Add support for encoding/decoding XPC connections

[amomchilov]

Add support for encoding/decoding XPC connections

[amomchilov]

Some observations:

• You can't retroactively add protocol conformances to xpc_object_t (and its variants):

  // error: extension of protocol 'OS_xpc_object' cannot have an inheritance clause
  extension xpc_connection_t: Codable {
  
  }

• You can use a property wrapper (which itself can be codable, even if its wrappedValue is not).

The only solution approaches I can think of involve wrapping the xpc_connection_t (or xpc_endpoint_t; I propose we make both codable). Either explicitly by the user, or implicitly via a property wrapper. I don't see any down-side to the latter over the former, so I propose we go with a property wrapper.

I see two ways we can make this work:

1. "Fake" the implementation of Codable on the property wrapper
   • Make the init(from:) and encode(to:) call fatalError().
     • The message should inform users that these values can only ever be encoded/decoded by the SecureXPC library.
   • Add special-cases to the XPCEncoder/XPCDecoder (well really, their containers), such that they intercept these values and never actually call their init(from:) or encode(to:) methods.
   • I haven't had time to spike this one out yet
2. Conform to Codable for real
   • Make the init(from:) and encode(to:) cast their Encoder/Decoder param to a concrete XPCEncoderImpl/XPCDecoderImpl. If the cast fails, fatalError() out.
     • Again, the message should inform users that these values can only ever be encoded/decoded by the SecureXPC library.
   • Make a single value container (in either case), and down cast to the XPCSingleValueEncodingContainer/XPCSingleValueDecodingContainer (this is safe to force-cast)
   • Don't special case anything. Let the default implementation of Codable take its course. init(from:) and encode(to:) will be called like normal.
   • Add methods to XPCEncoder/XPCDecoder (their containers) that handle xpc_connection_t and xpc_endpoint_t. Use these to implement init(from:) and encode(to:)
   • I made a proof of concept of this for xpc_connection_t, here: #13