IPPROTO_MAX has changed on newer versions of Linux

[Aaron1011]

In newer versions of Linux, an additional variant IPPROTO_MPTCP has been added: torvalds/linux@faf391c. This caused the value of IPPROTO_MAX to change from IPPROTO_MPLS + 1 = 256 to IPPROTO_MPTCP + 1 = 263

However, the libc crate currently defines IPPROTO_MAX as 256:

libc/src/unix/linux_like/linux/mod.rs

Line 1319 in 999e5e1

pub const IPPROTO_MAX: ::c_int = 256;

This causees libc-test to fail when run against a newer kernel version. I'm not really sure what IPPROTO_MPTCP is used for, but this seemed worth bringing up.

[tommythorn]

My just cloned repo (@61e5f91833272d8fa2a2fb6bede3add62218f627) fails libc-test on Ubuntu 20.10/AMD64 (beta) with

     Running /home/tommy/projects/Rust/libc/target/debug/deps/main-63261675787c2312
RUNNING ALL TESTS
bad IPPROTO_MAX value at byte 0: rust: 0 (0x0) != c 7 (0x7)
thread 'main' panicked at 'some tests failed', /home/tommy/projects/Rust/libc/target/debug/build/libc-test-e26fe6d8326278e5/out/main.rs:12:21

I presume this is related.

[joshtriplett]

I think, in the future, we should avoid exposing the placeholder _MAX constants at the ends of enumerations, unless there's a specific compatibility reason we need to do so.

In the meantime, I think we should update the value of this.

[nathaniel-bennett]

Would it be a good idea to remove this and similar _MAX constants in the 1.0 release to prevent this issue from happening again?