Darwin: correct ipv6_pktinfo to IPV6_3542PKTINFO (46)

[ianegordon]

Summary

The hardcoded Darwin ipv6_pktinfo constant is 50, which per the macOS SDK <netinet6/in6.h> is IPV6_3542DSTOPTS (destination-options header) — not pktinfo. The RFC 3542 value paired with the already-used IPV6_RECVPKTINFO (61) is IPV6_3542PKTINFO (46):

#define IPV6_3542PKTINFO        46 /* in6_pktinfo; send if, src addr */
#define IPV6_3542DSTOPTS        50 /* ip6_dest; send dst option befor rthdr */
#define IPV6_PKTINFO    IPV6_3542PKTINFO   // under __APPLE_USE_RFC_3542

Swift cannot define __APPLE_USE_RFC_3542 before importing Darwin, so these constants must be hardcoded — but with 50, getPacketInfoControl never matches the cmsg_type the kernel actually delivers (46), and every received IPv6 datagram Message carries interfaceIndex: nil, localAddress: nil on Darwin.

Changes

• Socket+Darwin.swift: ipv6_pktinfo 50 → 46; comments corrected from __APPLE_USE_RFC_2292 to __APPLE_USE_RFC_3542 (61 and 46 are RFC 3542 values).
• New test receiveMessage_ParsesIPv6PacketInfo: IPv6 loopback datagram round-trip asserting the received Message carries the parsed interface index and ::1 local address. Verified to fail with 50 and pass with 46 on macOS.

Notes

This is a prerequisite for a follow-up fix to the send-side pktinfo control message (its cmsg_level is currently SOL_SOCKET, which Darwin ignores; once corrected to IPPROTO_IPV6, the kernel interprets cmsg_type — and with 50 rejects IPv6 sends with EACCES).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.97%. Comparing base (ac24c58) to head (35b0b0a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   92.83%   92.97%   +0.13%     
==========================================
  Files          71       71              
  Lines        3743     3743              
==========================================
+ Hits         3475     3480       +5     
+ Misses        268      263       -5     

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[swhitty]

Great I think the original implementation was made and tested on Linux so Darwin was only quickly added and included in some tests. Thank you