[RFC wayland] server: Add special case destroy signal emitter

[Derek Foreman]

On 2018-02-23 02:15 AM, Pekka Paalanen wrote:
> On Thu, 22 Feb 2018 16:02:49 -0600
> Derek Foreman <derekf at osg.samsung.com> wrote:
> 
>> In the past much code (weston, efl/enlightenment, mutter) has
>> freed structures containing wl_listeners from destroy handlers
>> without first removing the listener from the signal.  As the
>> destroy notifier only fires once, this has largely gone
>> unnoticed until recently.
>>
>> Other code does not (Qt, wlroots) - and removes itself from
>> the signal before free.
>>
>> If somehow a destroy signal is listened to by code from both
>> kinds of callers, those that free will corrupt the lists for
>> those that don't, and Bad Things will happen.
>>
>> To avoid these bad things, remove every item from the signal list
>> during destroy emit, and put it in a list all its own.  This way
>> whether the listener is removed or not has no impact on the
>> following emits.
>>
>> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
>> ---
>>
>>   src/wayland-private.h |  3 +++
>>   src/wayland-server.c  | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
>>   2 files changed, 49 insertions(+), 3 deletions(-)
>>
>> diff --git a/src/wayland-private.h b/src/wayland-private.h
>> index 12b9032..29516ec 100644
>> --- a/src/wayland-private.h
>> +++ b/src/wayland-private.h
>> @@ -253,6 +253,9 @@ wl_priv_signal_get(struct wl_priv_signal *signal, wl_notify_func_t notify);
>>   void
>>   wl_priv_signal_emit(struct wl_priv_signal *signal, void *data);
>>   
>> +void
>> +wl_priv_signal_final_emit(struct wl_priv_signal *signal, void *data);
>> +
>>   void
>>   wl_connection_close_fds_in(struct wl_connection *connection, int max);
>>   
>> diff --git a/src/wayland-server.c b/src/wayland-server.c
>> index eb1e500..df5c16a 100644
>> --- a/src/wayland-server.c
>> +++ b/src/wayland-server.c
>> @@ -682,7 +682,7 @@ destroy_resource(void *element, void *data, uint32_t flags)
>>   	/* Don't emit the new signal for deprecated resources, as that would
>>   	 * access memory outside the bounds of the deprecated struct */
>>   	if (!resource_is_deprecated(resource))
>> -		wl_priv_signal_emit(&resource->destroy_signal, resource);
>> +		wl_priv_signal_final_emit(&resource->destroy_signal, resource);
>>   
>>   	if (resource->destroy)
>>   		resource->destroy(resource);
>> @@ -841,7 +841,7 @@ wl_client_destroy(struct wl_client *client)
>>   {
>>   	uint32_t serial = 0;
>>   
>> -	wl_priv_signal_emit(&client->destroy_signal, client);
>> +	wl_priv_signal_final_emit(&client->destroy_signal, client);
>>   
>>   	wl_client_flush(client);
>>   	wl_map_for_each(&client->objects, destroy_resource, &serial);
>> @@ -1089,7 +1089,7 @@ wl_display_destroy(struct wl_display *display)
>>   	struct wl_socket *s, *next;
>>   	struct wl_global *global, *gnext;
>>   
>> -	wl_priv_signal_emit(&display->destroy_signal, display);
>> +	wl_priv_signal_final_emit(&display->destroy_signal, display);
>>   
>>   	wl_list_for_each_safe(s, next, &display->socket_list, link) {
>>   		wl_socket_destroy(s);
>> @@ -2025,6 +2025,49 @@ wl_priv_signal_emit(struct wl_priv_signal *signal, void *data)
>>   	}
>>   }
>>   
>> +/** Emit the signal for the last time, calling all the installed listeners
>> + *
>> + * Iterate over all the listeners added to this \a signal and call
>> + * their \a notify function pointer, passing on the given \a data.
>> + * Removing or adding a listener from within wl_priv_signal_emit()
>> + * is safe, as is freeing the structure containing the listener.
>> + *
>> + * A large body of external code assumes it's ok to free a destruction
>> + * listener without removing that listener from the list.  Mixing code
>> + * that acts like this and code that doesn't will result in list
>> + * corruption.
>> + *
>> + * We resolve this by removing each item from the list and isolating it
>> + * in another list.  We discard it completely after firing the notifier.
>> + * This should allow interoperability between code that unlinks its
>> + * destruction listeners and code that just frees structures they're in.
>> + *
>> + */
>> +void
>> +wl_priv_signal_final_emit(struct wl_priv_signal *signal, void *data)
>> +{
>> +	struct wl_listener *l;
>> +	struct wl_list *pos;
>> +
>> +	wl_list_insert_list(&signal->emit_list, &signal->listener_list);
>> +
>> +	/* During a destructor notifier isolate every list item before
>> +	 * notifying.  This renders harmless the long standing misuse
>> +	 * of freeing listeners without removing them, but allows
>> +	 * callers that do choose to remove them to interoperate with
>> +	 * ones that don't. */
>> +	while (!wl_list_empty(&signal->emit_list)) {
>> +		wl_list_init(&signal->listener_list);
>> +		pos = signal->emit_list.next;
>> +		l = wl_container_of(pos, l, link);
>> +
>> +		wl_list_remove(pos);
>> +		wl_list_insert(&signal->listener_list, pos);
> 
> Just a quick fly-by comment; is there a reason to actually have a head
> in the personal list for 'pos'? Wouldn't wl_list_init(pos) be enough?

I was trying to keep wl_priv_signal_get() "working" - though I'm fairly 
solidly convinced it's totally useless for that case...

> Or would you want keep e.g. checks like
> wl_list_empty(&foofoo_destroy_listener->link) working? Not sure how that
> would be useful... you got called, so obviously the listener is in a
> list.
> 
> The idea seems fine to me, but I didn't look very hard.

Thanks for looking at all :)
Derek

> 
> Thanks,
> pq
> 
>> +
>> +		l->notify(l, data);
>> +	}
>> +}
>> +
>>   /** \endcond INTERNAL */
>>   
>>   /** \cond */ /* Deprecated functions below. */
>