Add bind_reusablemethod to UdpSocket

[danslapman]

This implements bind_reusable method for UdpSocket, which binds socket with SO_REUSEADDR set to true. Such behaviour is necessary in some cases, such as monitoring multiple multicast streams on the same machine. See also #11758

[rust-highfive]

r? @pcwalton

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[alexcrichton]

I made a similar comment on another PR, but I think that this may be best left off for now. We're actively redesigning the I/O system in a set of new RFCs. Currently one does not exist for std::net yet (it was part of the original monster-RFC), but I'll try to remember to ping this once that pops up!

We'll try to remember to handle this case as well, as I'm sure you're not the only one wanting this functionality! For now though I'm going to close this while we consider the new I/O system and we hope to have that RFC up soon!

[kali]

The new io is here, with no bind_reusable. Should we rebase/refactor this patch ? It would be a shame to freeze the io api with no reusable bind (it makes it impossible to implement things like SSDP and other multicast based protocol).

[danslapman]

Truly, @alexcrichton, I'm forced to support a fork of Rust with the only difference - "bind_reusable" :)

[alexcrichton]

@kali, @danslapman it is correct that there is currently no method for doing this, but this was explicitly discussed in the RFC. Adding this sort of behavior will likely not be done through a new bind_reusable constructor but rather a more general mechanism of starting with a Socket, settings some options, and then later binding or connecting it into a UdpSocket or TcpStream for example.

These sorts of additions are definitely planned for, but are all backwards compatible extensions to the API. An RFC will be required, however, to add this new API as it was not explicitly designed in rust-lang/rfcs#807

[kali]

Ok. Thanks for confirming this is still in the plan, and apologies for missing it in the monster io rfc.

[alexcrichton]

No worries! Even I often forget what's inside the RFCs...