Add support for privileged and unprivileged connections

[alessandrogario]

This PR adds support for separating the interface into privileged and unprivileged, depending on the permissions of the connecting client

[russellhancox]

I think this property can be removed and the presence of unprivilegedExportedInterface can be used to infer this. I'll elaborate more below.

[russellhancox]

This interface is nice and clean but to avoid breaking upgrading clients, I'd like to leave the exportedInterface property around for a release or two. Could you add it back and mark it as deprecated, like so:

/**
  Old interface property, please update to use privilegedExportedInterface and/or unprivilegedExportedInterface instead.
*/
@property(retain, nullable) NSXPCInterface *exportedInterface __attribute__((
   deprecated(Use privilegedExportedInterface and / or unprivilegedExportedInterface instead.)));

[russellhancox]

I think this could be simpler and more explicit about which interface to export; if we remove the allowUnprivilegedClients property as I suggested above and require that the right interface is used for privileged/unprivileged clients, this can become:

NSXPCInterface *interface;
if ([connection effectiveUserIdentifier] == 0) {
  interface = self.privilegedInterface;
} else {
  interface = self.unprivilegedInterface;
}

// TODO(any): Remove 1-2 releases after exportedInterface was marked deprecated.
if (!interface) {
  interface = self.exportedInterface;
}

if (!interface) return NO;

.....

connection.exportedInterface = interface;

Clients which then want the same interface exported for both privileged and unprivileged clients can use:
conn.privilegedExportedInterface = conn.unprivilegedExportedInterface = myNewInterface;

And if the appropriate interface is unset (and assuming exportedInterface isn't being used) the connection will be rejected.

[alessandrogario]

Hello @russellhancox!

Thanks for reviewing the PR! It's a lot better now with your corrections 👍

I also updated the tests by adding a dummy protocol because we are now failing the connection if there is no interface set.

[russellhancox]

Thanks again!

[russellhancox]

This has been published in v1.2 and pushed to CocoaPods. A 'pod update' in dependent projects will pick this up.