ART: Fix lower bound search within a node's children

Previously when doing a lower bound search on a node where the prefix was equal
and there was no child with an equal key chunk, we would move to the first
child. The correct thing to do is to move to the first _larger_ child.

src/art/art.c

@@ -1458,8 +1458,9 @@ static bool art_node_iterator_lower_bound(const art_node_t *node,
             return art_iterator_move(iterator, true);
         }
         if (indexed_child.key_chunk > key_chunk) {
-            // Only larger children, return the first leaf from the node.
-            return art_node_init_iterator(node, iterator, true);
+            // Only larger children, return the first larger child.
+            art_iterator_down(iterator, inner_node, indexed_child.index);
+            return art_node_init_iterator(indexed_child.child, iterator, true);
         }
         // We found a child with an equal prefix.
         art_iterator_down(iterator, inner_node, indexed_child.index);

tests/art_unit.cpp

@@ -254,22 +254,40 @@ DEFINE_TEST(test_art_iterator_prev) {
 }
 
 DEFINE_TEST(test_art_iterator_lower_bound) {
-    std::vector<const char*> keys = {
-        "000001", "000002", "000003", "000004", "001005",
-    };
-    std::vector<Value> values = {{1}, {2}, {3}, {4}, {5}};
-    art_t art{NULL};
-    for (size_t i = 0; i < keys.size(); ++i) {
-        art_insert(&art, (art_key_chunk_t*)keys[i], &values[i]);
-    }
+    {
+        std::vector<const char*> keys = {
+            "000001", "000002", "000003", "000004", "001005",
+        };
+        std::vector<Value> values = {{1}, {2}, {3}, {4}, {5}};
+        art_t art{NULL};
+        for (size_t i = 0; i < keys.size(); ++i) {
+            art_insert(&art, (art_key_chunk_t*)keys[i], &values[i]);
+        }
 
-    art_iterator_t iterator = art_init_iterator(&art, true);
-    assert_true(art_iterator_lower_bound(&iterator, (art_key_chunk_t*)keys[2]));
-    assert_key_eq(iterator.key, (art_key_chunk_t*)keys[2]);
-    const char* key = "000005";
-    assert_true(art_iterator_lower_bound(&iterator, (art_key_chunk_t*)key));
-    assert_key_eq(iterator.key, (art_key_chunk_t*)keys[4]);
-    art_free(&art);
+        art_iterator_t iterator = art_init_iterator(&art, true);
+        assert_true(
+            art_iterator_lower_bound(&iterator, (art_key_chunk_t*)keys[2]));
+        assert_key_eq(iterator.key, (art_key_chunk_t*)keys[2]);
+        const char* key = "000005";
+        assert_true(art_iterator_lower_bound(&iterator, (art_key_chunk_t*)key));
+        assert_key_eq(iterator.key, (art_key_chunk_t*)keys[4]);
+        art_free(&art);
+    }
+    {
+        // Lower bound search within a node's children.
+        std::vector<const char*> keys = {"000001", "000003", "000004",
+                                         "001005"};
+        std::vector<Value> values = {{1}, {3}, {4}, {5}};
+        art_t art{NULL};
+        for (size_t i = 0; i < keys.size(); ++i) {
+            art_insert(&art, (art_key_chunk_t*)keys[i], &values[i]);
+        }
+        art_iterator_t iterator = art_init_iterator(&art, true);
+        const char* key = "000002";
+        assert_true(art_iterator_lower_bound(&iterator, (art_key_chunk_t*)key));
+        assert_key_eq(iterator.key, (art_key_chunk_t*)keys[1]);
+        art_free(&art);
+    }
 }
 
 DEFINE_TEST(test_art_lower_bound) {