Re: [PATCH 6/6] tools/libxl: Fix datacopier POLLHUP handling to not always be fatal

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-02-20 at 13:55 +0000, Andrew Cooper wrote:

> >
> > It's a bit of a shame that callers which don't care about specific
> > pollhup handling have to provide two practically identical handlers.
> 
> Up until this patch, all users either provided no POLLHUP handler, or
> provided the same handler for both function pointers.

Yes, and I'm saying it is a shame that those in the latter class now
have to provide two callbacks instead of the one they used before.

> > Is there any mileage in suggesting that the default callback type used
> > for copyfail too might return a bool too in order that they can be
> > shared? Even if it must always return True.
> >
> > Or perhaps some method to indicate that on pollhup, if pollhip callback
> > is NULL, use the regular callback? (where some method might be the
> > pollhup_callback==NULL itself?)
> 
> Previously, every callback was issued after the datacopier had already
> been killed.  Now, the pollhup callback is called before the kill has
> occurred, and is able to prevent the kill from happening.
> 
> A different and far less invasive approach might be to have a per-fd
> revent ignore mask.  This would at least allow the callbacks to be made
> when the datacopier is in a consistent state.

Ian, any thoughts on this?