[PATCH] wayland-server: update the client fd when it failed to flush with EAGAIN

[Derek Foreman]

On 11/22/18 11:08 PM, Jeonghyun Kang wrote:
> When a wayland compositor gets an EAGAIN error whenever
> sending or receiving event(s) to a client in the
> wl_closure_send() or the wl_closure_queue(), the error
> variable of the wl_client for the client will be set to
> true and the client is going to be killed by the
> compositor later soon. This patch fixes the problem by
> updating the socket fd's event mask as we're doing
> in wl_display_flush_clients() for having another chance
> to do it again.
> 
> Actually, this kind of problem can be watched in an
> environment in which massive input events are coming
> from multi-touch screen or multi-pen devices. In this
> kind of environment, a client receiving the massive
> input events are trying to drawing something very hard
> but it's being killed sooner or later. With the given
> patch, the client receiving the input events is not being
> destroyed and is working well even though it doesn't get
> the whole input events from the compositor.

This is actually intentional behavior to avoid infinitely growing send
queues for clients that process events too slowly.

I don't think dropping events randomly (or at all) because the client
can't keep up is something we should do in libwayland. It'll just make
for nearly impossible to reproduce and debug problems when client and
compositor state fall out of sync.

What if it's a frame event that gets dropped and not an input event?  Or
a resource destroy?

The disconnect is a clear indication that there's a problem that needs
to be fixed somewhere else. The client needs to process events in a
timely fashion, or the compositor needs to send less events.

Thanks,
Derek

> Signed-off-by: jeon <jhyuni.kang at samsung.com
> <mailto:jhyuni.kang at samsung.com>>
> 
> ---
>  src/wayland-server.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/src/wayland-server.c b/src/wayland-server.c
> index eae8d2e..5afaa28 100644
> --- a/src/wayland-server.c
> +++ b/src/wayland-server.c
> @@ -222,8 +222,15 @@ handle_array(struct wl_resource *resource, uint32_t
> opcode,
>  
>         log_closure(resource, closure, true);
>  
> -       if (send_func(closure, resource->client->connection))
> -               resource->client->error = 1;
> +       if (send_func(closure, resource->client->connection)) {
> +               if (errno == EAGAIN) {
> +                       wl_event_source_fd_update(resource->client->source,
> +                                                 WL_EVENT_WRITABLE |
> +                                                 WL_EVENT_READABLE);
> +               } else {
> +                       resource->client->error = 1;
> +               }
> +       }
>  
>         wl_closure_destroy(closure);
>  }
> 
> -- 
> 2.7.4
> 
> 
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