Re: SIGCHLD ignored

[Vladimir Prus <ghost@cs.msu.su>]

On Thursday 12 June 2008 03:18:05 Pedro Alves wrote:
> [moved to gdb-patches@]
> 
> Original report here:
>  SIGCHLD ignored
>  http://sourceware.org/ml/gdb/2008-06/msg00099.html
> 
> A Wednesday 11 June 2008 23:45:22, Pedro Alves wrote:
> > A Wednesday 11 June 2008 18:21:05, Vladimir Prus wrote:
> > > Pedro, I think this SIGCHLD magic is your doing -- do you have any ideas
> > > how to fix it?
> >
> > Dang, I totally missed this could be a problem.
> >
> > Right, so, normal_mask has SIGCHLD blocked in _initialize_linux_nat,
> > and since forking a child inherits the signal mask, the child gets
> > SIGCHLD blocked by default.  This would have gone unnoticed
> > if Qt unblocked SIGCHLD in it's setup (one may argue it should).
> >
> > It's not safe to just remove that SIGCHLD blocking, we're not
> > taking care of blocking it in places we used to in sync mode
> > (we used to block it the first time we hit linux_nat_wait, which
> > is reached after forking an inferior).
> >
> > For async mode, we also need to do something about SIGCHLD
> > blocking.  We have:
> >
> > linux_nat_create_inferior
> >   -> linux_nat_async_mask
> >      -> linux_nat_async
> >         -> linux_nat_async_events
> >            -> block SIGCHLD.
> >
> >   -> linux_ops->to_create_inferior (forks a child)
> >
> > Attached is a patch to fix this.
> >
> > Basically, I turned linux_nat_async_events_enabled into a
> > 3-state instead of a bool, with the new state being
> > "SIGCHLD unblocked with action set to whatever we get
> > on startup".  Most of the SIGCHLD state management stays with
> > the same logic, except that linux_nat_create_inferior gets a
> > switch to the new state before forking a child, and
> > linux_nat_wait, gets an unconditional switch to the blocked
> > state.  The rest of the patch is mostly updating to the
> > new interface.
> >
> > Tested on x86-64-unknown-linux-gnu sync and async modes
> > without regressions.
> 
> Same code patch, but updated with a new testcase.  I imagine
> we're going to touch these issues again with full
> multi-process support.

This patch works fine for me, and (with my limited understanding of the code),
makes sense. Can we get it in?

- Volodya