Investigate and document UDP recv/peek(_from) behavior on *nix when using a smaller buffer.

[dtacalau]

On Windows, if the data is larger than the buffer specified, the buffer is filled with the first part of the data, and recv/peek(_from) returns the error WSAEMSGSIZE(10040). The excess data is lost.

What does *nix do?

Ref PR https://github.com/tokio-rs/mio/pull/1167/files#diff-c022871452a328dc8c9cd10130eb1572R233

[andrewtj]

In case you haven't seen it rust-lang/rust#55794 concerns the different behaviour of Windows and *nix. For my $0.02, it'd be better to make both platforms behave the same, probably by returning an error on truncation.

[Thomasdezeeuw]

Pr #1167 adds documentation to explain the difference in behaviour. And once rust-lang/rust#55794 is resolved we'll follow that approach. So I'm punting this to v1.

[Thomasdezeeuw]

This would be solved if we switch to socket2 (tracked in #1381) which has a struct RecvFlags to aid in this.

[Thomasdezeeuw]

This might depend on rust-lang/socket2#188.