refactor: throw when data provider returns more items than requested

Author: vursen

flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalDataCommunicator.java

@@ -259,7 +259,8 @@ public void refresh(T item, boolean refreshChildren) {
 
     @Override
     public Stream<T> fetchFromProvider(int offset, int limit) {
-        return fetchDataProviderChildren(null, Range.withLength(offset, limit));
+        return fetchDataProviderChildren(null, Range.withLength(offset, limit))
+                .stream();
     }
 
     @Override
@@ -516,8 +517,7 @@ && isExpanded(item)) {
     private void preloadRange(Cache<T> cache, int start, int length) {
         var range = Range.withLength(start, length)
                 .restrictTo(Range.withLength(0, cache.getSize()));
-        var items = fetchDataProviderChildren(cache.getParentItem(), range)
-                .toList();
+        var items = fetchDataProviderChildren(cache.getParentItem(), range);
         cache.setItems(range.getStart(), items);
     }
 
@@ -751,16 +751,21 @@ public HierarchicalQuery<T, Object> buildQuery(T parent, int offset,
     }
 
     @SuppressWarnings("unchecked")
-    private Stream<T> fetchDataProviderChildren(T parent, Range range) {
+    private List<T> fetchDataProviderChildren(T parent, Range range) {
         var query = buildQuery(parent, range.getStart(), range.length());
 
-        return ((HierarchicalDataProvider<T, Object>) getDataProvider())
-                .fetchChildren(query).peek((item) -> {
-                    if (item == null) {
-                        throw new IllegalStateException(
-                                "Data provider returned a null item. Null values are not supported");
-                    }
-                });
+        var items = ((HierarchicalDataProvider<T, Object>) getDataProvider())
+                .fetchChildren(query).toList();
+        if (items.size() > range.length()) {
+            throw new IllegalStateException(
+                    "Data provider returned more items than requested. Requested %d but got %d."
+                            .formatted(range.length(), items.size()));
+        }
+        if (items.contains(null)) {
+            throw new IllegalStateException(
+                    "Data provider returned a null item. Null values are not supported");
+        }
+        return items;
     }
 
     @SuppressWarnings("unchecked")