Perhaps fix an overflow bug relate to timer

[yulincoder]

Hello, I find a bug relates to timer system and try to fix it, which causes overflow task to execute and all of the other tasks to fail. The system will crash and the battery is quickly exhausted (not "slightly higher runtime cost" in the comments of code author in the function because 'remaining' will always be negative and cannot be corrected). The following code can trigger bugs,

call Timer2.startPeriodic( 111UL << 28);  

then you can see that the node is always running at full capacity, it will run the timer task with bugs forever.
Although the code author noticed the overflow problem, he/she did not solve it.
This problem appears to have been triggered in an early CTP protocol, but it has not really been resolved in underlying. [ 11ff964 ]

The idea my fix is to disable the timer that triggered the bug so that timer with the bug can not influent each other task. In this case, other tasks in the system can handle events normally, and the battery won't be drained quickly.

[yulincoder]

Any review?

[cire831]

I could have sworn I asked what machine you are building this for? But don't see any reference to it.

So what processor are you building this for?

I've looked at the code and don't understand why your code doesn't work. More investigation into the root cause is warranted.

[yulincoder]

@cire831 I am sorry to close another pull request so that did not notice your comment.
I have been building TinyOS on TelosB platform.
The cause of system crash is the following:

1. In the updateFromTimer task, the timer->dt may be a big value as unsigned int in some case, and the elapsed is not a big enough value. In this case, the value of remaining in int32_t remaining = timer->dt - elapsed; is a negative value forever because of the type conversion from uint32_t( timer->dt is a uint32_t variable) to int32_t.
2. As mentioned above, the following conditions are always satisfied. So the fireTimers will be always executed

if (min_remaining <= 0)
	  fireTimers(now);

3. Sadly, the updateFromTimer is posted in the function of fireTimers. The system caught up in this endless cycle:
   updateFromTimer ---> fireTimers --->post updateFromTimer ---> updateFromTimer ---> fireTimers --->post updateFromTimer ---> .....
   It results in the system runs at full capacity and other tasks in system are difficult to perform properly until the battery runs out.

[cire831]

okay. I'll see what I can do. I'll take a look at it in the next few days.

[yulincoder]

@cire831 Do you have a good fix?