sum_tree: fix SumTree::push to merge into existing leaves (APP-5507 memory fix)

crates/sum_tree/src/lib.rs

@@ -153,13 +153,13 @@ impl<T: Item> SumTree<T> {
 
     pub fn push(&mut self, item: T) {
         let summary = item.summary();
-        self.push_tree(SumTree::from_child_trees(vec![SumTree(Arc::new(
-            Node::Leaf {
-                summary: summary.clone(),
-                items: ArrayVec::from_iter(Some(item)),
-                item_summaries: ArrayVec::from_iter(Some(summary)),
-            },
-        ))]))
+        // An unwrapped single-item leaf is always underflowing, so `push_tree` descends into
+        // and tops up the last leaf that has room instead of appending a singleton sibling.
+        self.push_tree(SumTree(Arc::new(Node::Leaf {
+            summary: summary.clone(),
+            items: ArrayVec::from_iter(Some(item)),
+            item_summaries: ArrayVec::from_iter(Some(summary)),
+        })))
     }
 
     pub fn push_tree(&mut self, other: Self) {

crates/sum_tree/src/lib_tests.rs

@@ -14,6 +14,39 @@ fn test_extend_and_push_tree() {
     assert_eq!(tree1.items(), (0..20).chain(50..100).collect::<Vec<u8>>());
 }
 
+#[test]
+fn test_push_matches_extend_for_same_items() {
+    // TREE_BASE is 2 under the "test-util" feature (leaves hold up to 4 items), so 25 items
+    // forces multiple levels of the tree and several leaf splits.
+    let items: Vec<u8> = (0..25).collect();
+
+    let mut pushed = SumTree::new();
+    for item in items.iter().copied() {
+        pushed.push(item);
+    }
+
+    let mut extended = SumTree::new();
+    extended.extend(items.clone());
+
+    assert_eq!(pushed.items(), items);
+    assert_eq!(pushed.items(), extended.items());
+}
+
+#[test]
+fn test_repeated_push_packs_leaves_as_densely_as_extend() {
+    let items: Vec<u8> = (0..25).collect();
+
+    let mut pushed = SumTree::new();
+    for item in items.iter().copied() {
+        pushed.push(item);
+    }
+
+    let mut extended = SumTree::new();
+    extended.extend(items);
+
+    assert_eq!(pushed.0.height(), extended.0.height());
+}
+
 #[test]
 fn test_random() {
     for seed in 0..100 {