MulticastInterface_Set_AnyInterface_Succeeds fails on macOS 26 beta 6

[vcsjones]

This test is failing on macOS 26 beta 6.

System.Net.Sockets.Tests.SocketOptionNameTest.MulticastInterface_Set_AnyInterface_Succeeds [FAIL]
System.Net.Sockets.SocketException : Invalid argument
  Stack Trace:
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs(3913,0): at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, Boolean disconnectOnFailure, String callerName)
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs(3904,0): at System.Net.Sockets.Socket.UpdateStatusAfterSocketOptionErrorAndThrowException(SocketError error, String callerName)
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs(3638,0): at System.Net.Sockets.Socket.SetSocketOption(SocketOptionLevel optionLevel, SocketOptionName optionName, Int32 optionValue, Boolean silent)
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs(2083,0): at System.Net.Sockets.Socket.SetSocketOption(SocketOptionLevel optionLevel, SocketOptionName optionName, Int32 optionValue)
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/tests/FunctionalTests/SocketOptionNameTest.cs(104,0): at System.Net.Sockets.Tests.SocketOptionNameTest.MulticastInterface_Set_Helper(Int32 interfaceIndex)
  /Users/vcsjones/Projects/runtime/src/libraries/System.Net.Sockets/tests/FunctionalTests/SocketOptionNameTest.cs(77,0): at System.Net.Sockets.Tests.SocketOptionNameTest.MulticastInterface_Set_AnyInterface_Succeeds()
  --- End of stack trace from previous location ---

This is failing in the same way manner as #113827.

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[vcsjones]

@wfurt can you help us understand what this test failure means for macOS 26? Whether it be something that needs to be fixed, a release note, etc?

[liveans]

@wfurt can you help us understand what this test failure means for macOS 26? Whether it be something that needs to be fixed, a release note, etc?

I will look at this today.

[wfurt]

There was some discussion here #53767 and the multicast is broken for a while with newer versions. I'm wondering if they pulled trigger and remove the socket option completely...

[wfurt]

we should try to fix it for 10.x. It seems like we already have some insight and the fix should be small and safe.