Deprecate logged-in-user magic from permission checks

[juho-jaakkola]

Most of our permission checks default to the logged in user in case a user is not explicitly passed into the permission check functions.

Deprecating this feature will:

• Make permission checks safer as it forces devs to think what they're doing instead of relying on magic
• Reduce the complexity of core

[juho-jaakkola]

We might also type hint ElggUser to further reduce boilerplate.

[mrclay]

Transition might be tricky. We'd have to leave the default as 0 and interpret null as logged out, which might present a BC break.

[juho-jaakkola]

I forgot that sometimes we may need to check also non-logged-in users. Could an empty ElggEntity object represent a non-logged in user?

$container->canWriteToContainer(new ElggUser());

[mrclay]

A long time ago I proposed a UserState value object with just a few methods (getUser, getGuid, isLoggedIn). The value is in eliminating ambiguity. null means you don't have it, and the API can fetch it for you. If you pass it in, the API knows you intend for that state to be used.

Using a new ElggUser to mean logged out user I think would be dangerous as it leaked into other areas of the API. E.g. so many places we use a boolean check to see if there's an object or falsey.

Something like this would be BC safe:

function canEdit($user = ElggUser::NOT_GIVEN) {
    // here we can safely distinguish between these values:
    // ElggUser = logged in
    // null = logged out
    // ElggUser::NOT_GIVEN ('NOT_GIVEN') = pull it from session
    // anything else = throw exception
}

[mrclay]

Eh, that's no good either, same problem as using 0.

[mrclay]

Using entityTable->getUserForPermissionsCheck() internally, this seems no longer a big problem. If it's really worth breaking a ton of API to be more strict, we can use something like this, which can unambiguously represent a logged out user.

[jdalsem]

Going to leave this as is

[hypeJunction]

I don't think we should leave it as is

[jdalsem]

for now we will... both ideas were not solving the issue. Can reopen this when there is something better