Run world service policy in WASI init with pollable process observation

host/src/process/mod.rs

@@ -41,11 +41,14 @@ mod bindings {
         with: {
             "wasi:io": wasmtime_wasi::p2::bindings::io,
             "sausage:process/host.child": super::Child,
+            "sausage:process/host.watcher": super::Watcher,
         },
     });
 }
 use bindings::sausage::process::host::{Host, HostChild, Request, Stdio};
 mod services;
+mod watcher;
+pub use watcher::Watcher;
 mod stdio;
 pub use services::supervise;
 use stdio::Io;
@@ -470,7 +473,23 @@ impl Host for State {
         Ok(self.runtime.kill_pid(pid).map_err(|e| e.to_string()))
     }
 }
+#[async_trait::async_trait]
+impl wasmtime_wasi::p2::Pollable for Child {
+    async fn ready(&mut self) {
+        while self.code.borrow_and_update().is_none() {
+            if self.code.changed().await.is_err() {
+                break;
+            }
+        }
+    }
+}
 impl HostChild for State {
+    async fn subscribe(
+        &mut self,
+        child: Resource<Child>,
+    ) -> wasmtime::Result<Resource<wasmtime_wasi::p2::DynPollable>> {
+        wasmtime_wasi::p2::subscribe(&mut self.table, child)
+    }
     async fn pid(&mut self, child: Resource<Child>) -> wasmtime::Result<u32> {
         Ok(self.table.get(&child)?.pid)
     }