Mark sessions empty on final client removal

src/bin/dosh-server.rs

@@ -719,7 +719,11 @@ impl ServerState {
         if let Some(session_name) = self.client_index.remove(client_id)
             && let Some(session) = self.sessions.get_mut(&session_name)
         {
-            return session.clients.remove(client_id).is_some();
+            let removed = session.clients.remove(client_id).is_some();
+            if removed && session.clients.is_empty() {
+                session.empty_since.get_or_insert_with(Instant::now);
+            }
+            return removed;
         }
         false
     }
@@ -4536,6 +4540,15 @@ mod tests {
         assert!(state.lookup_client(&client_id).is_none());
         assert!(state.client_mut(&client_id).is_none());
         assert!(!state.sessions["work"].clients.contains_key(&client_id));
+        assert!(
+            state.sessions["work"].empty_since.is_none(),
+            "removing one of multiple clients must not mark the session empty"
+        );
+        assert!(state.remove_client_everywhere(&[7u8; 16]));
+        assert!(
+            state.sessions["work"].empty_since.is_some(),
+            "removing the final client should start empty-session cleanup grace immediately"
+        );
         // Removing an absent client is a no-op.
         assert!(!state.remove_client_everywhere(&client_id));
     }