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TestingAccessWrapper can't be used with methods that pass arguments by reference
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Description

TestingAccessWrapper can't be used with methods that pass arguments by reference. Attempting to do it causes a warning like "PHPUnit\Framework\Error\Warning: Parameter 1 to ... expected to be a reference, value given", and passes the parameter by value:

class A {
	private static function func( &$ref ) {
		$ref = 42;
	}
}

$a = Wikimedia\TestingAccessWrapper::newFromClass( A::class );

$b = 0;
$a->func( $b );
echo $b;

Expected output: 42
Actual output:

Warning: Parameter 1 to A::func() expected to be a reference, value given in ...TestingAccessWrapper.php on line 72                                                                
0

Event Timeline

Funnily enough, the following works in PHP 5.3, but it is a syntax error in all later versions where call-time pass-by-reference has been removed:

$a->func( &$b );

https://3v4l.org/EOJ07

We noticed the issue in code review of https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DiscussionTools/+/704835. I think it's an unavoidable limitation of __call (per PHP documentation: "None of the arguments of these magic methods can be passed by reference."), but @DannyS712 asked me to file a bug.


Using ReflectionClass / ReflectionMethod directly, we get the same error when doing the following:

$class = new ReflectionClass( A::class );
$func = $class->getMethod( 'func' );
$func->setAccessible( true );
$func->invokeArgs( null, [ $b ] );
echo $b;

…while the following works correctly:

$func->invokeArgs( null, [ &$b ] );

But even if you could determine at runtime which arguments need to be passed as references (seems tricky, ReflectionMethod doesn't seem to have a way to do it; you can maybe parse the output of __toString(), but it's probably a bad idea), you still can't make the arguments of the __call function behave like references.

I tried testing this out, and while you can pass references to the invokeArgs method, the testing access wrapper works by using a __call handler, and the parameters to that are never passed by reference (https://www.php.net/manual/en/language.oop5.overloading.php as noted above)

Perhaps we should add a functionality to TestingAccessWrapper to support calling method that require passing by reference, by using a dedicated method instead of __call ?

$obj = TestingAccessWrapper::newFromClass( Demo::class );
$obj->_callWithReference( 'targetMethod', [ $a, &$b ] );

would invoke Demo::targetMethod with $a and &$b as the parameters - its a bit uglier than how we using TestingAccessWrapper now, where we just call the desired private method, but it should support references.

This would also be useful in core[1] where the failure of TestingAccessWrapper to handle static params was worked around by using ReflectionClass/ReflectionMethod directly.

[1] https://gerrit.wikimedia.org/g/mediawiki/core/+/9dd696882db5117040ce9b38eb235c33270c4407/tests/phpunit/includes/session/SessionManagerTest.php#913