HASH_FLAG_ALLOW_COW_VIOLATION is not preserved by zend_hash_real_init_(mixed|packed)_ex()

[ju1ius]

Description

Hi,

Currently the zend_hash_real_init_mixed_ex and zend_hash_real_init_packed_ex functions overwrite the hashtable flags which causes the following example to fail:

// allocate a hashtable 
HashTable *ht = (HashTable*) emalloc(sizeof(HashTable));
_zend_hash_init(ht, HT_MIN_SIZE, NULL, false);
// then sometime later:
{
  GC_ADDREF(ht);
  HT_ALLOW_COW_VIOLATION(ht);

  zval *value;
  ZVAL_LONG(&value, 1);
  // HT_ASSERT_RC1(ht) succeeds here because HASH_FLAG_ALLOW_COW_VIOLATION is set.
  // However, since the hashtable is uninitialized at this point,
  // zend_hash_real_init_packed_ex() is called after the check, which unsets the flag.
  zend_hash_next_index_insert(ht, value);

  ZVAL_LONG(&value, 2);
  // HT_ASSERT_RC1(ht) fails here because HASH_FLAG_ALLOW_COW_VIOLATION
  // was cleared in the previous step.
  zend_hash_next_index_insert(ht, value);

  GC_DELREF(ht);
}

Would you consider a PR that preserves this flag through the initialization process ?

Thanks.

PHP Version

PHP 8.3-dev

Operating System

irrelevant

[SVGAnimate]

Hello,

It seems to me that your code is wrong.

  zval *value;
  ZVAL_LONG(value, 1);

consider :

  zval value;
  ZVAL_LONG(&value, 1);

Can you check on your side?

[ju1ius]

  zval value;
  ZVAL_LONG(&value, 1);

@SVGAnimate Yes, of course, thanks for fixing the typo. That is not the meat of the issue however. 😉

[iluuu1994]

What's your use-case? HT_ALLOW_COW_VIOLATION is used only in a very few distinct cases.

• The GC when destroying arrays that are part of a cycle.
• In php_auto_globals_create_server for PG(http_globals)[TRACK_VARS_SERVER] because it is modified in place.
• In unserialize with self-references.

HT_ALLOW_COW_VIOLATION is only used for debug builds and likely more of a workaround than something encouraged to be used. It's generally preferable not to violate RC-rules.

[bwoebi]

@iluuu1994 I recently encountered the very same issue while writing code, where I had to ensure the flag exists before every single insertion in order to ensure that my RC=2 array retains the flag, e.g. DataDog/dd-trace-php@b5d3b66 (probably could have called zend_hash_real_init manually too, but anyway...)

The use case in that code is having control over the destruction (zval_ptr_dtor will fail hard when called on a persistent array - but that's a separate issue).

This was quite ugly, so I'm totally agreeing with improving this here.

[ju1ius]

I'm closing the issue as WONTFIX.
See discussion in #13013.