This is pretty much just the API surface that is used in the demo code
for now, with a couple of minor things I tacked on while getting a
feel for how I was going to go about it. Unfortunately, the diff is
too messy to show the improvement provided by the wrapped API, but I'm
pretty pleased with how it is turning out so far. In general, there
have been no major hiccups.
Most of the time working on this was spent noodling about how to create
the subscription callback thunk to provide type safety and pointer
nullability guarantees for the subscription zig API. I'm satisfied
with the solution in place now: it seems to have good default
ergonomics and very few downsides except that the callback function
must be known at compile time. An earlier iteration of this code
supported runtime function pointers but had a showstopping problem
with memory management. In order to store the runtime callback
pointer, the thunk would need to be allocated, but there was no way of
attaching our allocated memory to the c-library subscription object.
No attachment meant that there were a number of error scenarios where
the allocated thunk couldn't be freed correctly.
Ultimately, if the user needs to use runtime callback pointers they can
write a comptime known callback that calls a runtime known callback.
This way all of the object lifetime and ownership is in the user's
hands, which is really where it needs to belong anyway for this.
