Confirm memcached is giving the correct response

[WPprodigy]

If you send an array, the memcache client will respond with an associative array: https://www.php.net/manual/en/memcache.get.php. With this, we can ensure that the memcached server is giving us the correct response.

This is just a proof of concept for now. Some further ideas include:

• Checking if it returns an array but with the wrong key, and log an error and retry once if so (helping keep tabs on the scale of the problem we're dealing with).
• Doing this in the memcache php client extension instead.

As a side note, we should really be making use of this functionality for the multi_get() logic that WP now uses - as it can greatly limit the # of roundtrips to the memcached server. But can come back to that another day :).

[rinatkhaziev]

isset is not reliable in this context because it'll be false if the value is NULL:

$keys = [
  'test' => null,
  'test2' => 'null',
  'test3' => 1,
];

foreach( $keys as $key => $val ) {
  var_dump( isset( $keys[ $key ] ) );
}
echo "\n";
foreach( $keys as $key => $val ) {
  var_dump(array_key_exists( $key , $keys ) );
}

=>

bool(false)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

array_key_exists would be safer.

[WPprodigy]

Documenting some of the edge-case behaviors:

## Non-existing keys

$flags = false;
$value = $memcache->get( KEY_THAT_DOES_NOT_EXIST, $flags );
var_dump( $flags, $value );
> bool(false) bool(false)

$flags = false;
$value = $memcache->get( [ KEY_THAT_DOES_NOT_EXIST ], $flags );
var_dump( $flags, $value );
> bool(false) array(0) { }

## Key with empty string value

$flags = false;
$value = $memcache->get( EMPTY_KEY, $flags );
var_dump( $flags, $value );
> int(0) string(0) ""

$flags = false;
$value = $memcache->get( [ EMPTY_KEY ], $flags );
var_dump( $flags, $value );
> array(1) { ["emptykey"]=> int(0) } array(1) { ["emptykey"]=> string(0) "" }

## Key with empty array value

$flags = false;
$value = $memcache->get( EMPTY_ARRAY_KEY, $flags );
var_dump( $flags, $value );
> int(1) array(0) { }

$flags = false;
$value = $memcache->get( [ EMPTY_ARRAY_KEY ], $flags );
var_dump( $flags, $value );
> array(1) { ["emptyarraykey"]=> int(1) } array(1) { ["emptyarraykey"]=> array(0) { } }

## Key with null value

$flags = false;
$value = $memcache->get( NULL_KEY, $flags );
var_dump( $flags, $value );
> int(1) bool(false)

$flags = false;
$value = $memcache->get( [ NULL_KEY ], $flags );
var_dump( $flags, $value );
> array(1) { ["nullkey"]=> int(1) } array(1) { ["nullkey"]=> NULL }

## Key with false value (same behavior with `true`, just flipped of course)

$flags = false;
$value = $memcache->get( FALSE_KEY, $flags );
var_dump( $flags, $value );
> int(256) bool(false)

$flags = false;
$value = $memcache->get( [ FALSE_KEY ], $flags );
var_dump( $flags, $value );
> array(1) { ["falsekey"]=> int(256) } array(1) { ["falsekey"]=> bool(false) }

## Key with normal string value

$flags = false;
$value = $memcache->get( CANARY_KEY, $flags );
var_dump( $flags, $value );
> int(0) string(6) "canary"

$flags = false;
$value = $memcache->get( [ CANARY_KEY ], $flags );
var_dump( $flags, $value );
> array(1) { ["canary"]=> int(0) } array(1) { ["canary"]=> string(6) "canary" }

• ✅ All that matters for $flags is that it returns false for "Non-existing keys" and for that only. So all good there .
• 🟥 NULL_KEY is a problem, as previously it would have just been seen as false but now it's NULL. Needs adjustment.
• ✅ The "Non-existing keys" differences are okay, as $found will cover that difference and set value to false anyway.
• ✅ Everything else ports over well.

[WPprodigy]

• Added logging so we can better track how often inconsistencies occurs.
• Handled the NULL edge case for backwards-compatibility.
• get_multi() didn't have a very accurate implementation for $found. Does now :)

The PR is a bit more complex now though, but some good news is that we can eventually deploy to object-cache-next.php first and conditionally test before a full rollout.

Side note: Held off on using PHP's new array destructuring & array_key_first(), as I'm not sure what minimum php version is being targeted here.

[sonarcloud]

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