Handle API evolution and additions

[kamalmarhubi]

In both #228 and #233, we're having CI failures for constants that aren't present in the headers that the test infrastructure uses.

Possible directions of improvement:

• verifying constants against the most recent kernel headers instead of whatever headers are installed in CI (suggested by @posborne)
• switch to generation of constant values instead of verification

With generation, there's still a need to pick the correct type for a constant, so it can't be done purely mechanically. Yay C preprocessor!

[kamalmarhubi]

This also relates to #121.

[kamalmarhubi]

Relevant: the wiki for predef: Pre-defined Compiler Macros, in particular the Operating Systems and Architectures pages. They list commonly predefined macros for OSes and architectures.

The wiki is also available as an export on GitHub if you prefer.

[alexcrichton]

I would personally not like to go down the route of generation just yet, but we're free to update whatever headers are on the system at any time in my opinion. They will likely inevitably break existing definitions as binary compatibility is now always upheld, but we can deal with it when it comes.

[kamalmarhubi]

@alexcrichton

They will likely inevitably break existing definitions as binary compatibility is now always upheld, but we can deal with it when it comes.

Could you clarify a little what you mean here?

[alexcrichton]

Ah just that whenever we upgrade the OS image to include a newer glibc in the past something's always broken because some constant changes somewhere.

[gnzlbg]

For each target_os, currently, libc only supports a very particular OS version. Some OSes preserve backward compatibility, and this works fine there (linux, windows). Others make very minor breaking changes (macos), others make larger breaking changes, but try to preserve ABI compatibility (FreeBSD), and others do not care at all, and each minor OS release is essentially a completely different target (OpenBSD).

How to fix this is being discussed in #570 , so I'm closing this issue. Please feel encouraged to chime up there if you care about this.