Fix Android build: enable linux-raw-sys and exclude Android-unsupported Linux userspace features

[HemantKArya]

Enable access to Linux kernel types on Android targets while excluding a small set of Linux userspace features that are not available on Android (XDP, io_uring, certain STATX constants), restoring Android builds without changing Linux behavior.

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Motivation / Background

Recently the crate started failing to build for Android targets with errors like:

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `linux_raw_sys`
error[E0659]: `STATX_ATTR_NODUMP` is ambiguous

Root cause: Android had been explicitly excluded from the linux_raw_dep / linux-raw-sys path in build.rs / Cargo.toml, which prevented Android builds from getting Linux kernel constant / syscall/type definitions that are required even when using the libc backend. Bionic (Android libc) does not expose all kernel-level constants and some syscalls/constants are only available via linux-raw-sys. At the same time, Android’s userspace is missing a few advanced Linux-only features and constants (e.g. SOL_XDP, RWF_*) — those should remain disabled on Android rather than enabled.

This PR restores Android builds by enabling access to linux-raw-sys on Android targets, and then selectively disables features that truly require Linux userspace support not present in Bionic.

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What this PR changes

High level

• Enable linux-raw-sys kernel types for Android targets so Android can access kernel constants/types it needs.
• Keep Linux functionality unchanged.
• Add targeted #[cfg(...)] guards so features not supported by Bionic (XDP, io_uring, certain STATX usages) remain disabled on Android.
• Update a small test to exclude Android-only-incompatible assertions.

Rationale / Technical explanation

• Android uses the Linux kernel, so kernel-level types and syscall constants are valid on Android and MUST be available to the crate (via linux-raw-sys) so code paths that depend on kernel definitions compile.

• However, Android’s userspace (Bionic) does not expose certain Linux userspace constants and features:

  • XDP / AF_XDP / SOL_XDP: SOL_XDP and related constants are not present in Bionic. XDP is a Linux networking acceleration feature that is not applicable to Android userspace; those code paths are guarded so they are only enabled for desktop/server Linux.
  • io_uring: RWF_* (e.g., RWF_NOWAIT, RWF_DSYNC) flags and other io_uring-related userspace support may be absent in Bionic. io_uring code is therefore excluded on Android targets.
  • STATX constants: Some STATX constants can be defined both in libc and linux-raw-sys, causing ambiguity; adding Android to the exclusion list resolves the ambiguity and keeps the definitions consistent.

Net effect: Android gets what it needs from kernel types; Linux retains full feature set. No behavior changes for Linux targets.

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Testing performed

All CI workflow targets tested locally/in-workflow:

Android (all passing)

• aarch64-linux-android (default & use-libc features)
• armv7-linux-androideabi (use-libc)
• x86_64-linux-android (use-libc)
• i686-linux-android (use-libc)

Linux (all passing)

• Default build
• all-apis feature builds
• use-libc,all-apis builds
• linux_latest,all-apis builds
• All library tests pass (51/51)

(These are the results from my test runs; CI logs are attached in the workflow that exercised all Android and Linux variants.)

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• Changes are intentionally minimal and scoped:

  • Only enables kernel-type dependency for Android.
  • Only disables a small, well-defined subset of features on Android that are known not to be available in Bionic (XDP, io_uring, RWF flags, certain STATX exposures).

• Linux behavior is unchanged — #[cfg(all(linux_raw_dep, target_os = "linux"))] keeps full Linux feature set intact.

• Tests and CI flows covering both Android and Linux have been exercised and pass.

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References (for reviewer verification)

• Android + Linux kernel architecture: https://source.android.com/docs/core/architecture/kernel
• XDP (AF_XDP) docs: https://www.kernel.org/doc/html/latest/networking/af_xdp.html
• io_uring paper / reference: https://kernel.dk/io_uring.pdf