convert setPktInfo() to SocketOption

[swhitty]

There is currently an issue on the main branch where constructing a UDP socket with IP6 fails every time on both Darwin and Linux:

try Socket(domain: AF_INET6, type: .datagram)

Darin: SocketError.failed(type: "SetOption", errno: 13, message: "Permission denied")
Linux: SocketError.failed(type: "SetPktInfoOption", errno: 22, message: "Invalid argument")

While this PR does not fix the underlying issue, it moves the logic from init() so sockets can opt in to setting the packet info allowing users to sidestep the issue.

The private func setPktInfo() has now been removed completely as some recent refactoring allows these options to be set via the existing protocol SocketOption / setValue() API.

Users now enable the packet info options like so:

let socket = try Socket(domain: AF_INET, type: .datagram)
try socket.setValue(true, for: .packetInfoIP)

Unit tests have been added to cover this case, and because it still fails when using IPv6 the tests currently assert expected failures.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.08%. Comparing base (5a20fdc) to head (854a952).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   90.02%   90.08%   +0.05%     
==========================================
  Files          60       60              
  Lines        3951     3933      -18     
==========================================
- Hits         3557     3543      -14     
+ Misses        394      390       -4     

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[lhoward]

If setPktInfo() is not called then the caller will need to explicitly call it, otherwise receive() returning the local interface address will fail (well, it won't fail, but it won't return the local address). I think it's better if it's set by default, so let me look at what the correct fix is.

[lhoward]

Will aim to have an answer by lunchtime.

[swhitty]

thanks, take your time though

[lhoward]

OK, the fix is the sock opt is IPV6_RECVPKTINFO not IPV6_PKTINFO. However the latter is still the message type. I'll prepare a fix.

[lhoward]

See #132