Re: [PATCH v2 6/6] tools/libxl: Adjust datacopiers POLLHUP handling when the fd is also readable

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

On Fri, 2015-03-06 at 19:05 +0000, Andrew Cooper wrote:
> POLLHUP|POLLIN is a valid revent to receive when there is readable data in a
> pipe, but the writable fd has been closed.  This occurs in migration v2 when
> the legacy conversion process (which transforms the data inline) completes and
> exits successfully.
> 
> In the case that there is data to read, suppress the POLLHUP.  POSIX states
> that the hangup state is latched[1], which means it will reoccur on subsequent
> poll() calls.  The datacopier is thus provided the opportunity to read until
> EOF, if possible.
> 
> A POLLHUP on its own is treated exactly as before, indicating a different
> error with the fd.
> 
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> CC: Ian Campbell <Ian.Campbell@citrix.com>
> CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
> CC: Wei Liu <wei.liu2@citrix.com>

Having looked at the previous thread I'm satisfied that this matches
what was discussed there, but I think the final ack needs to come from
Ian.

> 
> ---
> v2: Rework solution from scratch
> ---
>  tools/libxl/libxl_aoutils.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/libxl/libxl_aoutils.c b/tools/libxl/libxl_aoutils.c
> index a402e5c..a5ad3e5 100644
> --- a/tools/libxl/libxl_aoutils.c
> +++ b/tools/libxl/libxl_aoutils.c
> @@ -204,6 +204,9 @@ static void datacopier_readable(libxl__egc *egc, libxl__ev_fd *ev,
>      libxl__datacopier_state *dc = CONTAINER_OF(ev, *dc, toread);
>      STATE_AO_GC(dc->ao);
>  
> +    if ((revents & (POLLHUP|POLLIN)) == (POLLHUP|POLLIN))
> +        revents &= ~POLLHUP;
> +
>      if (datacopier_pollhup_handled(egc, dc, revents, 0))
>          return;
>