Trouble building Bazel+pico_lwip_contrib_freertos

[jaguilar]

Besides #1796, I'm still having some issues building with bazel and LWIP. I'm writing down the results of what investigation I've done for either myself or someone else to follow up on later.

I think the issue stems from a dependency that is not completely expressed in the Bazel-accepted DAG-ish way. So, pico_lwip_contrib_freertos in lwip.BUILD depends on pico_lwip_core. Let's look at one compile-time dependency chain there:

• pico_lwip_core -> mem.c
• mem.c -> arch.h
• arch.h -> sys_arch.h

All well and good, right? We're building pico_lwip_contrib_freertos, and that will provide sys_arch.h. Sadly not. The includes directories from cc_library get added to -isystem only for the dependents of the library that defines it. Not the library's dependencies. So when we compile mem.c, contrib/ports/freertos/include is nowhere to be found on the compiler command-line.

This kind of makes sense. If a library is depending on a thing, it can't be necessary for the thing depended upon to require the library's existence. But you can actually do that in C. So I think we'll have to figure out how to express it.

Intuitively, it seems to me like maybe the solution is something along the lines of "treat sys_arch.h as an interface and put it on the hdrs line of pico_lwip_core, but the definition is only available when pico_lwip_contrib_freertos is linked. But I'm not an expert on adapting C libraries to bazel so I'm open to other ideas.