XCB & XKB: it compiles!

While trying to autogenerate C interfaces for XKB during the last 2 months it sometimes felt like an attempt to "behead the Hydra": Whenever I fixed a problem, a number of new issues arose (deep from the intestines of the python code generator). At the moment it seems I have reached a state where no new heads emerge, and I even managed to cut off a number of the most ugliest...

The cause of most (all?) troubles is a very basic way of distinguishing data types into fixed size (everything you can feed to sizeof), variable size data types (lists, valueparams, ...) and padding. In the X protocol fixed size fields are normally grouped together, so e.g. in a request all fixed size fields are collected before any variable size fields.

XCB, whose purpose it is to translate between X server and client, takes advantage of this ordering: Several grouped fixed size fields are conveniently mapped to a C struct, so it is fairly easy to deal with. The treatment of variable size data is more difficult and involves the use of (autogenerated) accessors and iterators. Also the specific number of elements in a variable size data type can depend on expressions that are specified in the protocol, but need to be evaluated at runtime.
Now XKB breaks with the "pure doctrine" of the X core protocol: Fixed and variable size fields are intermixed in several cases and new expressions are introduced. Further problematic cases include a variable size union, lists with a variable number of variable size elements, ... And finally, the keyboard extension defines a new datatype (ListOfItems), which is referred to as 'switch' in XCB.

Switch is a kind of list, but the 'items' are not necessarily of the same type, and whether an item is included or not depends on expressions that have to be evaluated for each item separately. Defining a C mapping for 'switch' was one of the main goals of my work, and it turned out that a set of new functions was needed. These new functions must either 'serialize' a switch into a buffer, depending on the concrete switch conditions, or 'unserialize' it to some C struct. As 'switch' can contain any other data type valid in the X protocol (which means especially that it also can contain other switches...), 'serialize'/'unserialize' had to be defined for all those data types.
Once I had the autogenerator spill out '_serialize()' and '_unserialize()', it turned out they can be used to deal with some other special cases as well - most notably the (above-mentioned) intermixing of fixed and variable size fields.

But probably the most appealing feature of the serializers is that they are invisible to anyone who wants to use XCB as they get called automatically in the autogenerated helper functions.

A last note on the current status:
+ xkb.c (autogenerated from xkb.xml) compiles!
+ the request side should be finished (more tests needed however)
+ simple XKB test programs run
- on the reply side, special cases still need special treatment
- _unserialize() should be renamed to _sizeof() in some cases
- some special cases also need special accessors

The code is currently hosted on annarchy, but I will export the repos shortly.