Re: Annoying problem with subprocesses

[sdupont@xxxxxxx]

On Mon, 27 Jan 2003 11:04:21 -0500
"John E. Davis" <davis@xxxxxxxxxxxxx> wrote:

> sdupont@xxxxxxx <sdupont@xxxxxxx> wrote:
> >1. the process (0) gets killed in jed_close_process(),
> >   signal_handler will do cleanup (but not yet)
> >2. new process (1) is created with open_process(),
> >   no cleanup done so far
> >3. sys_input_pending() is trying to read_process_input() 
> >   and than it happens "what never should happen":
> >   Since cleanup wasn't done or wasn't ready, Num_Subprocesses
> >   is 2 (0 and 1) though in fact there is only one (1).
> 
> "what should never happen" happens in the get_process function.  And
> that only happens if p->flags is 0.  It is not clear to me how
> p->flags is getting set to 0.  The only place I can see this happening
> is in the child_signal_handler after the call to waitpid.  Does adding
> a check for status==0 after waitpid help?  That is, use:
> 
>         pid = (int) waitpid (-1, &status, WNOHANG | WUNTRACED);
>         if (pid == -1)
>           {
>              if (errno == ECHILD) break;
>              continue;
>           }
> 	   .
> 	   .
> 	 /* What else?? */
> 
> /* ADD CHECK HERE */
> 	if (status == 0)  
> 	  continue;
> 
>         p = Processes;
> 	pmax = p + MAX_PROCESSES;
> 
> If that solves the problem, my next question is what does status==0
> mean?
> [...]  

I tried the above but it doesn't help. In fact, in all the tests
I did, status is != 0.
So I spent another couple of hours trying to track this error down.
After a lot more printfs I found that it isn't necessary to force
the call to get_process_status(), since the problem appears *after*
the cleanup is done. The problem is that sys_input_pending() is trying
to read from the wrong fd (read descriptor), though get_process_status() 
*has* done the cleanup (adjustment of Subprocess_Read_fds array). 
So I checked get_process_status() again and there seems to be a bug
in the adjustment since Subprocess_Read_fds[i][0] (the actual fd)
is checked and not Subprocess_Read_fds[i][1] (the jed rep.) as it should(?):

static void get_process_status (Process_Type *p) /*{{{*/
{
   [...]
   /* Adjust the array of read descriptors */

   i = 0;
   
   while (i < Num_Subprocesses)
     { 
	/* if (Subprocess_Read_fds[i][0] == slfd) WRONG? */
        if (Subprocess_Read_fds[i][1] == slfd)
	  break;
	  
	i++;
     }
   /*fd = Subprocess_Read_fds [i][0]; WRONG? */
   fd = Subprocess_Read_fds [i][1];

   [...]

   if (Max_Subprocess_FD == fd)
     {
	i = 0;
	fd = -1;
	while (i < Num_Subprocesses)
	  {
	     /*if (Subprocess_Read_fds[i][0] > fd) 
	       fd = Subprocess_Read_fds[i][0]; WRONG? */
             if (Subprocess_Read_fds[i][1] > fd) 
	       fd = Subprocess_Read_fds[i][1];
	     i++;
	  }
	Max_Subprocess_FD = fd;
     }

Doing this little change, subprocesses work as expected (=GREAT!)

Greetings

Stefan