[retry] Use a separate flag to indicate that a retry timer is running

Using start==0 to indicate a stopped timer is dangerous, because 0 is a
valid value for the current tick counter.

src/include/gpxe/retry.h

 struct retry_timer {
 	/** List of active timers */
 	struct list_head list;
+	/** Timer is currently running */
+	unsigned int running;
 	/** Timeout value (in ticks) */
 	unsigned long timeout;
 	/** Minimum timeout value (in ticks)
 	 * A value of zero means "use default timeout."
 	 */
 	unsigned long max_timeout;
-	/** Start time (in ticks)
-	 *
-	 * A start time of zero indicates a stopped timer.
-	 */
+	/** Start time (in ticks) */
 	unsigned long start;
 	/** Retry count */
 	unsigned int count;
  */
 static inline __attribute__ (( always_inline )) unsigned long
 timer_running ( struct retry_timer *timer ) {
-	return ( timer->start );
+	return ( timer->running );
 }
 
 #endif /* _GPXE_RETRY_H */

src/net/retry.c

  * be stopped and the timer's callback function will be called.
  */
 void start_timer ( struct retry_timer *timer ) {
-	if ( ! timer_running ( timer ) )
+	if ( ! timer->running )
 		list_add ( &timer->list, &timers );
 	timer->start = currticks();
+	timer->running = 1;
 
 	/* 0 means "use default timeout" */
 	if ( timer->min_timeout == 0 )
 void start_timer_fixed ( struct retry_timer *timer, unsigned long timeout ) {
 	start_timer ( timer );
 	timer->timeout = timeout;
+	DBG2 ( "Timer %p expiry time changed to %ld\n",
+	       timer, ( timer->start + timer->timeout ) );
 }
 
 /**
 	unsigned long runtime;
 
 	/* If timer was already stopped, do nothing */
-	if ( ! timer_running ( timer ) )
+	if ( ! timer->running )
 		return;
 
 	list_del ( &timer->list );
 	runtime = ( now - timer->start );
-	timer->start = 0;
+	timer->running = 0;
 	DBG2 ( "Timer %p stopped at time %ld (ran for %ld)\n",
 	       timer, now, runtime );
 
 	/* Stop timer without performing RTT calculations */
 	DBG2 ( "Timer %p stopped at time %ld on expiry\n",
 	       timer, currticks() );
+	assert ( timer->running );
 	list_del ( &timer->list );
-	timer->start = 0;
+	timer->running = 0;
 	timer->count++;
 
 	/* Back off the timeout value */