[ptrace] please review follow fork/exec changes

[Konstantin Belousov]

On Mon, Feb 13, 2012 at 02:50:45PM -0800, Dmitry Mikulin wrote:
> >>>It seems that now wait4(2) can be called from the real (non-debugger)
> >>>parent first and result in the call to proc_reap(), isn't it ? We would
> >>>then just reparent the child back to the caller, still leaving the
> >>>zombie and confusing debugger.
> >>When either gdb or the real parent gets to proc_reap() the process 
> >>wouldn't
> >>get destroyed, it'll get caught by the following clause:
> >>     if (p->p_oppid&&  (t = pfind(p->p_oppid)) != NULL) {
> >>
> >>and the real parent with get the child back into the children's list while
> >>gdb will get it into the orphan list. The second time around when
> >>proc_reap() is entered, p->p_oppid will be 0 and the process will get
> >>really reaped. Does it make sense? And proc_reparent() attempts to keep 
> >>the
> >>orphan list clean and not have the same entries and the list of siblings.
> >Right, this is what I figured. But I asked about some further implication
> >of this change:
> >
> >if real parent spuriosly calls wait4(2) on the child pid after the child
> >exited, but before the debugger called the wait4(), then exactly the
> >code you noted above will be run. This results in the child being fully
> >returned to the original parent.
> >
> >Next, the wait4() call from debugger gets an error, and zombie will be
> >kept around until parent calls wait4() for this pid once more.
> >
> >Am I missed something ?
> 
> In this case the process will move from gdb's child list to gdb's orphan 
> list when the real parent does a wait4(). Next time around the wait loop in 
> gdb it'll be caught by the orphan's proc_reap().

I do not see how the next debugger loop could find this process at all,
since the first wait4() call reparented it to the original parent.
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