hashmap: allow re-use after hashmap_free()

Previously, once map->table had been freed, any calls to hashmap_put(),
hashmap_get(), or hashmap_remove() would cause a NULL pointer
dereference (since hashmap_free_() also zeros the memory; without that
zeroing, calling these functions would cause a use-after-free problem).

Modify these functions to check for a NULL table and automatically
allocate as needed.

Also add a HASHMAP_INIT(fn, data) macro for initializing hashmaps on the
stack without calling hashmap_init().

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hashmap.c

@@ -114,6 +114,7 @@ int hashmap_bucket(const struct hashmap *map, unsigned int hash)
 
 static void rehash(struct hashmap *map, unsigned int newsize)
 {
+	/* map->table MUST NOT be NULL when this function is called */
 	unsigned int i, oldsize = map->tablesize;
 	struct hashmap_entry **oldtable = map->table;
 
@@ -134,6 +135,7 @@ static void rehash(struct hashmap *map, unsigned int newsize)
 static inline struct hashmap_entry **find_entry_ptr(const struct hashmap *map,
 		const struct hashmap_entry *key, const void *keydata)
 {
+	/* map->table MUST NOT be NULL when this function is called */
 	struct hashmap_entry **e = &map->table[bucket(map, key)];
 	while (*e && !entry_equals(map, *e, key, keydata))
 		e = &(*e)->next;
@@ -196,6 +198,8 @@ struct hashmap_entry *hashmap_get(const struct hashmap *map,
 				const struct hashmap_entry *key,
 				const void *keydata)
 {
+	if (!map->table)
+		return NULL;
 	return *find_entry_ptr(map, key, keydata);
 }
 
@@ -211,8 +215,12 @@ struct hashmap_entry *hashmap_get_next(const struct hashmap *map,
 
 void hashmap_add(struct hashmap *map, struct hashmap_entry *entry)
 {
-	unsigned int b = bucket(map, entry);
+	unsigned int b;
 
+	if (!map->table)
+		alloc_table(map, HASHMAP_INITIAL_SIZE);
+
+	b = bucket(map, entry);
 	/* add entry */
 	entry->next = map->table[b];
 	map->table[b] = entry;
@@ -230,7 +238,11 @@ struct hashmap_entry *hashmap_remove(struct hashmap *map,
 				     const void *keydata)
 {
 	struct hashmap_entry *old;
-	struct hashmap_entry **e = find_entry_ptr(map, key, keydata);
+	struct hashmap_entry **e;
+
+	if (!map->table)
+		return NULL;
+	e = find_entry_ptr(map, key, keydata);
 	if (!*e)
 		return NULL;
 

hashmap.h

@@ -210,6 +210,9 @@ struct hashmap {
 
 /* hashmap functions */
 
+#define HASHMAP_INIT(fn, data) { .cmpfn = fn, .cmpfn_data = data, \
+				 .do_count_items = 1 }
+
 /*
  * Initializes a hashmap structure.
  *