vmm: Create breadth-first search iterator for the DeviceTree

This iterator will let the VMM enumerate the resources associated
with the DeviceManager, allowing for introspection.

Moreover, by implementing a double ended iterator, we can get the
hierarchy from the leaves to the root of the tree, which is very
helpful in the context of restoring the devices in the right order.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/device_manager.rs

@@ -604,6 +604,68 @@ impl DeviceTree {
     fn iter(&self) -> std::collections::hash_map::Iter<String, DeviceNode> {
         self.0.iter()
     }
+    fn breadth_first_traversal(&self) -> BftIter {
+        BftIter::new(&self.0)
+    }
+}
+
+// Breadth first traversal iterator.
+struct BftIter<'a> {
+    nodes: Vec<&'a DeviceNode>,
+}
+
+impl<'a> BftIter<'a> {
+    fn new(hash_map: &'a HashMap<String, DeviceNode>) -> Self {
+        let mut nodes = Vec::new();
+
+        for (_, node) in hash_map.iter() {
+            if node.parent.is_none() {
+                nodes.push(node);
+            }
+        }
+
+        let mut node_layer = nodes.as_slice();
+        loop {
+            let mut next_node_layer = Vec::new();
+
+            for node in node_layer.iter() {
+                for child_node_id in node.children.iter() {
+                    if let Some(child_node) = hash_map.get(child_node_id) {
+                        next_node_layer.push(child_node);
+                    }
+                }
+            }
+
+            if next_node_layer.is_empty() {
+                break;
+            }
+
+            let pos = nodes.len();
+            nodes.extend(next_node_layer);
+
+            node_layer = &nodes[pos..];
+        }
+
+        BftIter { nodes }
+    }
+}
+
+impl<'a> Iterator for BftIter<'a> {
+    type Item = &'a DeviceNode;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.nodes.is_empty() {
+            None
+        } else {
+            Some(self.nodes.remove(0))
+        }
+    }
+}
+
+impl<'a> DoubleEndedIterator for BftIter<'a> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.nodes.pop()
+    }
 }
 
 #[derive(Serialize, Deserialize)]
@@ -3159,35 +3221,17 @@ impl Snapshottable for DeviceManager {
         // It's important to restore devices in the right order, that's why
         // the device tree is the right way to ensure we restore a child before
         // its parent node.
-        for (id, node) in self.device_tree.iter() {
-            if !node.children.is_empty() {
-                continue;
-            }
-
+        for node in self.device_tree.breadth_first_traversal().rev() {
             // Restore the node
             if let Some(migratable) = &node.migratable {
-                debug!("Restoring {} from DeviceManager", id);
-                if let Some(snapshot) = snapshot.snapshots.get(id) {
+                debug!("Restoring {} from DeviceManager", node.id);
+                if let Some(snapshot) = snapshot.snapshots.get(&node.id) {
                     migratable.lock().unwrap().restore(*snapshot.clone())?;
                 } else {
-                    return Err(MigratableError::Restore(anyhow!("Missing device {}", id)));
-                }
-            }
-
-            // Restore the parent if it exists
-            if let Some(parent) = &node.parent {
-                if let Some(node) = self.device_tree.get(parent) {
-                    if let Some(migratable) = &node.migratable {
-                        debug!("Restoring {} from DeviceManager", parent);
-                        if let Some(snapshot) = snapshot.snapshots.get(parent) {
-                            migratable.lock().unwrap().restore(*snapshot.clone())?;
-                        } else {
-                            return Err(MigratableError::Restore(anyhow!(
-                                "Missing parent device {}",
-                                id
-                            )));
-                        }
-                    }
+                    return Err(MigratableError::Restore(anyhow!(
+                        "Missing device {}",
+                        node.id
+                    )));
                 }
             }
         }