Revert libc::memfd_create for gnu and musl

[StephenWakely]

Closes #1972

cf15c2b introduced changes to force a hard dependency on libc::memfd_create. This symbol doesn't exist in Ubuntu 16.04 or CentOS 7 and thus it breaks our builds. Neither of these are EOL for a few more years.

 = note: /target/x86_64-unknown-linux-gnu/debug/deps/libnix-0e1e3ef93965abb0.rlib(nix-0e1e3ef93965abb0.nix.194bd3d9-cgu.7.rcgu.o): In function `nix::sys::memfd::memfd_create::h8f53f62aa4e9fe80':
          /cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/nix-0.26.2/src/sys/memfd.rs:56: undefined reference to `memfd_create'
          collect2: error: ld returned 1 exit status

This change pulls out using libc::memfd_create for gnu and musl. I believe the original commit was intended just for freebsd, so the original intent of that commit remains unchanged.

[asomers]

I don't want to hold back development for the sake of six year old distros. The correct solution is to figure out the righth way to enable as-needed in the linker.

[jszwedko]

I don't want to hold back development for the sake of six year old distros.

That is completely reasonable. Do you have a minimum glibc that you are targeting? That would help with setting expectations. With https://github.com/vectordotdev/vector, we have users deploying on systems with older libcs (in particular CentOS 7) and so have a desire to maintain compatibility with them.

[asomers]

No, we don't target a minimum glibc. Have you checked to see what, if any, minimum glibc the Rust standard library targets?

[asomers]

Actually, I don't think glibc is really the problem. The problem is more likely the old linker that you're using.

[jszwedko]

No, we don't target a minimum glibc. Have you checked to see what, if any, minimum glibc the Rust standard library targets?

It looks like 2.17, which matches what we are targeting: https://github.com/rust-lang/rust/blob/7d782b7ff4d57170e110211565209ecc5bbb3907/src/doc/rustc/src/platform-support.md?plain=1#L40

[nekopsykose]

does RUSTFLAGS="-Clink-arg=-Wl,--as-needed" fix it for you, in case it's not the default?

[jszwedko]

does RUSTFLAGS="-Clink-arg=-Wl,--as-needed" fix it for you, in case it's not the default?

We did try that, but it didn't seem to make a difference. See #1972 (comment)

[nekopsykose]

it's link-arg (linkarg isn't recognised?), but no worries, just a guess :)

[asomers]

Superseded by #2049