Fixes to delete algorithm.

python/repeated.c

@@ -354,11 +354,18 @@ static int PyUpb_RepeatedContainer_DeleteSubscript(upb_array* arr,
     step = -step;
   }
   size_t dst = start;
-  size_t src = start + 1;
-  for (Py_ssize_t i = 0; i < count; i++, dst += step, src += step + 1) {
-    upb_array_move(arr, dst, src, step);
+  size_t src = step ? start + 1 : start + count;
+  if (step) {
+    // Move elements between steps.
+    for (Py_ssize_t i = 0; i < count; i++, dst += step, src += step + 1) {
+      upb_array_move(arr, dst, src, step);
+    }
   }
-  upb_array_resize(arr, upb_array_size(arr) - count, NULL);
+  // Move tail.
+  size_t tail = upb_array_size(arr) - src;
+  assert(dst + tail == upb_array_size(arr) - count);
+  upb_array_move(arr, dst, src, upb_array_size(arr) - src);
+  upb_array_resize(arr, dst + tail, NULL);
   return 0;
 }
 

upb/reflection.c

@@ -373,7 +373,7 @@ bool upb_array_append(upb_array *arr, upb_msgval val, upb_arena *arena) {
   return true;
 }
 
-bool upb_array_move(upb_array* arr, size_t dst_idx, size_t src_idx,
+void upb_array_move(upb_array* arr, size_t dst_idx, size_t src_idx,
                     size_t count) {
   char* data = _upb_array_ptr(arr);
   int lg2 = arr->data & 7;

upb/reflection.h

@@ -136,7 +136,7 @@ bool upb_array_append(upb_array *array, upb_msgval val, upb_arena *arena);
 
 /* Moves elements within the array using memmove(). Like memmove(), the source
  * and destination elements may be overlapping. */
-bool upb_array_move(upb_array* array, size_t dst_idx, size_t src_idx,
+void upb_array_move(upb_array* array, size_t dst_idx, size_t src_idx,
                     size_t count);
 
 /* Inserts one or more empty elements into the array.  Existing elements are