptr->callflag==CTXT_TOPLEVEL, so the test succeeds, but as there are
no other contexts we have cptr->nextcontext==NULL and a segfault.


Thomas Lumley
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