hashmap: ensure hashmaps are reusable after hashmap_clear()

In the series merged at bf0a430 (Merge branch 'en/strmap',
2020-11-21), strmap was built on top of hashmap and hashmap was extended
in a few ways to support strmap and be more generally useful.  One of
the extensions was that hashmap_partial_clear() was introduced to allow
reuse of the hashmap without freeing the table.  Peff believed that it
also made sense to introduce a hashmap_clear() which freed everything
while allowing reuse.

I added hashmap_clear(), but in doing so, overlooked the fact that for
a hashmap to be reusable, it needs a defined cmpfn and data (the
HASHMAP_INIT macro requires these fields as parameters, for example).
So, if we want the hashmap to be reusable, we shouldn't zero out those
fields.  We probably also shouldn't zero out do_count_items.  (We could
zero out grow_at and shrink_at, but whether we zero those or not is
irrelevant as they'll be automatically updated whenever a new entry is
inserted.)

Since clearing is associated with freeing map->table, and the only thing
required for consistency after freeing map->table is zeroing tablesize
and private_size, let's only zero those fields out.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

hashmap.c

@@ -205,8 +205,9 @@ void hashmap_clear_(struct hashmap *map, ssize_t entry_offset)
 		return;
 	if (entry_offset >= 0)  /* called by hashmap_clear_and_free */
 		free_individual_entries(map, entry_offset);
-	free(map->table);
-	memset(map, 0, sizeof(*map));
+	FREE_AND_NULL(map->table);
+	map->tablesize = 0;
+	map->private_size = 0;
 }
 
 struct hashmap_entry *hashmap_get(const struct hashmap *map,