[lhoward]

Summary

Socket.withPacketInfoControl sends IPv4 datagrams with an uninitialised source address. On Linux, sendMessage_WithPacketInfo_RoundTripsDatagram() then fails with SendMsg errno 101 (Network is unreachable). Whether it fails depends on what happens to be in memory, so it passes on some builds and fails on others:

• On SocketPool: throw CancellationError when cancelled while waiting #242's CI it failed on Linux Swift 6.0 and 6.2, and passed on 6.1 and 6.3.
• On one Linux machine with Swift 6.3.3 it failed 10 times out of 10.

Cause

The in_pktinfo and in6_pktinfo are built in the element storage of a ManagedBuffer, which starts out uninitialised.

1. IPv4 source in the wrong field. The local address is written to ipi_addr, and ipi_spec_dst is never set. ip(7): "If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not zero, then it is used as the local source address for the routing table lookup". ipi_addr is the destination of a received packet. So the source used is whatever was in memory.
2. The address is read from the wrong bytes. It's read by reinterpreting the bytes of the any SocketAddress existential as a sockaddr_in or sockaddr_in6. Those bytes are the address only when the value is stored inline in the existential. A sockaddr_in6 (28 bytes) or a sockaddr_storage is boxed, so what gets read is the box. This affects every IPv6 send with a local address.

Changes

• Zero-initialise the in_pktinfo or in6_pktinfo.
• Read the address through makeStorage(), checking its family.
• Put an IPv4 source in ipi_spec_dst. For IPv6, ipi6_addr was already the right field (RFC 3542 §6.1).

Tests

Two new unit tests read back the control message that's built, with the address passed both as a plain value and as a boxed sockaddr_storage:

• withPacketInfoControl_IP4_SetsSourceAddressInSpecDst: checks ipi_spec_dst is the source address, ipi_addr is 0, and ipi_ifindex is set. Without this change it fails: ipi_spec_dst is 0, and ipi_addr is 127.0.0.1 for the plain value but garbage for the boxed one.
• withPacketInfoControl_IP6_SetsSourceAddress: checks ipi6_addr is the source address. Without this change it fails: ipi6_addr holds the existential's box pointer rather than ::1.

Test plan:

• macOS (Swift 6.3.3): full suite, 474 tests pass
• Linux (x86_64, Swift 6.3.3): the pktinfo unit tests and both sendMessage_WithPacketInfo… round trips pass in 10 of 10 runs. On the same machine and toolchain, sendMessage_WithPacketInfo_RoundTripsDatagram failed 10 of 10 without this change.

🤖 Generated with Claude Code

https://claude.ai/code/session_012Fq7ie1LzJERrh9uRuZZWE

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.73%. Comparing base (bd32d43) to head (49356ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   93.72%   93.73%   +0.01%     
==========================================
  Files          72       72              
  Lines        3777     3785       +8     
==========================================
+ Hits         3540     3548       +8     
  Misses        237      237              

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