[RFC wayland 1/2] client: Introduce 'proxy wrapper' concept
Marek Chalupa
mchqwerty at gmail.com
Tue Jun 30 23:57:05 PDT 2015
On Mon, Jun 29, 2015 at 3:37 PM, Giulio Camuffo <giuliocamuffo at gmail.com>
wrote:
> 2015-06-29 14:30 GMT+03:00 Jonas Ådahl <jadahl at gmail.com>:
> > Arnaud Vrac discovered an issue in the libwayland client API causing
> > race conditions when doing round trips using the wl_display object using
> > non-default proxy queues.
> >
> > The problem is that one thread can read and dispatch events after
> > another thread creates the wl_callback object but before it sets the
> > queue. This could potentially cause the events to be dropped completely
> > if the event is dispatched before the creating thread sets the
> > implementation, or that the event is dispatched on the wrong queue which
> > might run on another thread.
> >
> > This patch introduces API to solve this case by introducing "proxy
> > wrappers". In short, a proxy wrapper is a struct wl_proxy * that will
> > never itself emit any events, but the client can set a queue, and use it
> > when sending requests that creates new proxy objects. When sending
> > requests, the wrapper will work in the same way as the normal proxy
> > object, but the proxy created by sending a request (for example
> > wl_display.sync) will default to the same proxy queue as the wrapper.
> >
> > Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
> > ---
> >
> > Hi,
> >
> > This is one of the two alternative solutions that was discussed on
> #wayland.
> >
> > The other one being introducing API alternatives to
> > wl_proxy_marshal_constructor and wl_proxy_marshal_array_constructor that
> > also take a queue and also changing scanner.c to produce functions that
> > take a queue for every request that creates a proxy.
> >
> > In short, instead of
> >
> > bar = wl_foo_get_bar(foo);
> > wl_proxy_set_queue((struct wl_proxy *) bar, queue);
> > wl_bar_add_listener(bar, ...);
> >
> > with this RFC a client does
> >
> > foo_wrapper = wl_proxy_create_wrapper((struct wl_proxy *) foo);
> > wl_proxy_set_queue((struct wl_proxy *) foo_wrapper, queue);
> >
> > bar = wl_foo_get(foo_wrapper);
> > wl_bar_add_listener(bar, ...);
> >
> > and the with other idea that is implemented anywhere yet AFAIK
> >
> > bar = wl_foo_get_bar_with_queue(foo, queue)
> > wl_bar_add_listener(bar, ...);
>
> Hi, i think this second option is better. The proxy approach looks to
> me like a clunky and unnecessary workaround, while i don't see
> downsides to new api taking a queue.
>
>
With this second approach there's still a race before setting listener.
We should be able to set queue and listener atomically to get rid of this
race.
So something like:
wl_proxy_create_full(..., queue, listener)
+ the scanner thing?
>
> --
> Giulio
>
> >
> > Opinions?
> >
> >
> > Jonas
> >
> >
> >
> > src/wayland-client-core.h | 1 +
> > src/wayland-client.c | 106
> +++++++++++++++++++++++++++++++++++++++++-----
> > src/wayland-private.h | 7 +++
> > 3 files changed, 103 insertions(+), 11 deletions(-)
> >
> > diff --git a/src/wayland-client-core.h b/src/wayland-client-core.h
> > index dea70d9..0db8b9c 100644
> > --- a/src/wayland-client-core.h
> > +++ b/src/wayland-client-core.h
> > @@ -125,6 +125,7 @@ void wl_proxy_marshal_array(struct wl_proxy *p,
> uint32_t opcode,
> > union wl_argument *args);
> > struct wl_proxy *wl_proxy_create(struct wl_proxy *factory,
> > const struct wl_interface *interface);
> > +struct wl_proxy *wl_proxy_create_wrapper(struct wl_proxy *proxy);
> > struct wl_proxy *wl_proxy_marshal_constructor(struct wl_proxy *proxy,
> > uint32_t opcode,
> > const struct wl_interface
> *interface,
> > diff --git a/src/wayland-client.c b/src/wayland-client.c
> > index 6450b67..f8c64c4 100644
> > --- a/src/wayland-client.c
> > +++ b/src/wayland-client.c
> > @@ -51,7 +51,8 @@
> >
> > enum wl_proxy_flag {
> > WL_PROXY_FLAG_ID_DELETED = (1 << 0),
> > - WL_PROXY_FLAG_DESTROYED = (1 << 1)
> > + WL_PROXY_FLAG_DESTROYED = (1 << 1),
> > + WL_PROXY_FLAG_WRAPPER = (1 << 2),
> > };
> >
> > struct wl_proxy {
> > @@ -405,17 +406,19 @@ wl_proxy_create_for_id(struct wl_proxy *factory,
> > void
> > proxy_destroy(struct wl_proxy *proxy)
> > {
> > - if (proxy->flags & WL_PROXY_FLAG_ID_DELETED)
> > - wl_map_remove(&proxy->display->objects, proxy->object.id
> );
> > - else if (proxy->object.id < WL_SERVER_ID_START)
> > - wl_map_insert_at(&proxy->display->objects, 0,
> > - proxy->object.id, WL_ZOMBIE_OBJECT);
> > - else
> > - wl_map_insert_at(&proxy->display->objects, 0,
> > - proxy->object.id, NULL);
> > -
> > + if (proxy->flags != WL_PROXY_FLAG_WRAPPER) {
> > + if (proxy->flags & WL_PROXY_FLAG_ID_DELETED)
> > + wl_map_remove(&proxy->display->objects,
> > + proxy->object.id);
> > + else if (proxy->object.id < WL_SERVER_ID_START)
> > + wl_map_insert_at(&proxy->display->objects, 0,
> > + proxy->object.id,
> WL_ZOMBIE_OBJECT);
> > + else
> > + wl_map_insert_at(&proxy->display->objects, 0,
> > + proxy->object.id, NULL);
> >
> > - proxy->flags |= WL_PROXY_FLAG_DESTROYED;
> > + proxy->flags |= WL_PROXY_FLAG_DESTROYED;
> > + }
> >
> > proxy->refcount--;
> > if (!proxy->refcount)
> > @@ -459,6 +462,11 @@ WL_EXPORT int
> > wl_proxy_add_listener(struct wl_proxy *proxy,
> > void (**implementation)(void), void *data)
> > {
> > + if (proxy->flags == WL_PROXY_FLAG_WRAPPER) {
> > + wl_log("proxy %p is a wrapper\n", proxy);
> > + return -1;
> > + }
> > +
> > if (proxy->object.implementation || proxy->dispatcher) {
> > wl_log("proxy %p already has listener\n", proxy);
> > return -1;
> > @@ -512,6 +520,11 @@ wl_proxy_add_dispatcher(struct wl_proxy *proxy,
> > wl_dispatcher_func_t dispatcher,
> > const void *implementation, void *data)
> > {
> > + if (proxy->flags == WL_PROXY_FLAG_WRAPPER) {
> > + wl_log("proxy %p is a wrapper\n", proxy);
> > + return -1;
> > + }
> > +
> > if (proxy->object.implementation || proxy->dispatcher) {
> > wl_log("proxy %p already has listener\n");
> > return -1;
> > @@ -1885,6 +1898,77 @@ wl_proxy_set_queue(struct wl_proxy *proxy, struct
> wl_event_queue *queue)
> > proxy->queue = &proxy->display->default_queue;
> > }
> >
> > +/** Create a proxy wrapper
> > + *
> > + * \param proxy The proxy object to be wrapped
> > + * \return A proxy wrapper for the given proxy
> > + *
> > + * A proxy wrapper is type of 'struct wl_proxy' instance that can be
> used for
> > + * sending requests instead of using the original proxy. A proxy
> wrapper does
> > + * not have an implementation or dispatcher, and events received on the
> > + * object is still emitted on the original proxy. Trying to set an
> > + * implementation or dispatcher will have no effect but result in a
> warning
> > + * being logged.
> > + *
> > + * Even though there is no implementation nor dispatcher, the proxy
> queue can
> > + * be changed. This will affect the default queue of new objects
> created by
> > + * requests sent via the proxy wrapper.
> > + *
> > + * A proxy wrapper must be destroyed before the proxy it was created
> from.
> > + * Creating a proxy wrapper on a proxy that has either been destroyed or
> > + * removed will fail, and NULL will be returned.
> > + *
> > + * If a user reads and dispatches events on more than one thread, it is
> > + * necessary to use a proxy wrapper when sending requests on objects
> when the
> > + * intention is that a newly created proxy is to use a proxy queue
> different
> > + * from the proxy the request was sent on, as creating the new proxy
> and then
> > + * setting the queue is not thread safe.
> > + *
> > + * For example, a module that runs using its own proxy queue that needs
> to
> > + * do display roundtrip must wrap the wl_display proxy object before
> sending
> > + * the wl_display.sync request. For example:
> > + *
> > + * struct wl_proxy *wrapped_display;
> > + * struct wl_callback *callback;
> > + *
> > + * wrapped_display = wl_proxy_create_wrapper((struct wl_proxy *)
> display);
> > + * callback = wl_display_sync((struct wl_display *) wrapped_display);
> > + * wl_proxy_destroy(wrapped_display);
> > + * wl_callback_add_listener(callback, ...);
> > + *
> > + * \memberof wl_proxy
> > + */
> > +WL_EXPORT struct wl_proxy *
> > +wl_proxy_create_wrapper(struct wl_proxy *proxy)
> > +{
> > + struct wl_proxy *wrapper;
> > +
> > + wrapper = zalloc(sizeof *wrapper);
> > + if (!wrapper)
> > + return NULL;
> > +
> > + pthread_mutex_lock(&proxy->display->mutex);
> > +
> > + if ((proxy->flags & WL_PROXY_FLAG_ID_DELETED) ||
> > + (proxy->flags & WL_PROXY_FLAG_DESTROYED)) {
> > + pthread_mutex_unlock(&proxy->display->mutex);
> > + wl_log("proxy %p already deleted or destroyed flag:
> 0x%x\n",
> > + proxy, proxy->flags);
> > + return NULL;
> > + }
> > +
> > + wrapper->object.interface = proxy->object.interface;
> > + wrapper->object.id = proxy->object.id;
> > + wrapper->display = proxy->display;
> > + wrapper->queue = proxy->queue;
> > + wrapper->flags = WL_PROXY_FLAG_WRAPPER;
> > + wrapper->refcount = 1;
> > +
> > + pthread_mutex_unlock(&proxy->display->mutex);
> > +
> > + return wrapper;
> > +}
> > +
> > WL_EXPORT void
> > wl_log_set_handler_client(wl_log_func_t handler)
> > {
> > diff --git a/src/wayland-private.h b/src/wayland-private.h
> > index 17c507c..f072811 100644
> > --- a/src/wayland-private.h
> > +++ b/src/wayland-private.h
> > @@ -29,6 +29,7 @@
> > #define WAYLAND_PRIVATE_H
> >
> > #include <stdarg.h>
> > +#include <stdlib.h>
> >
> > #define WL_HIDE_DEPRECATED 1
> >
> > @@ -71,6 +72,12 @@ struct wl_map {
> >
> > typedef void (*wl_iterator_func_t)(void *element, void *data);
> >
> > +static inline void *
> > +zalloc(size_t size)
> > +{
> > + return calloc(1, size);
> > +}
> > +
> > void wl_map_init(struct wl_map *map, uint32_t side);
> > void wl_map_release(struct wl_map *map);
> > uint32_t wl_map_insert_new(struct wl_map *map, uint32_t flags, void
> *data);
> > --
> > 2.1.4
> >
> > _______________________________________________
> > wayland-devel mailing list
> > wayland-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/wayland-devel
> _______________________________________________
> wayland-devel mailing list
> wayland-devel at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/wayland-devel
>
Regards,
Marek
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