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emulating SO_REUSEPORT in Linux userspace

The SO_REUSEPORT socket option can be a useful feature. Not only for multicasting applications (where it supposedly came from), or for load balancing servers (which seems to be what most people are interested in regarding this flag). It also allows for clean and simple solutions for some problems which are approached in a rather hacky way otherwise. For example when updating servers, it allows to easily avoid unavailability between stopping and starting the server, as the new server can just be started before stopping the old one.

Unfortunately, it is a non-standard socket option. And for the widely spread Linux kernels, it has not yet been integrated (apparently there was a patch to do so, but it wasn't accepted). And even if it was, it would still mean to sacrifice portability to use it.

However, POSIX allows to emulate something similar with a little effort. The listening socket descriptor can be shared between multiple processes, and it can be passed to new processes through a local domain socket. Thus, while kernels cannot always be depended on for allowing to reuse ports, applications can share these themselves for reuse.

Using boost::asio as a socket abstraction layer, I built a small library to automate this, together with an echo server implementation to demonstrate its use. The API is loosely modeled after boost::asio's acceptors (async_accept works the same way, but to start listening, an asynchronous async_connect call is needed as the process has to wait for another process to share the socket). You can check it out yourself here. And be sure to let me know your comments.