host: add a shared writable /config world mount

Reviewed-on: #3

PROTOCOL.md

@@ -59,13 +59,19 @@ it exits. After 5 seconds, it kills the instance.
 
 ## Filesystem
 
-Each instance sees three folders:
+Each instance sees four folders:
 
 | Path | Access | Contents |
 | --- | --- | --- |
-| `/data` | read-write | the actor's state |
+| `/data` | read-write by its actor; readable by every actor | the actor's state |
 | `/public` | read-write | the files that the actor publishes |
 | `/actor` | read-only | the `actor/` folder of the world |
+| `/config` | read-write by every actor | the world's shared `config/` folder; persists across restarts |
+
+`/config` is the same world folder for every instance, including candidates;
+writers coordinate by convention, not permissions. Do not put secrets there.
+
+An actor can write only its own `/data`, but can read every actor's `/data` through `/actor/<id>/data/`. `/data` is not private: any unprotected secret file there is readable by every actor.
 
 `/actor/<id>/` holds everything about an actor: `actor.wasm`, `actor.json`
 (installed version, running, subscriptions, tapped, watches), `source/`, `public/`, `data/`,
@@ -123,16 +129,18 @@ the request: the request's `type` in past tense (`spawned`, `upgraded`,
 | `type` | Fields | Effect |
 | --- | --- | --- |
 | `spawn` | `actor`, `wasm`, `source`?, `built_by`? | test, then start a new actor |
-| `upgrade` | `actor`, `wasm`, `source`?, `built_by`? | test, then swap the version |
+| `upgrade` | `actor`, `wasm`, `source`?, `built_by`? | test, then swap the version: a running actor is stopped (normal shutdown) and starts the new version; a stopped actor gets the new version and stays stopped, with its mailbox untouched, until `start`. The answer has `running` |
-| `rollback` | `actor`, `version` | go back to an older version |
+| `rollback` | `actor`, `version` | go back to an older version; like `upgrade`, a stopped actor stays stopped. The answer has `running` |
 | `stop`, `start` | `actor` | stop or start an actor |
 | `remove` | `actor` | stop the actor (normal shutdown), then delete its whole folder `/actor/<id>/` (all versions, `data/`, `public/`, logs); answer `{"type": "removed", "actor"}`. Messages to it are then undeliverable, and the ID can be spawned again. An unknown actor or a native one (`broker`, `console`, `cargo`, `fs`) is an `error` |
 | `subscribe`, `unsubscribe` | `actor` | receive (or not) what `actor` publishes |
 | `tap`, `untap` | | receive (or not) a copy of every message the broker routes |
 | `watch`, `unwatch` | `path` | receive (or not) `fs_changed` for every change of `path` or anything under it; answer `{"type": "watching" \| "unwatched", "path"}` with the path as an `/actor/...` path |
-| `exit` | | answer `exiting`, stop every actor, and end the `sausage` process (a wrapper like `nix run .#loop` starts it again, updated) |
+| `exit` | | answer `exiting`, stop every actor, and end the `sausage` process with exit code 75 (a wrapper like `nix run .#loop` starts it again, updated) |
 
-Paths are `/actor/...`, or the sender's `/public/...` or `/data/...`.
+**Headless.** `sausage run --headless [dir]` (or `sausage --headless`, or `SAUSAGE_HEADLESS=1`) turns the console input off: the host does not read stdin, so end of input (`/dev/null`) does not stop the world. `nix run .#loop -- --headless <world>` passes it through the loop. It runs until SIGTERM or SIGINT (stops every actor, exit code 0) or a broker `exit` request (exit code 75, as above). Without the flag, EOF on stdin or `/quit` ends the host.
+
+Paths are `/actor/...`, `/config/...`, or the sender's `/public/...` or `/data/...`.
 
 **Taps.** After `tap`, the sender gets a copy of every message that the
 broker routes (direct messages, replies, and publishes, between any actors,
@@ -208,7 +216,8 @@ instance. A version without test files passes.
 
 **Swap.** The broker holds the mailbox, stops the old instance, makes the
 top-level `actor.wasm` and `source/` a copy of the new version, and starts
-it. A rollback does the same with the older version.
+it. A stopped actor is not started: it keeps its mailbox and runs the new
+version at the next `start`. A rollback does the same with the older version.
 
 **Crashes.** The broker restarts a crashed actor after 0.5 s, then doubles
 the delay up to 30 s. A new version that crashes 3 times within 30 seconds

README.md

@@ -48,6 +48,18 @@ From a clone (`git clone https://gitea.surma.technology/surma/sausage`), the
 same commands are `nix run`, `nix run -- init`, `nix run -- run` and
 `nix run .#loop`.
 
+### Headless
+
+`--headless` (or `SAUSAGE_HEADLESS=1`) turns the console input off: the host
+never reads stdin, so end of input does not stop the world. It runs until
+SIGTERM/SIGINT (clean stop, exit code 0) or a broker `exit` request (exit
+code 75, so the loop starts the newest version). Create the world first
+(`init`), then pass the flag through the loop:
+
+```sh
+nix run git+https://gitea.surma.technology/surma/sausage#loop -- --headless my-world
+```
+
 In the console:
 
 ```text
@@ -66,7 +78,7 @@ actor.wasm     the current version
 actor.json     installed version, running, subscriptions
 source/        the source of the current version
 public/        files that the actor publishes
-data/          the actor's state
+data/          actor state; owner-writable, world-readable via /actor/<id>/data/ (not private)
 stderr.log     what the actor and the broker write about it
 versions/<n>/  every installed version: actor.wasm, source/, meta.json
 ```

agent/src/prompt.md

@@ -3,3 +3,5 @@ People talk to you through other actors. Your text goes to your subscribers.
 Before writing an actor, read `/actor/broker/public/PROTOCOL.md` for messages, the broker, `cargo`, and tests.
 Read the SDK at `/actor/cargo/public/sdk/` before you write an actor.
 You can change your own source at `/actor/agent/source`.
+
+The global filesystem is readable by every actor, including every actor's `/data` at `/actor/<id>/data/`. An actor can write only its own `/data` and `/public`. `/data` is not private: any unprotected secret file there is readable by every actor.

cli/src/broker.rs

@@ -70,6 +70,7 @@ pub struct Broker {
 
 impl Broker {
     pub fn new(world: World, runtime: Arc<Runtime>) -> Result<Arc<Self>> {
+        std::fs::create_dir_all(world.config_dir())?;
         let metas = world.actor_ids()?.into_iter().map(|id| (id.clone(), world.read_meta(&id))).collect();
         Ok(Arc::new_cyclic(|me| Self {
             world,
@@ -258,6 +259,7 @@ impl Broker {
                 Mount { guest: "/actor".into(), host: self.world.actors_root(), writable: false },
                 Mount { guest: "/public".into(), host: own.join("public"), writable: true },
                 Mount { guest: "/data".into(), host: own.join("data"), writable: true },
+                Mount { guest: "/config".into(), host: self.world.config_dir(), writable: true },
             ],
             network: scratch.is_none(),
             stderr_log: own.join("stderr.log"),
@@ -331,12 +333,13 @@ impl Broker {
         Ok((component, tests))
     }
 
-    /// Make `version` the current `actor.wasm`, `source/`, and `actor.json`.
+    /// Make `version` the current `actor.wasm`, `source/`, and `actor.json`,
-    pub fn install(&self, id: &str, version: u64) -> Result<()> {
+    /// and record whether the actor runs.
+    pub fn install(&self, id: &str, version: u64, running: bool) -> Result<()> {
         self.world.install_files(id, version)?;
         self.update_meta(id, |m| {
             m.version = Some(version);
-            m.running = true;
+            m.running = running;
         })
     }
 
@@ -360,7 +363,7 @@ impl Broker {
 
     async fn create_version(self: &Arc<Self>, id: &str, meta: &VersionMeta, wasm: &Path, source: Option<&Path>) -> Result<Value> {
         let (_, tests) = self.try_version(id, meta, wasm, source).await?;
-        self.install(id, meta.version)?;
+        self.install(id, meta.version, true)?;
         self.start_supervisor(id, meta.version, true);
         Ok(json!({"type": "spawned", "actor": id, "version": meta.version, "tests": tests}))
     }

cli/src/console.rs

@@ -3,7 +3,8 @@
 //! - `@<actor> <json>` sends a JSON body. `@<actor> <text>` sends
 //!   `{"type": "text", "text": ...}`.
 //! - A line without `@` goes to `agent` as text.
-//! - `/help`, `/quit`. EOF on stdin also quits.
+//! - `/help`, `/quit`. EOF on stdin also quits (not with `--headless`, which
+//!   never reads stdin).
 
 use std::sync::Arc;
 

cli/src/main.rs

@@ -5,6 +5,7 @@ use anyhow::{Context, Result, bail};
 use clap::{Parser, Subcommand};
 use include_dir::{Dir, include_dir};
 use serde_json::json;
+use tokio::signal::unix::Signal;
 
 use sausage::world::{ActorMeta, World};
 use sausage::{Host, cargo_actor, console};
@@ -17,6 +18,11 @@ static AGENT_SOURCE: Dir<'_> = include_dir!("$CARGO_MANIFEST_DIR/../agent");
 #[derive(Parser)]
 #[command(version, about)]
 struct Cli {
+    /// Do not read stdin, so end of input does not stop the world. It runs
+    /// until SIGTERM (or SIGINT), or until an actor sends
+    /// `@broker {"type":"exit"}`. Also set by SAUSAGE_HEADLESS=1.
+    #[arg(long, global = true)]
+    headless: bool,
     #[command(subcommand)]
     command: Option<Cmd>,
 }
@@ -91,13 +97,41 @@ async fn init(dir: PathBuf) -> Result<()> {
     Ok(())
 }
 
-async fn run(dir: PathBuf) -> Result<()> {
+/// Install the SIGTERM and SIGINT handlers now (not when first polled).
+fn termination_signals() -> Result<(Signal, Signal)> {
+    use tokio::signal::unix::{SignalKind, signal};
+    let term = signal(SignalKind::terminate()).context("install SIGTERM handler")?;
+    let int = signal(SignalKind::interrupt()).context("install SIGINT handler")?;
+    Ok((term, int))
+}
+
+/// Resolves on SIGTERM or SIGINT.
+async fn termination((mut term, mut int): (Signal, Signal)) {
+    tokio::select! {
+        _ = term.recv() => {}
+        _ = int.recv() => {}
+    }
+}
+
+async fn run(dir: PathBuf, headless: bool) -> Result<()> {
+    // Register the signal handlers before the host starts, so a signal that
+    // arrives early is never the default (kill) action.
+    let signals = if headless { Some(termination_signals()?) } else { None };
     let host = Host::start(&dir).await?;
     tokio::spawn(console::run_mailbox(host.mailbox(console::ID)));
     let broker = host.broker.clone();
-    let exit = tokio::select! {
+    let exit = match signals {
+        Some(signals) => {
+            println!("Sausage headless: running until SIGTERM or an exit request.");
+            tokio::select! {
+                _ = termination(signals) => false,
+                _ = broker.exit_requested() => true,
+            }
+        }
+        None => tokio::select! {
             _ = console::run_input(host.broker.clone()) => false,
             _ = broker.exit_requested() => true,
+        },
     };
     println!("Stopping actors …");
     host.stop().await;
@@ -113,16 +147,18 @@ async fn run(dir: PathBuf) -> Result<()> {
 
 #[tokio::main]
 async fn main() -> Result<()> {
-    match Cli::parse().command {
+    let cli = Cli::parse();
+    let headless = cli.headless || std::env::var("SAUSAGE_HEADLESS").is_ok_and(|v| !v.is_empty() && v != "0");
+    match cli.command {
         Some(Cmd::Init { dir }) => init(dir).await,
-        Some(Cmd::Run { dir }) => run(dir).await,
+        Some(Cmd::Run { dir }) => run(dir, headless).await,
         None => {
             let dir = PathBuf::from("world");
             if !dir.join("actor").is_dir() {
                 println!("No world in ./world yet, creating it (same as `sausage init`).");
                 init(dir.clone()).await?;
             }
-            run(dir).await
+            run(dir, headless).await
         }
     }
 }

cli/src/supervisor.rs

@@ -220,17 +220,16 @@ impl Supervisor {
 
     /// Make `version` current. It is compiled first (unless `component` is
     /// given): if that fails, nothing changes. `data/` and `public/` stay as
-    /// they are.
+    /// they are. A stopped actor stays stopped.
     async fn install(&mut self, version: u64, component: Option<Component>) -> Result<()> {
         let component = match component {
             Some(c) => c,
             None => self.broker.load_version(&self.id, version)?,
         };
         self.stop().await;
-        self.broker.install(&self.id, version)?;
+        self.broker.install(&self.id, version, self.running)?;
         self.version = version;
         self.component = Some(component);
-        self.running = true;
         self.restarts = 0;
         self.restart_at = None;
         Ok(())
@@ -239,20 +238,26 @@ impl Supervisor {
     async fn upgrade(&mut self, version: u64, component: Component, requester: String) -> Result<Value> {
         let previous = self.version;
         self.install(version, Some(component)).await?;
+        if !self.running {
+            // Installed, not started: main runs and mail is read at the next `start`.
+            return Ok(json!({"type": "upgraded", "actor": self.id, "version": version, "previous": previous, "running": false}));
+        }
         self.probation = Some(Probation { previous, since: Instant::now(), crashes: 0, requester });
         if let Err(e) = self.start().await {
             self.rollback(previous).await?;
             bail!("version {version} did not start ({e:#}); rolled back to {previous}");
         }
-        Ok(json!({"type": "upgraded", "actor": self.id, "version": version, "previous": previous}))
+        Ok(json!({"type": "upgraded", "actor": self.id, "version": version, "previous": previous, "running": true}))
     }
 
     async fn rollback(&mut self, version: u64) -> Result<Value> {
         let from = self.version;
         self.install(version, None).await?;
         self.probation = None;
+        if self.running {
             self.start().await?;
-        Ok(json!({"type": "rolled_back", "actor": self.id, "from": from, "to": version}))
+        }
+        Ok(json!({"type": "rolled_back", "actor": self.id, "from": from, "to": version, "running": self.running}))
     }
 
     /// Handle a control request. Returns `true` when the supervisor ends.

cli/src/world.rs

@@ -4,6 +4,7 @@
 //!
 //! ```text
 //! <world>/
+//!   config/              shared configuration, writable by every actor
 //!   actor/<id>/
 //!     actor.wasm        the current version
 //!     actor.json        version, running, subscriptions, tapped, watches (broker)
@@ -89,6 +90,9 @@ impl World {
         Self { root: root.into() }
     }
 
+    pub fn config_dir(&self) -> PathBuf {
+        self.root.join("config")
+    }
     pub fn actors_root(&self) -> PathBuf {
         self.root.join("actor")
     }
@@ -191,7 +195,7 @@ impl World {
 
     /// Map a path in an actor's namespace to a host path.
     ///
-    /// - `/actor/<id>/...` is the global filesystem.
+    /// - `/actor/<id>/...` and `/config/...` are world paths.
     /// - `/public/...` and `/data/...` belong to `sender`.
     /// - Native senders (the console) can also give absolute host paths.
     pub fn resolve(&self, sender: &str, path: &str) -> Result<PathBuf> {
@@ -212,6 +216,9 @@ impl World {
         if let Some(rest) = prefixed("/actor") {
             return under(self.actors_root(), rest);
         }
+        if let Some(rest) = prefixed("/config") {
+            return under(self.config_dir(), rest);
+        }
         if let Some(rest) = prefixed("/public") {
             return under(self.public_dir(sender), rest);
         }
@@ -221,7 +228,7 @@ impl World {
         if NATIVE.contains(&sender) && Path::new(path).is_absolute() {
             return Ok(PathBuf::from(path));
         }
-        bail!("cannot resolve path {path} for {sender}: use /actor/<id>/..., /public/... or /data/...")
+        bail!("cannot resolve path {path} for {sender}: use /actor/<id>/..., /config/..., /public/... or /data/...")
     }
 
     /// The path of a host file in the global filesystem, if it has one.

cli/tests/cli.rs

@@ -39,3 +39,47 @@ fn bare_command_without_a_world_starts_init() {
     // It asks before it creates anything.
     assert!(!dir.join("world").exists());
 }
+
+/// Start a headless host on a fresh world with stdin at EOF; returns once it
+/// printed its ready line (printed after the signal handlers are installed).
+fn start_headless(label: &str) -> (std::process::Child, std::io::Lines<std::io::BufReader<std::process::ChildStdout>>) {
+    use std::io::{BufRead, BufReader};
+    let dir = empty_dir(label);
+    let world = dir.join("world");
+    sausage::World::create(&world).unwrap();
+    let mut child = Command::new(env!("CARGO_BIN_EXE_sausage"))
+        .args(["run", "--headless"])
+        .arg(&world)
+        .stdin(Stdio::null())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
+        .unwrap();
+    let mut lines = BufReader::new(child.stdout.take().unwrap()).lines();
+    let line = lines.next().expect("host exited before it started").unwrap();
+    assert!(line.contains("headless"), "unexpected first line: {line}");
+    (child, lines)
+}
+
+/// With stdin at EOF (as soon as the host starts), a headless host stays up.
+#[test]
+fn headless_ignores_stdin_eof() {
+    let (mut child, _lines) = start_headless("headless-eof");
+    std::thread::sleep(std::time::Duration::from_millis(500));
+    let alive = child.try_wait().unwrap().is_none();
+    let _ = child.kill();
+    let _ = child.wait();
+    assert!(alive, "headless host exited on stdin EOF");
+}
+
+/// SIGTERM right after the ready line stops the actors and exits with 0.
+#[test]
+fn headless_stops_cleanly_on_sigterm_right_after_ready() {
+    let (mut child, lines) = start_headless("headless-term");
+    let pid = child.id().to_string();
+    assert!(Command::new("kill").args(["-TERM", &pid]).status().unwrap().success());
+    let rest: Vec<String> = lines.map(|l| l.unwrap()).collect();
+    let status = child.wait().unwrap();
+    assert!(status.success(), "exit status {status:?}, output {rest:?}");
+    assert!(rest.iter().any(|l| l.contains("Stopping actors")), "output {rest:?}");
+}

cli/tests/host.rs

@@ -175,6 +175,42 @@ async fn filesystem_permissions_and_public_sharing() {
     host.stop().await;
 }
 
+#[tokio::test]
+async fn config_is_shared_writable_and_survives_host_restart() {
+    let (world, host, mut mailbox) = filesystem_world("world-config").await;
+    let path = "/config/global.json";
+    let first = r#"{"model":"first"}"#;
+    let second = r#"{"model":"second"}"#;
+    ask_type(
+        &host, &mut mailbox, "writer",
+        json!({"type": "write", "path": path, "text": first}), "ok",
+    ).await;
+    let read = ask_type(
+        &host, &mut mailbox, "reader",
+        json!({"type": "read", "path": path}), "content",
+    ).await;
+    assert_eq!(read.body["text"], first);
+    // The second actor can replace the same file too.
+    ask_type(
+        &host, &mut mailbox, "reader",
+        json!({"type": "write", "path": path, "text": second}), "ok",
+    ).await;
+    assert_eq!(std::fs::read_to_string(world.config_dir().join("global.json")).unwrap(), second);
+    assert_eq!(world.resolve("writer", path).unwrap(), world.config_dir().join("global.json"));
+    assert_eq!(world.resolve("console", path).unwrap(), world.config_dir().join("global.json"));
+    host.stop().await;
+
+    let (host, mut mailbox) = start(&world).await;
+    for actor in ["writer", "reader"] {
+        let read = ask_type(
+            &host, &mut mailbox, actor,
+            json!({"type": "read", "path": path}), "content",
+        ).await;
+        assert_eq!(read.body["text"], second);
+    }
+    host.stop().await;
+}
+
 #[tokio::test]
 async fn actors_can_read_but_not_write_other_actor_data() {
     let (_, host, mut mailbox) = filesystem_world("actor-data-sharing").await;
@@ -506,6 +542,50 @@ async fn rolls_back_a_crash_loop_to_the_previous_version_without_restoring_data(
     host.stop().await;
 }
 
+#[tokio::test]
+async fn upgrade_and_rollback_keep_a_stopped_actor_stopped() {
+    let world = create_world("upgrade-stopped");
+    let wasm = toolbox_wasm().clone();
+    let (host, mut mailbox) = start(&world).await;
+    let source = passing_source(&world, "source");
+    spawn_type(&host, &mut mailbox, "sleeper", &wasm, &source).await;
+    let stopped = request(&host, &mut mailbox, json!({"type": "stop", "actor": "sleeper"})).await;
+    assert_eq!(stopped.body["type"], "stopped");
+
+    let upgraded = upgrade_actor(&host, &mut mailbox, "sleeper", &wasm, &source).await;
+    assert_eq!(upgraded.body["type"], "upgraded", "{upgraded:?}");
+    assert_eq!(upgraded.body["version"], 2);
+    assert_eq!(upgraded.body["running"], false);
+    let meta = world.read_meta("sleeper");
+    assert_eq!((meta.version, meta.running), (Some(2), false));
+
+    // Mail waits in the mailbox: the stopped actor reads nothing.
+    let waiting = host.broker.send("console", Some("sleeper"), None, None, json!({"type": "version"}));
+    assert!(
+        tokio::time::timeout(Duration::from_millis(500), receive_reply(&mut mailbox, &waiting, Duration::from_secs(10)))
+            .await
+            .is_err(),
+        "a stopped actor must not answer"
+    );
+
+    let rolled_back = request(&host, &mut mailbox, json!({"type": "rollback", "actor": "sleeper", "version": 1})).await;
+    assert_eq!(rolled_back.body["type"], "rolled_back", "{rolled_back:?}");
+    assert_eq!(rolled_back.body["running"], false);
+    assert!(!world.read_meta("sleeper").running);
+
+    // `start` runs the installed version and delivers the waiting message.
+    let started = request(&host, &mut mailbox, json!({"type": "start", "actor": "sleeper"})).await;
+    assert_eq!(started.body["type"], "started");
+    let answer = receive_reply(&mut mailbox, &waiting, Duration::from_secs(10)).await;
+    assert_eq!(answer.body["version"], "1");
+
+    // A running actor still restarts on the new version.
+    let running = upgrade_actor(&host, &mut mailbox, "sleeper", &wasm, &source).await;
+    assert_eq!(running.body["running"], true);
+    assert_eq!(ask(&host, &mut mailbox, "sleeper", json!({"type": "version"})).await.body["version"], "3");
+    host.stop().await;
+}
+
 #[tokio::test]
 async fn explicit_rollback_keeps_current_data() {
     let world = create_world("explicit-rollback");

flake.nix

@@ -21,9 +21,11 @@
           targets = [ "wasm32-wasip2" ];
           extensions = [ "rust-src" ];
         };
-        # Crane builds the dependencies once (`cargoArtifacts`, keyed only on
+        # The runtime cargo actor needs Cargo, rustc (with wasm32-wasip2), and
-        # Cargo.toml/Cargo.lock), so a change to our own crates rebuilds
+        # the native linker, even when the host itself is started with a bare
-        # just them.
+        # PATH. Keep this same runtime path in the packaged host and exercise
+        # the installed binary below with PATH deliberately empty.
+        runtimePath = pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ];
         craneLib = (crane.mkLib pkgs).overrideToolchain rust;
         common = {
           pname = "sausage";
@@ -42,12 +44,13 @@
         });
         sausage = craneLib.buildPackage (common // {
           inherit cargoArtifacts;
+          doNotRemoveReferencesToRustToolchain = true;
           cargoExtraArgs = "--locked -p sausage";
           doCheck = false;
           nativeBuildInputs = [ pkgs.makeWrapper ];
           # The `cargo` actor runs cargo at runtime. Build scripts need a C linker.
           postInstall = ''
-            wrapProgram $out/bin/sausage --prefix PATH : ${pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ]}
+            wrapProgram $out/bin/sausage --prefix PATH : ${runtimePath}
           '';
           meta.mainProgram = "sausage";
         });
@@ -91,6 +94,69 @@
             rm -f cli/tests/actors/toolbox/Cargo.lock
           '';
         });
+        # Exercise the installed host exactly as the loop launches it: the
+        # caller has no Cargo on PATH, so only the packaged runtime toolchain
+        # can run the native tests and the wasm32-wasip2 build.
+        checks.runtime-toolchain = pkgs.runCommand "sausage-runtime-toolchain" { } ''
+          set -eu
+          root="$TMPDIR/runtime-toolchain"
+          world="$root/world"
+          emptyPath="$root/empty-bin"
+          mkdir -p "$emptyPath" "$root/home" "$root/cargo-home"
+          for actor in broker console cargo fs; do
+            mkdir -p "$world/actor/$actor/public" "$world/actor/$actor/data"
+          done
+          source="$world/actor/console/public/probe"
+          mkdir -p "$source/src" "$source/tests"
+          cat > "$source/Cargo.toml" <<'TOML'
+          [package]
+          name = "runtime-probe"
+          version = "0.1.0"
+          edition = "2024"
+          TOML
+          printf 'fn main() {}\n' > "$source/src/main.rs"
+          cat > "$source/tests/probe.rs" <<'RUST'
+          #[test]
+          fn native_test_harness_runs() {
+              assert_eq!(2 + 2, 4);
+          }
+          RUST
+          echo "runtime-check: run packaged host with an empty PATH"
+          coproc host {
+            ${pkgs.coreutils}/bin/env -i PATH="$emptyPath" HOME="$root/home" TMPDIR="$root" \
+              CARGO_HOME="$root/cargo-home" ${sausage}/bin/sausage run "$world"
+          }
+          hostInput=''${host[1]}
+          hostOutput=''${host[0]}
+          hostPid=$host_PID
+          trap 'kill "$hostPid" 2>/dev/null || true' EXIT
+          printf '%s\n' '@cargo {"type":"build","source":"/actor/console/public/probe"}' >&"$hostInput"
+          built=0
+          while IFS= read -r -t 300 line <&"$hostOutput"; do
+            echo "$line"
+            case "$line" in
+              *'"type":"build_failed"'*)
+                echo "packaged cargo actor build failed with an empty caller PATH" >&2
+                exit 1
+                ;;
+              *'"type":"built"'*)
+                case "$line" in
+                  *'"passed":1'*) built=1 ;;
+                  *) echo "native test did not pass" >&2; exit 1 ;;
+                esac
+                break
+                ;;
+            esac
+          done
+          if [ "$built" -ne 1 ]; then
+            echo "packaged host did not build the probe" >&2
+            exit 1
+          fi
+          printf '%s\n' '/quit' >&"$hostInput"
+          wait "$hostPid"
+          mkdir -p "$out"
+          touch "$out/passed"
+        '';
 
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
         apps.loop = { type = "app"; program = "${loop}/bin/sausage-loop"; };