v3: world directory layout (world/ is the VFS root, host/ is host state)

host/src/world.rs

@@ -1,18 +1,32 @@
-//! A world folder and the one virtual root that host code and guests share.
+//! A world directory and the one virtual root that host code and guests share.
+//!
+//! Layout of a world directory:
+//! - `world/`: the virtual root `/` that every guest sees. Agents change everything in it.
+//! - `host/`: host state outside the sandbox (harness storage, later the vault). No guest sees it.
 use sausage_vfs::{Host, Layer, Mounts, Process};
-use std::{path::Path, sync::Arc};
+use std::{
+    path::{Path, PathBuf},
+    sync::Arc,
+};
 
 /// An open world. `root` is the filesystem every guest sees as `/`; host code (the harness's file
 /// tools) reads and writes through the same layer, so both always see the same files.
 pub struct World {
     pub root: Arc<dyn Layer>,
+    /// The host state folder, `<dir>/host`.
+    pub host_dir: PathBuf,
 }
 
 impl World {
-    /// Open the world folder at `path`. The process table adds more mounts (`/proc`, `/run`).
-    pub fn open(path: &Path) -> anyhow::Result<Self> {
+    /// Open (and create if missing) the world directory `dir`. The process table adds more mounts
+    /// (`/proc`, `/run`).
+    pub fn open(dir: &Path) -> anyhow::Result<Self> {
+        let root_dir = dir.join("world");
+        let host_dir = dir.join("host");
+        std::fs::create_dir_all(&root_dir)?;
+        std::fs::create_dir_all(&host_dir)?;
         let mut mounts = Mounts::default();
-        mounts.mount("/", Arc::new(Host::new(path, Process::default())?), false);
-        Ok(World { root: Arc::new(mounts) })
+        mounts.mount("/", Arc::new(Host::new(&root_dir, Process::default())?), false);
+        Ok(World { root: Arc::new(mounts), host_dir })
     }
 }