Autoloading: Introduce compatibility shim for spl_autoload_register

[rmccue]

Split from #36335.

To ensure support for multiple autoloaders when SPL isn't available (5.2 with SPL disabled; 5.3+ require SPL), we should polyfill the autoloader registration code.

[rmccue]

SPL autoload compatibility shim/polyfill

[rmccue]

Initial patch to add to compat.php

[rmccue]

Remove spl_autoload_register check

[rmccue]

Added an actual patch for core; renamed from $spl_autoloaders to $_wp_spl_autoloaders to indicate that a) it's part of a shim and shouldn't be used, and b) to avoid any potential variable conflicts. While we could introduce a class with protected/private vars to actually protect it, that's too much overhead for just a shim, especially one that's only loaded on a small number of installs.

This doesn't really introduce any new stuff into core, apart from the compatibility shim for 5.2, so it should be pretty uncontroversial. Core does actually use this with the introduction of Requests, plus we can remove the check in the SimplePie loader (see 36926.2.diff​). Based on this, I'd like to get it into core this cycle, regardless of progress on #36335 (although I hope we can get that in too!).

[tfrommen]

Hey Ryan,

since you copied stuff from the Autoloader ticket, I do that as well. ;)

We actually spoke about it, but it hasn't made it into your code yet.

Improve __autoload()
$spl_autoloaders is global, so anyone can put into it whatever they like. So we should check for is_callable for each autoloader function inside __autoload(), and continue if it isn't.

Improve spl_autoload_functions()
Since $spl_autoloaders is, again, a global, and since we don't want to argue with SPL, we should make sure we're returning an array.

<?php

function spl_autoload_functions() {
        global $spl_autoloaders;
        
        return is_array( $spl_autoloaders ) ? $spl_autoloaders : array();
}

[dd32]

@rmccue 36926.2.diff​ seems pretty sane to me.
The obvious breakage is when something else is defining __autoload(), but I think that's something we're just going to have to expect - there's no way to polyfill it in those cases.

Replying to tfrommen:

Improve __autoload()
$spl_autoloaders is global, so anyone can put into it whatever they like. So we should check for is_callable for each autoloader function inside __autoload(), and continue if it isn't.

Improve spl_autoload_functions()
Since $spl_autoloaders is, again, a global, and since we don't want to argue with SPL, we should make sure we're returning an array.

Both of these seem like needless checks in this case, if outside code modifies the global their warranty is void, just as if they modify the global $wp_filters arrays, or almost any other global in WordPress for that matter.

[rmccue]

Replying to tfrommen:

We actually spoke about it, but it hasn't made it into your code yet.

Oops, sorry about that! Forgot to incorporate it here.

While I agree with @dd32 that your warranty is void if you do that, I think it makes sense to be defensive here. As it turns out, ​PHP throws an exception if the callback can't be found, so we should match that. The array check is also cheap, so makes sense to do that.

[dd32]

Replying to rmccue:

As it turns out, ​PHP throws an exception if the callback can't be found, so we should match that.

You already support that :) PHP doesn't throw an exception when it's calling the autoloaders (​https://3v4l.org/Fj7I1), only when you first try to register it.

[rmccue]

Replying to dd32:

You already support that :) PHP doesn't throw an exception when it's calling the autoloaders (​https://3v4l.org/Fj7I1), only when you first try to register it.

Oh you're totally right, I misread the exception! However, the current patch will throw a warning from call_user_func, which doesn't match the PHP implementation.

[rmccue]

Add is_callable check

[rmccue]

Added an is_callable check to __autoload to ensure we don't generate excess warnings. I didn't add the is_array check, because a) if you directly change that, you've voided your warranty, and b) someone might want to replace it with an array-like object or something, and I don't think there's a need to be super strict.

While this isn't absolutely optimal (I'd be concerned about performance with is_callable usually), this is only a shim for 5.2, so I don't think performance matters. If you need more performant code, use 5.6+ (ideally, PHP 7).

I think this is ready for commit.

[dd32]

Looks good to me, commit it.

[rmccue]

In 37636:

Autoload: Introduce shim for SPL autoloading.

For PHP 5.2, SPL can be disabled. As SPL provides the support for multiple autoloaders, this needs to be shimmed if not available.

Fixes #36926.

[rmccue]

In 37637:

Autoload: Add missed @since tags to SPL shim.

See #36926.