Re: [RFC 0/4] meson: Enable -Wundef

On a Thursday in 2024, Andrea Bolognani wrote: > A few days ago I have posted a patch[1] that addresses an issue > introduced when a meson check was dropped but some uses of the > corresponding WITH_ macro were not removed at the same time. > > That got me thinking about what we can do to prevent such scenarios > from happening again in the future. I have come up with something > that I think would be effective, but since applying the approach > throughout the entire codebase would require a non-trivial amount of > work, I figured I'd ask for feedback before embarking on it. > > The idea is that there are two types of macros we can use for > conditional compilation: external ones, coming from the OS or other > libraries, and internal ones, which are the result of meson tests. > > The external ones (e.g. SIOCSIFFLAGS, __APPLE__) are usually only > defined if they apply, so it is correct to check for their presence > with #ifdef. Using #if will also work, as undefined macros evaluate > to zero, but it's not good practice to use them that way. If -Wundef > has been passed to the compiler, those incorrect uses will be > reported (only on platforms where they are not defined, of course). > > The internal ones (e.g. WITH_QEMU, WITH_STRUCT_IFREQ) are similar, > but in this case we control their definition. This means that using > means that the feature is not available on the machine we're building > on, but it could also mean that we've removed the meson check and > forgot to update all users of the macro. In this case, -Wundef would > work 100% reliably to detect the issue: if the meson check doesn't > exist, neither will the macro, regardless of what platform we're > building on. > > So the approach I'm suggesting is to use a syntax-check rule to > ensure that internal macros are only ever checked with #if instead of > > Of course this requires a full sweep to fix all cases in which we're > not already doing things according to the proposal. Should be fairly > easy, if annoying. A couple of examples are included here for > demonstration purposes. > > The bigger impact is going to be on the build system. Right now we > generally only define WITH_ macros if the check passed, but that will > have to change and the result is going to be quite a bit of > additional meson code I'm afraid. > > Thoughts? I don't like all the extra additional meson code. Can it be somehow simplified? Calling:   foreach function : functions     conf.set('WITH_@0(a)'.format(function.to_upper()), cc.has_function(function))   endforeach does not seem to work.