hive-priv: never leave an agent dir or the agents root root-owned on a fresh host

hive-priv/src/storage.rs

@@ -244,17 +244,31 @@ async fn ensure_agent_subvolume(
     let root = env.state_root;
     let agent_root = root.join(agent_name);
 
-    // Progressive: existing agents (plain dir OR already a subvol) are left
-    // untouched — never auto-migrated.
+    // Progressive: existing agents (plain dir OR already a subvol) are never
+    // auto-migrated. The one exception is an agent dir this op itself left
+    // root-owned on a fresh host: hive-c0re cannot create below it.
     if agent_root.exists() {
+        if let (Ok(agent), Ok(root_meta)) = (
+            std::fs::symlink_metadata(&agent_root),
+            std::fs::metadata(root),
+        ) && is_stranded_root_dir(agent.uid(), root_meta.uid())
+        {
+            std::os::unix::fs::lchown(&agent_root, Some(root_meta.uid()), Some(root_meta.gid()))
+                .with_context(|| {
+                    format!(
+                        "chown stranded {} to agents-root owner",
+                        agent_root.display()
+                    )
+                })?;
+            tracing::warn!(agent = %agent_name, path = %agent_root.display(), "handed a root-owned agent dir back to the agents-root owner");
+        }
         return Ok((String::new(), String::new()));
     }
 
     // Only btrfs supports subvolumes; on anything else hive-c0re's normal
     // `create_dir_all` makes a plain dir (the pre-subvolume behaviour). The
     // parent must exist for both statfs and `btrfs subvolume create`.
-    std::fs::create_dir_all(root)
-        .with_context(|| format!("create agents root {}", root.display()))?;
+    ensure_agents_root(root)?;
     if !env.probes.on_btrfs(root)? {
         return Ok((String::new(), String::new()));
     }
@@ -306,6 +320,30 @@ async fn ensure_agent_subvolume(
     Ok((String::new(), String::new()))
 }
 
+/// Make the agents root if it is missing. Made by root here, so it is handed
+/// to the owner of its parent (hive-core's state dir); the subvolume takes the
+/// root's owner, and a root-owned root would strand every agent dir under it.
+/// One made meanwhile by the tmpfiles rule is left as that rule made it.
+fn ensure_agents_root(root: &Path) -> Result<()> {
+    use std::os::unix::fs::MetadataExt as _;
+
+    match std::fs::create_dir(root) {
+        Ok(()) => {}
+        Err(e) if e.kind() == std::io::ErrorKind::AlreadyExists => return Ok(()),
+        Err(e) => return Err(e).with_context(|| format!("create agents root {}", root.display())),
+    }
+    let parent = std::fs::metadata(root.parent().unwrap_or(root))
+        .with_context(|| format!("stat parent of {}", root.display()))?;
+    std::os::unix::fs::chown(root, Some(parent.uid()), Some(parent.gid()))
+        .with_context(|| format!("chown agents root {} to its parent's owner", root.display()))
+}
+
+/// An agent dir owned by root under an agents root that is not: hive-c0re
+/// (hive-core) cannot create `claude/` or `state/` in it.
+fn is_stranded_root_dir(agent_uid: u32, root_uid: u32) -> bool {
+    agent_uid == 0 && root_uid != 0
+}
+
 /// `DeleteAgentSubvolume` — delete the agent's state root iff it's a btrfs
 /// subvolume (purge path only). No-op for plain dirs / missing paths;
 /// hive-c0re's own `remove_dir_all` covers those. See the wire doc.
@@ -1506,6 +1544,37 @@ esac"#;
         assert!(!fx.agent("iris").exists());
     }
 
+    #[test]
+    fn the_agents_root_is_made_once_and_a_racing_creator_is_not_an_error() {
+        use super::ensure_agents_root;
+        use std::os::unix::fs::MetadataExt as _;
+        let fx = Fx::new();
+        let root = fx.state.join("made");
+        ensure_agents_root(&root).unwrap();
+        let (dir, parent) = (
+            std::fs::metadata(&root).unwrap(),
+            std::fs::metadata(&fx.state).unwrap(),
+        );
+        assert_eq!((dir.uid(), dir.gid()), (parent.uid(), parent.gid()));
+        ensure_agents_root(&root).unwrap();
+        assert!(ensure_agents_root(&root.join("no/parent")).is_err());
+    }
+
+    #[test]
+    fn only_a_root_owned_dir_under_a_non_root_root_is_stranded() {
+        use super::is_stranded_root_dir;
+        assert!(is_stranded_root_dir(0, 998));
+        assert!(
+            !is_stranded_root_dir(0, 0),
+            "a root-run host is not stranded"
+        );
+        assert!(!is_stranded_root_dir(998, 998));
+        assert!(
+            !is_stranded_root_dir(1000, 998),
+            "never touch an agent-owned dir"
+        );
+    }
+
     #[tokio::test]
     async fn ensure_creates_the_subvolume_owned_like_the_root() {
         use std::os::unix::fs::MetadataExt as _;