[Xcb] [PATCH] _xcb_conn_wait(): Do better at detecting closed sockets

[Ben Hildred]

On Thu, Oct 13, 2016 at 9:02 AM, Adam Jackson <ajax at redhat.com> wrote:

> Currently, when the X server crashes or a client is disconnected with
> XKillClient, you get a somewhat confusing error message from libX11
> along the lines of:
>
> XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
>       after 98 requests (40 known processed) with 0 events remaining.
>
> I had been wondering about a similar although less verbose message from
gvim during server crash.


> What's happening here is the previous recvmsg has thrown EAGAIN, and
> since errno is not cleared on success that's the errno that the I/O
> error handler sees.
>
> excellent troubleshooting.


> To fix this, check for POLLHUP explicitly, and if there's no more data
> in the socket buffer to read, treat that as an error. We coerce errno to
> EPIPE in this case, which triggers the existing EPIPE path in libX11 and
> thus generates the much more honest error message:
>
> X connection to :0 broken (explicit kill or server shutdown).
>
> Signed-off-by: Adam Jackson <ajax at redhat.com>
> ---
>  src/xcb_conn.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/src/xcb_conn.c b/src/xcb_conn.c
> index 7d09637..ad94443 100644
> --- a/src/xcb_conn.c
> +++ b/src/xcb_conn.c
> @@ -45,6 +45,7 @@
>  #elif !defined _WIN32
>  #include <sys/select.h>
>  #endif
> +#include <sys/ioctl.h>
>
>  #ifdef _WIN32
>  #include "xcb_windefs.h"
> @@ -451,7 +452,7 @@ int _xcb_conn_wait(xcb_connection_t *c,
> pthread_cond_t *cond, struct iovec **vec
>  #if USE_POLL
>      memset(&fd, 0, sizeof(fd));
>      fd.fd = c->fd;
> -    fd.events = POLLIN;
> +    fd.events = POLLIN | POLLHUP;
>  #else
>      FD_ZERO(&rfds);
>      FD_SET(c->fd, &rfds);
> @@ -484,6 +485,19 @@ int _xcb_conn_wait(xcb_connection_t *c,
> pthread_cond_t *cond, struct iovec **vec
>              ret = -1;
>              break;
>          }
> +
> +       /* hangup with no data left to read is an error */
> +       if (fd.revents & POLLHUP)
> +       {
> +           int unread = -1;
> +           ioctl(c->fd, FIONREAD, &unread);
> +           if (unread <= 0)
> +           {
> +               /* coerce errno to not be EAGAIN */
> +               errno = EPIPE;
> +               ret = -1;
> +           }
> +       }
>  #else
>          ret = select(c->fd + 1, &rfds, &wfds, 0, 0);
>  #endif
> --
> 2.9.3
>
> _______________________________________________
> Xcb mailing list
> Xcb at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/xcb




-- 
--
Ben Hildred
Automation Support Services
303 815 6721
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