feat(agent): systemd service install/uninstall subcommands (alpha.11)

For Saturday's Ubuntu host + Linux VM: 'corrosion-host-agent install' writes a
systemd unit (Type=simple — the agent already handles SIGTERM cleanly),
daemon-reloads, and enables+starts the service; 'uninstall' reverses it.

- new service.rs: pure unit_file_contents() generator (unit-tested) + Linux
  install/uninstall via systemctl; non-Linux returns a clear 'Linux only' error
  (Windows SCM is the follow-up).
- ExecStart honors the resolved --config path (default or explicit).
- Runs as root: the agent supervises game processes + their files, needs broad
  filesystem access.

cargo check + service unit test green. Tag agent-v2.0.0-alpha.11 -> CI signs ->
CDN /host-agent/alpha/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

corrosion-host-agent/Cargo.lock

@@ -287,7 +287,7 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.10"
+version = "2.0.0-alpha.11"
 dependencies = [
  "anyhow",
  "async-nats",

corrosion-host-agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.10"
+version = "2.0.0-alpha.11"
 edition = "2021"
 description = "Corrosion Host Agent — multi-game ops runtime for self-hosted game servers"
 license = "UNLICENSED"

corrosion-host-agent/src/lib.rs

@@ -11,6 +11,7 @@ pub mod instancecmd;
 pub mod prober;
 pub mod process;
 pub mod rcon;
+pub mod service;
 pub mod steamcmd;
 pub mod subjects;
 pub mod supervisor;

corrosion-host-agent/src/main.rs

@@ -6,7 +6,7 @@
 
 use corrosion_host_agent::{
     agent, bus, config, docker_compose, filemanager, hostcmd, instancecmd, prober, process,
-    subjects, supervisor, telemetry, version,
+    service, subjects, supervisor, telemetry, version,
 };
 
 use anyhow::{Context, Result};
@@ -37,6 +37,10 @@ enum Command {
     Check,
     /// Print full version (semver, git hash, build timestamp) and exit.
     Version,
+    /// Install as a systemd service and start it (Linux; requires root).
+    Install,
+    /// Stop and remove the systemd service (Linux; requires root).
+    Uninstall,
 }
 
 fn main() -> Result<()> {
@@ -58,6 +62,8 @@ fn main() -> Result<()> {
             );
             Ok(())
         }
+        Some(Command::Install) => service::install(&config_path),
+        Some(Command::Uninstall) => service::uninstall(),
         None => {
             let settings = config::load(&config_path)?;
             init_logging(&settings.log_level);

corrosion-host-agent/src/service.rs

@@ -0,0 +1,129 @@
+//! systemd service installation for the host agent (Linux).
+//!
+//! `corrosion-host-agent install` writes a systemd unit pointing at the current
+//! binary + config, reloads systemd, and enables + starts the service.
+//! `uninstall` reverses it. Windows SCM support is a follow-up; on non-Linux
+//! these return a clear "Linux only" error rather than silently doing nothing.
+//!
+//! The agent already handles SIGTERM (see main::wait_for_shutdown_signal), so a
+//! plain `Type=simple` unit gives systemd clean start/stop semantics.
+
+use anyhow::{bail, Result};
+use std::path::Path;
+
+#[cfg(target_os = "linux")]
+use anyhow::Context;
+
+pub const SERVICE_NAME: &str = "corrosion-host-agent";
+
+#[cfg(target_os = "linux")]
+const UNIT_PATH: &str = "/etc/systemd/system/corrosion-host-agent.service";
+
+/// Render the systemd unit. Pure (no I/O) so it is unit-testable.
+pub fn unit_file_contents(exec_path: &str, config_path: &str) -> String {
+    format!(
+        "[Unit]\n\
+         Description=Corrosion Host Agent (multi-game ops runtime)\n\
+         Documentation=https://corrosionmgmt.com\n\
+         After=network-online.target\n\
+         Wants=network-online.target\n\
+         \n\
+         [Service]\n\
+         Type=simple\n\
+         ExecStart={exec} --config {cfg}\n\
+         Restart=on-failure\n\
+         RestartSec=5\n\
+         # The agent supervises game-server processes and their files, so it\n\
+         # needs broad filesystem access and runs as root by default.\n\
+         User=root\n\
+         \n\
+         [Install]\n\
+         WantedBy=multi-user.target\n",
+        exec = exec_path,
+        cfg = config_path,
+    )
+}
+
+#[cfg(target_os = "linux")]
+pub fn install(config_path: &Path) -> Result<()> {
+    let exec = std::env::current_exe().context("resolving current executable path")?;
+    let exec_str = exec.to_string_lossy();
+    let cfg_str = config_path.to_string_lossy();
+
+    let unit = unit_file_contents(&exec_str, &cfg_str);
+    std::fs::write(UNIT_PATH, unit)
+        .with_context(|| format!("writing {UNIT_PATH} (are you root?)"))?;
+    println!("wrote {UNIT_PATH}");
+
+    run("systemctl", &["daemon-reload"])?;
+    run("systemctl", &["enable", "--now", SERVICE_NAME])?;
+
+    println!(
+        "service '{SERVICE_NAME}' installed and started.\n  \
+         status: systemctl status {SERVICE_NAME}\n  \
+         logs:   journalctl -u {SERVICE_NAME} -f"
+    );
+    Ok(())
+}
+
+#[cfg(target_os = "linux")]
+pub fn uninstall() -> Result<()> {
+    // Best-effort stop+disable; don't fail if it isn't currently active.
+    let _ = std::process::Command::new("systemctl")
+        .args(["disable", "--now", SERVICE_NAME])
+        .status();
+
+    if Path::new(UNIT_PATH).exists() {
+        std::fs::remove_file(UNIT_PATH)
+            .with_context(|| format!("removing {UNIT_PATH} (are you root?)"))?;
+        println!("removed {UNIT_PATH}");
+    }
+    run("systemctl", &["daemon-reload"])?;
+    println!("service '{SERVICE_NAME}' uninstalled.");
+    Ok(())
+}
+
+#[cfg(target_os = "linux")]
+fn run(cmd: &str, args: &[&str]) -> Result<()> {
+    let status = std::process::Command::new(cmd)
+        .args(args)
+        .status()
+        .with_context(|| format!("running {cmd} {}", args.join(" ")))?;
+    if !status.success() {
+        bail!("{cmd} {} failed with {status}", args.join(" "));
+    }
+    Ok(())
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn install(_config_path: &Path) -> Result<()> {
+    bail!(
+        "`install` is only supported on Linux (systemd). Windows SCM support is \
+         coming; for now run the agent directly or via your platform's service manager."
+    );
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn uninstall() -> Result<()> {
+    bail!("`uninstall` is only supported on Linux (systemd).");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn unit_contains_exec_config_and_install_target() {
+        let u = unit_file_contents(
+            "/usr/local/bin/corrosion-host-agent",
+            "/etc/corrosion/agent.toml",
+        );
+        assert!(u.contains(
+            "ExecStart=/usr/local/bin/corrosion-host-agent --config /etc/corrosion/agent.toml"
+        ));
+        assert!(u.contains("Type=simple"));
+        assert!(u.contains("Restart=on-failure"));
+        assert!(u.contains("WantedBy=multi-user.target"));
+        assert!(u.contains("After=network-online.target"));
+    }
+}