Remove Linux-Only Features

[JonasGessner]

I'm using SCION on Darwin OSes (iOS and macOS) and dispatcher and sciond can be almost compiled straight out of the box if it weren't for one unsupported sycall flag in underlay/conn.go: MSG_WAITFORONE

In this case the affected code can simply be commented out because it appears to be unused anyway. But going forward I suggest removing this specific syscall flag so that dispatcher can be compiled with no modifications on macOS and other OSes that aren't Linux. Surely there must be some way to work without this flag.

Generally it should be avoided to use OS-specific features such as this MSG_WAITFORONE flag – I think it just unnecessarily hurts the possibilities for broader adoption.

[matzf]

Which part of SCION are you running on this macOS? This underlay/conn.go is (only) used by the router and dispatcher components.

The router and dispatcher use x/net library's ReadBatch, as this (supposedly) allows to achieve higher throughput. As the documentation for ReadBatch notes, this only uses recmmsg on Linux, so this is normally portable: "On Linux, a batch read will be optimized. On other platforms, this method will read only a single message."
Now, setting MSG_WAITFORONE is critical when recvmmsg is used; otherwise, the call would block indefinitely until the full batch was read which is obviously undesirable when there is a lower rate of incoming packets.
You could move the definition for the flags passed to ReadBatch to a platform specific file (// +build linux) to make this more portable.

[JonasGessner]

Which part of SCION are you running on this macOS? This underlay/conn.go is (only) used by the router and dispatcher components.

I'm running the full stack, so sciond and dispatcher.

The router and dispatcher use x/net library's ReadBatch, as this (supposedly) allows to achieve higher throughput. As the documentation for ReadBatch notes, this only uses recmmsg on Linux, so this is normally portable: "On Linux, a batch read will be optimized. On other platforms, this method will read only a single message."
Now, setting MSG_WAITFORONE is critical when recvmmsg is used; otherwise, the call would block indefinitely until the full batch was read which is obviously undesirable when there is a lower rate of incoming packets.
You could move the definition for the flags passed to ReadBatch to a platform specific file (// +build linux) to make this more portable.

Sounds good to me!