Remove Mono.Unix dependency

[lucaspimentel]

Summary

Removes the Mono.Unix NuGet dependency, which was incompatible with some target runtimes (e.g. .NET Framework 4.6.1) and blocked customers from upgrading the client (#220).

Only two pieces of Mono.Unix were actually used, and both are replaced with self-contained alternatives:

• UnixEndPoint (Unix domain socket transport): vendored directly into the repo as src/StatsdClient/UnixEndPoint.cs, copied from the Mono source under its MIT license. The public API and namespace (Mono.Unix) are preserved, so transport code is unchanged.
• Syscall.stat (inode lookup for cgroup/container origin detection on Linux): replaced with a direct statx system call via P/Invoke in src/StatsdClient/NativeMethods.cs. struct statx has a fixed, architecture-independent layout defined by the kernel, so a single managed struct definition works across x64/arm64 and glibc/musl. No native binaries are shipped, and the package now has no native assets or third-party runtime dependencies.

FileSystem.TryStat is narrowed from any Unix platform to Linux only (the sole consumer, cgroup origin detection, reads Linux-only /proc paths) and delegates the platform check to NativeMethods.TryGetInode. Inode lookup is compiled out on .NET Framework (#if !NETFRAMEWORK), which always runs on Windows.

Changes

• Remove the Mono.Unix PackageReference from StatsdClient.csproj.
• Add src/StatsdClient/UnixEndPoint.cs (vendored, MIT).
• Add src/StatsdClient/NativeMethods.cs (statx P/Invoke wrapper).
• Update src/StatsdClient/IFileSystem.cs to use the new inode lookup.
• Add tests/StatsdClient.Tests/NativeLibraryTests.cs exercising the real statx call on Linux.
• Add BUILD.md and CLAUDE.md developer docs.
• Add .gitattributes to force LF line endings on shell scripts.

Testing

• Built the library across all four target frameworks (net461, netstandard2.0, netcoreapp3.1, net6.0); the net461 build exercises the #if NETFRAMEWORK fallback path.
• NativeLibraryTests exercises the real statx syscall on Linux (inode for files, directories, symlinks, the cgroup namespace link, and non-existent-file failure) and verifies the non-Linux path returns false.

Compatibility

• No public API changes.
• No native assets in the package; no third-party runtime dependencies.

"Turns out the best way to depend on Mono.Unix is to not." — Claude 🤖

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EDIT 2026-06-04. Summary from 🤖:

Why Mono.Unix is there at all: it's only used to call stat() and read the inode number for origin detection (OriginDetection.cs) — specifically the host cgroup-namespace check (/proc/self/ns/cgroup, line 104) and the cgroup-inode container-ID fallback GetCgroupInode, line 168). That's it.

This PR initially replaced Mono.Unix with a tiny native C library that wraps stat(), called via P/Invoke. It shiped a .so for 4 RIDs (linux-x64, linux-musl-x64, linux-arm64, linux-musl-arm64), with Docker cross-builds, CI changes, and NuGet packaging work.

Why not P/Invoke libc's stat() directly? Because struct stat's layout varies by architecture (x86_64 vs arm64) and libc (glibc vs musl), so a single C# struct definition can't safely marshal it — which is why we were wrapping it in C in early commits in this PR.

@atanzu suggested using statx instead. That's a genuinely good idea, and it sidesteps the whole problem:

• struct statx has a fixed, architecture-independent layout defined by the kernel (fixed-width fields + explicit padding, 256 bytes). One C# struct works everywhere — no per-arch/per-libc variants.
• statx(2) is a Linux syscall since kernel 4.11 (2017); the glibc wrapper exists since glibc 2.28 and musl since 1.1.20. You can P/Invoke [LibraryImport("libc")] statx(...) directly, request only STATX_INO, and read stx_ino.
• This eliminates the entire native library: no C code, no CMake, no Docker cross-builds, no 4-RID packaging, no arm64 test flakiness, no NuGet native-asset plumbing. It's pure managed P/Invoke, like the old Mono.Unix path but with zero external dependency.

The one tradeoff: kernels older than 4.11 (e.g. CentOS 7 / kernel 3.10) lack statx. But origin detection already degrades gracefully (returns empty / false), so on those hosts you'd just lose container-ID-by-inode fallback — the same as today on any unsupported platform. For containerized workloads (the only place this matters) 4.11+ is essentially universal in 2026.

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[Copilot]

Pull request overview

This PR removes the external Mono.Unix NuGet dependency by vendoring the required UnixEndPoint implementation and replacing inode lookup based on Mono.Unix.Native.Syscall.stat with a Linux statx P/Invoke helper, plus adds documentation and integration tests around inode lookup.

Changes:

• Removed Mono.Unix package reference and added an in-repo Mono.Unix.UnixEndPoint implementation.
• Introduced NativeMethods (statx P/Invoke) and updated FileSystem.TryStat to use it on Linux (non-NETFRAMEWORK builds).
• Added Linux integration tests for inode lookup and new build/testing guidance docs.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/StatsdClient.Tests/NativeLibraryTests.cs	Adds Linux-only integration tests validating inode lookup behavior.
src/StatsdClient/UnixEndPoint.cs	Vendors UnixEndPoint to avoid relying on the Mono.Unix package.
src/StatsdClient/StatsdClient.csproj	Removes the Mono.Unix package reference from the library project.
src/StatsdClient/NativeMethods.cs	Adds statx-based inode lookup via P/Invoke on Linux.
src/StatsdClient/IFileSystem.cs	Switches TryStat implementation to use the new inode lookup on Linux.
CLAUDE.md	Adds repository build/test guidance for agent-based workflows.
BUILD.md	Adds general build and test instructions (including framework selection guidance).
.gitattributes	Normalizes line endings and enforces LF for shell scripts.

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[zacharycmontoya]

LGTM

[andrewlock]

LGTM - I think we should mention somewhere th as t this is incompatible with a bunch of glibc versions and musk - maybe in the PR and in the source code (so anyone wondering "why doesn't this work" has an easy answer)