[ns] bug fix in scheduler.{h,cc}

[Shashi Guruprasad <shash@cs.utah.edu>]

Problem Description:
====================

Trying to change the scheduler to anything other than the default Calender
scheduler (using Tcl Simulation class's use-scheduler method) causes a seg
fault in wireless scenarii.

The static Scheduler::instance_ field never gets reinitialized back to
NULL when the Scheduler class's destructor is called.

Solution:
=========

Scheduler being a base class needs a virtual destructor with 1 line
reinitializing the field to NULL. 

The diffs with the cvs version of the files is included in this email to
be incorporated by the NS team@ISI

Notes:
======

This fix enables wireless scenarii to be used in NS emulation.

--scheduler.h diff with CVS version 1.20 in the NSNAM cvs tree--
101d100
<       virtual ~Scheduler();
> 
--end scheduler.h diff --

--scheduler.cc diff with CVS version 1.62 in the NSNAM cvs tree--
64,68d63
< Scheduler::~Scheduler()
< {
<     instance_ = NULL ;
< }
< 
--end scheduler.cc diff --


---------- Forwarded message ----------
Date: Fri, 15 Jun 2001 11:58:47 -0600 (MDT)
From: Shashi Guruprasad <shash@famine.cs.utah.edu>
To: Padmaparna Haldar <haldar@ISI.EDU>
Subject: NS wireless examples segfault on changing the scheduler (looks
    like some memory overwrite)

Hi,

I was informed that you have done a lot of porting and hacking of the NS
wireless code. I'm trying to get wireless scenarii to work under emulation
(in allinone 2.1b7a). As a first step, I just tried changing the scheduler
from the default calender to realtime. However, I get a seg fault when
creating a new wireless node. A gdb "where" tells me that the offending
line is 93 in scheduler.cc in the function Scheduler::schedule(...) which
is a virtual function call to the function insert(). The _vptr$ appears to
be corrupted coz its NULL when it should be pointing to the VTBL of class
RealTimeScheduler. The problem occurs for any scheduler (Heap, List) other
than Calendar. I did try putting a watchpoint on the _vptr$'s location but
the watchpoint was never triggered and I'm essentially lost. I built NS
with dmalloc and enabled the highest level of debugging possible
(including ones that are supposed to out of bounds writes). However, I
couldn't solve the problem.

It also appears that 2.1b8 doesn't solve this seg fault although I could't
compile it due to tclsh version mismatch.

Could you please take a quick look at the problem? You'll probably be able
to narrow down the problem a lot quicker. My work's been blocked for the
past 2 days.

Thanks in advance,
Shashi