diff --git a/string-list.c b/string-list.c
index 5f5b60fe1c..0f8ac117fd 100644
--- a/string-list.c
+++ b/string-list.c
@@ -203,6 +203,15 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
 	list->nr = list->alloc = 0;
 }
 
+void string_list_setlen(struct string_list *list, size_t nr)
+{
+	if (list->strdup_strings)
+		BUG("cannot setlen a string_list which owns its entries");
+	if (nr > list->nr)
+		BUG("cannot grow a string_list with setlen");
+	list->nr = nr;
+}
+
 struct string_list_item *string_list_append_nodup(struct string_list *list,
 						  char *string)
 {
diff --git a/string-list.h b/string-list.h
index 77854840f7..122b318641 100644
--- a/string-list.h
+++ b/string-list.h
@@ -134,6 +134,16 @@ typedef void (*string_list_clear_func_t)(void *p, const char *str);
 /** Call a custom clear function on each util pointer */
 void string_list_clear_func(struct string_list *list, string_list_clear_func_t clearfunc);
 
+/*
+ * Set the length of a string_list to `nr`, provided that (a) `list`
+ * does not own its own storage, and (b) that `nr` is no larger than
+ * `list->nr`.
+ *
+ * Useful when "shrinking" `list` to write over existing entries that
+ * are no longer used without reallocating.
+ */
+void string_list_setlen(struct string_list *list, size_t nr);
+
 /**
  * Apply `func` to each item. If `func` returns nonzero, the
  * iteration aborts and the return value is propagated.