vhost-device-sound: add --socket-fd argument

This allows a service manager to start vhost-device-sound with an
already listening socket.  With this, the service manager can create
the socket in advance of starting any services, so there's no race in
between vhost-device-sound being started and being ready to accept
connections.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost-device-sound/CHANGELOG.md

@@ -3,6 +3,9 @@
 
 ### Added
 
+- [[#909]](https://github.com/rust-vmm/vhost-device/pull/909)
+  `vhost-device-sound` now supports a `--socket-fd` argument.
+
 ### Changed
 
 - [[#907]](https://github.com/rust-vmm/vhost-device/pull/907)

vhost-device-sound/README.md

@@ -15,6 +15,10 @@ generated with help2man target/debug/vhost-device-sound |mandoc
      --socket <SOCKET>
             vhost-user Unix domain socket path
 
+     --socket-fd <FD>
+            listening vhost-user Unix domain socket file descriptor
+            (e.g. from a service manager)
+
      --backend <BACKEND>
             audio backend to be used [possible values: null, pipewire, alsa, gstreamer]
 

vhost-device-sound/src/args.rs

@@ -1,16 +1,25 @@
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 //! An arguments type for the binary interface of this library.
 
+use std::os::fd::RawFd;
 use std::path::PathBuf;
 
-use clap::{Parser, ValueEnum};
+use clap::{ArgGroup, Parser, ValueEnum};
 
 #[derive(Parser, Debug)]
-#[clap(version, about, long_about = None)]
+#[clap(
+    version,
+    about,
+    long_about = None,
+    group(ArgGroup::new("socket group").required(true).args(&["socket", "socket_fd"])),
+)]
 pub struct SoundArgs {
     /// vhost-user Unix domain socket path.
     #[clap(long)]
-    pub socket: PathBuf,
+    pub socket: Option<PathBuf>,
+    /// vhost-user Unix domain socket FD.
+    #[clap(long)]
+    pub socket_fd: Option<RawFd>,
     /// audio backend to be used
     #[clap(long)]
     #[clap(value_enum)]

vhost-device-sound/src/main.rs

@@ -2,6 +2,9 @@
 // Stefano Garzarella <sgarzare@redhat.com>
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
+use std::os::unix::net::UnixListener;
+use std::os::unix::prelude::*;
+
 use clap::Parser;
 use vhost::vhost_user::Listener;
 use vhost_device_sound::{args::SoundArgs, start_backend_server, SoundConfig};
@@ -11,7 +14,15 @@ fn main() {
 
     let args = SoundArgs::parse();
     let config = SoundConfig::new(false, args.backend);
-    let mut listener = Listener::new(args.socket, true).unwrap();
+
+    let mut listener = if let Some(fd) = args.socket_fd {
+        // SAFETY: user has assured us this is safe.
+        unsafe { UnixListener::from_raw_fd(fd) }.into()
+    } else if let Some(path) = args.socket {
+        Listener::new(path, true).unwrap()
+    } else {
+        unreachable!()
+    };
 
     loop {
         start_backend_server(&mut listener, config.clone());