[RFC wayland 2/2] protocol: Don't silently fail to create seat device objects

[Jonas Ådahl]

One of the principles of Wayland is that object creation never fails. If
the object cannot be created, it should be reported as a protocol error
and an offending client should be terminated.

A current violation to this principle is the input device creation in
wl_seat, as it may silently ignore a request if the set of seat
capabilities at the time of the request does not contain the capability
associated with the object that is to be created.

This violation was introduced in the protocol in 53b57a63eeee06, and I
believe it may have been introduced in error, due to the fact that
weston implemented the violation.

The interface specifies that "the request only takes effect if the seat
has the [given] capability", meaning if the seat does not advertise a
given capability, a request creating the corresponding object will fail
silently.

This is racy, as a client may potentially create an object without
knowing whether it succeeded or not, and in effect may unknowingly never
receive any input events for the given capability.

This is worked around in weston by always succeed if the capability has
ever been advertised. This goes against the protocol specification.

This patch updates the protocol to get rid of the discussed race and the
protocol violation. The change should not cause any backward
compatibility issues, especially since "half" of the new semantics are
already implemented by for example weston.

Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
---

While backward compatibility is not effected, it might be necessary to
make it explicit what version these semantics would be guaranteed so that
a client can rely on it knowing it won't create defunct objects. I'll let
this RFC out so it can be discussed whether this general change seems
like a desirable thing to do or not.


Jonas

 protocol/wayland.xml | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index 42c9309..eef95a7 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -1361,8 +1361,8 @@
         The ID provided will be initialized to the wl_pointer interface
 	for this seat.
 
-        This request only takes effect if the seat has the pointer
-        capability.
+	This request always takes effect, but the created wl_pointer object
+	will only emit events as long as the seat has the pointer capability.
       </description>
       <arg name="id" type="new_id" interface="wl_pointer"/>
     </request>
@@ -1372,8 +1372,8 @@
         The ID provided will be initialized to the wl_keyboard interface
 	for this seat.
 
-        This request only takes effect if the seat has the keyboard
-        capability.
+	This request always takes effect, but the created wl_keyboard object
+	will only emit events as long as the seat has the keyboard capability.
       </description>
       <arg name="id" type="new_id" interface="wl_keyboard"/>
     </request>
@@ -1383,8 +1383,8 @@
         The ID provided will be initialized to the wl_touch interface
 	for this seat.
 
-        This request only takes effect if the seat has the touch
-        capability.
+	This request always takes effect, but the created wl_touch object
+	will only emit events as long as the seat has the touch capability.
       </description>
       <arg name="id" type="new_id" interface="wl_touch"/>
     </request>
-- 
2.1.4