macos: add signal shim for downstream rust-vmm crates

Adds macOS compatibility for the `signal` module so downstream rust-vmm
crates (`vhost`, `vhost-user-backend`, `virtiofsd`, …) can build and
run on macOS without code changes. All macOS code is gated behind
`#[cfg(target_os = "macos")]` and adds no new dependencies; Linux and
Android builds are unaffected. The crate stays at `BSD-3-Clause` and no
third-party code is vendored.

Host-signal validation uses `1..=SIGUSR2` because macOS signal numbers
do not match Linux's: SIGSYS=12, SIGTERM=15, SIGUSR2=31. The
`SIGHUP..=SIGSYS` range used on Linux would reject valid macOS signals.
Pending-signal clearing goes via unblock-then-reblock because macOS
does not expose `sigtimedwait`.

What this PR no longer ships, and why:

- No `eventfd` shim. The cross-platform `EventConsumer` /
  `EventNotifier` primitive added in #244 already covers the daemon
  side of `vhost-user-backend`. The remaining macOS callers of
  `vmm_sys_util::eventfd::EventFd` are inside the `vhost` crate's
  `VhostBackend` / `VhostBackendMut` traits, whose only consumers are
  Linux-only (kernel vhost drivers, vhost-user frontends inside
  Linux-only VMMs). A small follow-up PR to `vhost` gates those trait
  methods on Linux, after which nothing in the macOS build path
  references `vmm_sys_util::eventfd` and a shim is genuinely
  unnecessary.

- No `epoll` shim. Consumers needing epoll-shaped polling on macOS
  should use `mio` (see vhost #316), which gives proper cross-platform
  event polling backed by `kqueue` on macOS — a cleaner architectural
  answer than shimming the Linux API.

Tests:

- `macos::signal::tests` — validator range, register/block/unblock/kill
  round-trips, pending-signal clearing.

Pre-existing failures in `rand` and `unix::tempdir` on macOS are out of
scope for this PR.

Signed-off-by: Chris Thomas <chris.thomas@antimatter-studios.com>

Author: christhomas

.gitignore

@@ -3,3 +3,4 @@
 **/*.rs.bk
 Cargo.lock
 
+.DS_Store

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Added
 
 - [[#254](https://github.com/rust-vmm/vmm-sys-util/pull/254)]: Support `TFD_NONBLOCK` for `timerfd::TimerFd`.
+- [[#268](https://github.com/rust-vmm/vmm-sys-util/pull/268)]: Add macOS support for the `signal` module. The shim is gated behind `#[cfg(target_os = "macos")]` and adds no new dependencies; Linux and Android builds are unaffected. macOS consumers needing epoll-shaped polling should use `mio` (see vhost #316). `eventfd` is not provided: the cross-platform `EventConsumer`/`EventNotifier` primitive added in #244 already covers the daemon use case, and `vhost`'s public traits that take `&EventFd` are Linux-only callers (a follow-up gates them on Linux).
 
 ## v0.15.0
 

src/lib.rs

@@ -12,6 +12,11 @@ mod linux;
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub use crate::linux::*;
 
+#[cfg(target_os = "macos")]
+mod macos;
+#[cfg(target_os = "macos")]
+pub use crate::macos::*;
+
 #[cfg(unix)]
 mod unix;
 #[cfg(unix)]

src/macos/mod.rs

@@ -0,0 +1,6 @@
+// Copyright 2024 rust-vmm Authors. All Rights Reserved.
+// SPDX-License-Identifier: BSD-3-Clause
+
+//! macOS-specific modules providing compatibility shims for Linux-only APIs.
+
+pub mod signal;