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Reduce memory usage when unserializing packed arrays of size 1 #7691

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Reduce memory usage when unserializing packed arrays of size 1 #7691

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Reduce memory usage when unserializing packed arrays of size 1 
Previously, PHP's unserialize() would unconditionally convert a packed array to
an associative(hashed) array to solve the problem of `[0 => $v1, 'key' => $v2]`
starting out as a packed array but becoming an associative array,
causing the raw zval pointer to the value for $v1 to have changed.

- This can only happen when there's at least two elements, though.

Additionally, reduce memory usage when calling `__unserialize` with arrays of
size 0 or 1
(e.g. for user-defined functions that store the passed in array as a property)
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@TysonAndre
TysonAndre committed Nov 26, 2021
commit 0e9bce65e43bb2b813fa33ed45de7f64e2c88dcd
32 changes: 22 additions & 10 deletions ext/standard/var_unserializer.re
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Original file line number Diff line number Diff line change
Expand Up @@ -769,14 +769,22 @@ static inline int object_common(UNSERIALIZE_PARAMETER, zend_long elements, bool
return 0;
}

array_init_size(&ary, elements);
/* Avoid reallocation due to packed -> mixed conversion. */
zend_hash_real_init_mixed(Z_ARRVAL(ary));
if (!process_nested_array_data(UNSERIALIZE_PASSTHRU, Z_ARRVAL(ary), elements)) {
ZVAL_DEREF(rval);
GC_ADD_FLAGS(Z_OBJ_P(rval), IS_OBJ_DESTRUCTOR_CALLED);
zval_ptr_dtor(&ary);
return 0;
if (elements > 0) {
/* Avoid reallocation due to packed -> mixed conversion.
* Note that an array must have at least 2 elements in order for the packed -> mixed conversion to change zval locations.
*/
array_init_size(&ary, elements);
if (elements > 1) {
zend_hash_real_init_mixed(Z_ARRVAL(ary));
}
if (!process_nested_array_data(UNSERIALIZE_PASSTHRU, Z_ARRVAL(ary), elements)) {
ZVAL_DEREF(rval);
GC_ADD_FLAGS(Z_OBJ_P(rval), IS_OBJ_DESTRUCTOR_CALLED);
zval_ptr_dtor(&ary);
return 0;
}
} else {
ZVAL_EMPTY_ARRAY(&ary);
}

/* Delay __unserialize() call until end of serialization. We use two slots here to
Expand Down Expand Up @@ -1078,8 +1086,12 @@ use_double:
if (elements) {
array_init_size(rval, elements);
/* we can't convert from packed to hash during unserialization, because
reference to some zvals might be kept in var_hash (to support references) */
zend_hash_real_init_mixed(Z_ARRVAL_P(rval));
* references to some zvals might be kept in var_hash (to support references).
* This will only change zval addresses if there are at least 2 values
* (the value being referenced, and the value being added afterwards) */
if (elements > 1) {
zend_hash_real_init_mixed(Z_ARRVAL_P(rval));
}
} else {
ZVAL_EMPTY_ARRAY(rval);
return finish_nested_data(UNSERIALIZE_PASSTHRU);
Expand Down