MDEV-26637: (hash) ASAN: main.metadata and user_variables.basic MTR failures after MDEV-26572

Explicitly devide two function of 0 length in the hash keys comparing.

Author: sanja-byelkin

mysys/hash.c

@@ -282,6 +282,8 @@ uchar* my_hash_first_from_hash_value(const HASH *hash,
  uint flag= 1;
  uint idx= my_hash_mask(hash_value,
  hash->blength, hash->records);
+ if (!length)
+ length= hash->key_length; // length for fixed length keys or 0
  do
  {
  pos= dynamic_element(&hash->array,idx,HASH_LINK*);
@@ -316,6 +318,8 @@ uchar* my_hash_next(const HASH *hash, const uchar *key, size_t length,
  if (*current_record != NO_RECORD)
  {
  HASH_LINK *data=dynamic_element(&hash->array,0,HASH_LINK*);
+ if (!length)
+ length= hash->key_length; // length for fixed length keys or 0
  for (idx=data[*current_record].next; idx != NO_RECORD ; idx=pos->next)
  {
  pos=data+idx;
@@ -356,8 +360,11 @@ static void movelink(HASH_LINK *array,uint find,uint next_link,uint newlink)
  length length of key
 
  NOTES:
- If length is 0, comparison is done using the length of the
- record being compared against.
+ length equal 0 can mean 2 things:
+ 1) it is fixed key length hash (HASH::key_length != 0) and
+ default length should be taken in this case
+ 2) it is really 0 length key for variable key length hash
+ (HASH::key_length == 0)
 
  RETURN
  = 0 key of record == key
@@ -368,10 +375,11 @@ static int hashcmp(const HASH *hash, HASH_LINK *pos, const uchar *key,
  size_t length)
 {
  size_t rec_keylength;
- uchar *rec_key= (uchar*) my_hash_key(hash, pos->data, &rec_keylength, 1);
- return ((length && length != rec_keylength) ||
+ uchar *rec_key;
+ rec_key= (uchar*) my_hash_key(hash, pos->data, &rec_keylength, 1);
+ return (length != rec_keylength) ||
  my_strnncoll(hash->charset, (uchar*) rec_key, rec_keylength,
- (uchar*) key, rec_keylength));
+ (uchar*) key, rec_keylength);
 }