feat: Get tcp_info structure

[b-parikh]

Implement TCP_INFO option for getsockopt.

What does this PR do

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[b-parikh]

Hello, it looks like the CI is failing for two reasons:

1. A unit test is using the deprecated libc::SOCK_PACKET type.
2. The tcp_info struct doesn't exist in the libc implementation of uClibc-ng.

The former is out of this PR's scope, so I won't address that here. However, I'd like advice on how to resolve the latter. Thank you!

[SteveLauC]

Hi there

However, I'd like advice on how to resolve the latter.

If uClibc has that symbol, you can add it to the libc crate. Otherwise, we need to filter it out:

Suggested change

	#[cfg(target_os = "linux")]
	#[cfg(all(target_os = "linux", not(target_env = "uclibc")))]

[b-parikh]

Added tcp_info to the uClibc symbols in libc: rust-lang/libc@4a34937. It looks like the CI still isn't happy though, do you know if that's because the libc change isn't published to crates.io yet?

[SteveLauC]

That PR rust-lang/libc#4347 added it to the main branch, Nix uses the libc-0.2 branch, so you need to ask the maintainer to backport it to this branch.

Once the backport PR gets merged, update the Cargo.toml file to use it from git:

nix/Cargo.toml

Line 31 in ea012be

libc = { version = "0.2.171", features = ["extra_traits"] }

- libc = { version = "0.2.171", features = ["extra_traits"] } 
+ libc = { git = "https://github.com/rust-lang/libc", rev = "the commit includes your libc PR", features = ... }

[b-parikh]

Got it, thanks! I'll update this PR when the change is backported.

[SteveLauC]

A unit test is using the deprecated libc::SOCK_PACKET type.

Will be addressed in #2610