binding KeyboardListener results in a segfault

[abooij]

me being lazy and ignoring "array" type arguments has turned against me: the KeyboardListener now doesn't have the right number of entries, causing a segfault when trying to bind to it.

[nh2]

Can you give me a hint where precisely the problem is and what would have to be done about it?

[abooij]

Wayland message arguments can have one of a number of types, as detailed in the wayland documentation. These bindings use that information to properly marshal data to Haskell types - e.g. an fd type argument is converted into a System.Posix.IO.Fd.

The array type argument is an opaque array-like structure. However, the type of its elements is not specified by the protocol, giving us little chance to properly marshal the data to Haskell types. So I just ignored all messages containing such arguments.

But the wl_keyboard interface makes use of an array argument in its enter event, so we can't set a callback for enter: indeed, KeyboardListener doesn't even offer a place to set it.

But now the C structure that we pass to the wayland library (notice that the KeyboardListener is Storable, a fact that we don't want the user to exploit but can't really hide either AFAIA) has the wrong length (since it's missing the enter field), causing a segfault.

The solution is to either add that field to the C struct we store and set it to null (ie. properly disable interfacing with messages which have an array type argument), or invent a smart way to marshal array type arguments into Haskell, which would be the preferred thing to do, but takes some, well, let's say "creative" solutions.

The former would simply be done by remembering that there is some disabled message in an interface, and then pass a nullFunPtr in Graphics/Wayland/Scanner.chs.

The latter would consist of, if possible, use heuristics to figure out how to present the data neatly (e.g. the wl_keyboard.enter.keys argument is a list of ints specifying the currently pressed keys), or otherwise just return the underlying length and a data pointer, since officially it is indeed just a blob of binary data.

[nh2]

Thanks!