Good Morning, PHP Internals Folks!
I have a design question about caching of IS_REFERENCE zval values, and want to
understand the most correct way to implement a memory Cache for zvals in PHP7.
Background:
-----------
WinCache's user cache does a 'deep copy' of zvals into a shared memory segment
that can start at different offsets in the processes which share the memory
segment. This means that all internal pointers must be converted into offsets
relative to the starting address of the shared memory segment. Further, flags
are altered such that the cached zval is considered immutable and not ref
counted.
So, for example, with an IS_STRING zval, a zval in shared memory is created
(zv_cached), the zend_string is cached, and then the Z_STR_P(zv_cached) is set
to the offset of the cached zend_string.
When the user fetches the cached IS_STRING zval, the reverse is performed: a
zval is created in request-temporary memory (zv_fetched), and initialized from
zv_cached. Z_STR_P(zv_fetched) is translated back into a pointer (to the
cached zend_string) which is correct in the current process. The caller won't
attempt to free or modify the cached zend_string since the type flags were
modified when it was cached.
For IS_REFERENCE objects, I'm not clear on what I should do. Should I cache the
zend_reference struct, with its included zval? Or should I "flatten" the
IS_REFERENCE, and just convert it to the zval that's in the zend_reference
struct?
Consider the following:
<?php
$bar = "BAR";
$arr2 = array('black' => &$bar, 'black2' => &$bar );
wincache_ucache_add('foo', $arr2);
$fetched_arr = wincache_ucache_get('foo');
?>
The $fetched_arr won't have the internal references between elements (e.g.
$fetched_arr['black'] and $fetched_arr['black2'] won't point at the same actual
value).
Questions:
----------
* How should one persist/intern an IS_REFERENCE zval?
* Is it acceptable to "flatten" IS_REFERENCE zvals when persisting/interning
them? If not, what's the reason for not doing so?
* If IS_REFERENCE zvals have to be maintained, does that mean that the
zend_reference will need to be copied out of the shared memory cache each time
it's fetched?
* Is there an example of a PHP7 extension which is doing something similar?
APC doesn't appear to have been updated for PHP7.
Thx!
--E.
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