agent: use socket activation, port credential + provide nixos module

sidebus-agent/Cargo.toml

@@ -5,7 +5,6 @@ edition = "2024"
 
 [dependencies]
 sidebus-common = { workspace = true }
-clap = { version = "4.5.40", features = ["derive"] }
 eyre = "0.6.12"
 tokio = { version = "1.46.0", features = ["full"] }
 tokio-stream = "0.1.17"
@@ -13,3 +12,4 @@ tokio-vsock = "0.7.1"
 tracing = "0.1.41"
 tracing-subscriber = "0.3.19"
 zbus = { workspace = true }
+listenfd = "1.0.2"

sidebus-agent/src/main.rs

@@ -1,27 +1,28 @@
-use clap::Parser;
+use eyre::OptionExt;
 use tokio::net::UnixListener;
 use tracing::info;
 
-#[derive(Parser)]
-#[command(version, about, long_about = None)]
-struct AgentCli {
-    listen_path: String,
-}
-
 #[tokio::main]
 async fn main() -> eyre::Result<()> {
     tracing_subscriber::fmt::init();
 
-    let cli = AgentCli::parse();
+    let creds_dir = std::path::PathBuf::try_from(std::env::var("CREDENTIALS_DIRECTORY")?)?;
+    let vsock_port = std::fs::read_to_string(creds_dir.join("sidebus.port"))?
+        .trim()
+        .parse::<u32>()?;
+
+    let unix_listener = UnixListener::from_std(
+        listenfd::ListenFd::from_env()
+            .take_unix_listener(0)?
+            .ok_or_eyre("no unix listener provided")?,
+    )?;
+    info!("listening for unix clients");
 
-    let unix_addr = cli.listen_path;
-    let unix_listener = UnixListener::bind(unix_addr.clone())?;
-    info!(%unix_addr, "listening for unix clients");
     while let Ok((unix_client, client_addr)) = unix_listener.accept().await {
         info!(?client_addr, "new unix client");
         tokio::spawn(async move {
             let vsock_addr = zbus::Address::from(zbus::address::Transport::Vsock(
-                zbus::address::transport::Vsock::new(2, 4269),
+                zbus::address::transport::Vsock::new(2, vsock_port),
             ));
             let vsock_conn = zbus::connection::Builder::address(vsock_addr)
                 .unwrap()