Verify that exporting a connection for a Mach service works correctly

[amomchilov]

https://github.com/trilemma-dev/SecureXPC/blob/ab4e612fd83ae3afef76e455831ae3ce9fe07f0d/Sources/SecureXPC/Server/XPCMachServer.swift#L204-L208

Fatal-error this code path to say that it's untested, and treat it as effectively not-yet-implemented.

...

Let's go with option 2 for now. I'd volunteer to try testing it out, but between end of year busyness at work and heading off on holiday soon I just won't have time before early January. Can revisit it then.

_Originally posted by @jakaplan in https://github.com/trilemma-dev/SecureXPC/pull/23#discussion_r769131546_

[jakaplan]

I've very briefly started looking into this, but haven't done any actual testing yet. I'd be rather surprised if what we currently have works in all cases as it's inconsistent:

• XPCAnonymousServer returns the listener connection, which absolutely works as demonstrated by our tests
• XPCMachServer returns the listener connection if the server has been started, otherwise fatalErrors out
  • I haven't tested this yet, but I can imagine it work as it's consistent with XPCAnonymousServer
  • It'd be good to redesign this so that the endpoint can always be retrieved, just like it can for XPCAnonymousServer
• XPCServiceServer returns the most recent client connection it's received (but in practice I think it'd only ever receive one?)
  • My hunch is this doesn't actually work, although I don't currently have an XPC Service implementation so it'll take me a bit to create a setup to test this out
  • There is no concept of a listener connection for an XPC Service; xpc_main is the one and only entry point

If it's true that there's no equivalent for XPC Services and therefore no way to return an XPCServerEndpoint, I think we could do one or both of the following:

• Move the endpoint property to NonBlockingStartable (and rename it)
• Have the start() function defined by NonBlockingStartable return the NonBlockingStartable
  • Only doing this might turn out to be inconvenient for anyone trying to pass around an endpoint for a Mach service

[jakaplan]

I've confirmed that exporting a connection for an XPCMachServer works correctly. So in terms of exactly this issue, I've put up PR #45 for this.

Since this does work and it differs from XPCServiceServer's behavior, educated guess that the endpoint for the XPCServiceServer doesn't work properly (and probably can't work). I've created issue #46 for this.