Fix list_copy_head() with empty Lists

list_copy_head() given an empty List would crash from trying to
dereference the List to obtain its length.  Since NIL is how we represent
an empty List, we should just be returning another empty List in this
case.

list_copy_head() is new to v16, so let's fix it now before too many people
start coding around the buggy NIL behavior.

Reported-by: Miroslav Bendik
Discussion: https://postgr.es/m/CAPoEpV02WhawuWnmnKet6BqU63bEu7oec0pJc=nKMtPsHMzTXQ@mail.gmail.com

diff --git a/src/backend/nodes/list.c b/src/backend/nodes/list.c
index a709d23ef1c6fd3da9fc426b3140bc31a8c86fc7..750ee5a7e559eb2a8540cc935e89ac3d14cb86dd 100644 (file)
--- a/src/backend/nodes/list.c
+++ b/src/backend/nodes/list.c
@@ -1593,11 +1593,11 @@ list_copy_head(const List *oldlist, int len)
 {
        List       *newlist;
 
-       len = Min(oldlist->length, len);
-
-       if (len <= 0)
+       if (oldlist == NIL || len <= 0)
                return NIL;
 
+       len = Min(oldlist->length, len);
+
        newlist = new_list(oldlist->type, len);
        memcpy(newlist->elements, oldlist->elements, len * sizeof(ListCell));