host: add a shared writable /config world mount

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;