Add graceful shutdown to TcpServer

[alexcrichton]

This commit prototypes support for graceful shutdown in TcpServer
through some new methods. A TcpServer::bind method was acced which is
used to construct a server and unlike serve returns the value
immediately.

The returned value can then be used to inspect the state of the server,
finally calling run to execute indefinitely or run_until to run
until a shutdown signal is received.

[alexcrichton]

This is very similar to the support I added to Hyper and is an alternative to #115.

[seanmonstar]

One thing I'd like to add to what exists in hyper is the ability to also update the keep-alive status of the currently running services. This would allow hyper to stop accepting new requests on existing connections while trying to shutdown. hyper already has the concept of a shared keep-alive status, which is used by the client, and I could make similar use for the server, but if I were to shift to using the TcpServer shutdown, I'd need some way to be alerted that shutdown is desired.

[alexcrichton]

@seanmonstar yeah that's a good point. In general it may be useful to propagate the shutdown signal to services being run so they can run custom logic themselves.

How one might represent that, though, is... tricky

[casey]

This is definitely better than #115. TcpServer should probably now be called something else, like TcpServerBuilder, but that's obviously a breaking change.

[tomusdrw]

It should increase info.active when creating new NotifyService.

[alexcrichton]

Oops, thanks!

[tikue]

Could you expand on the semantics of graceful shutdown a bit? I assume it doesn't allow new connections, but does it allow existing connections to continue to make requests? I've also been working on graceful shutdown for tarpc. It'd be nice to just reuse this capability, but I don't want to be required to use the tcp server itself, because I don't want to require users to hand over management of the reactor.

[seanmonstar]

@tikue it currently will stop accepting new connections, and let the current ones run (and hopefully shutdown) up until a time limit.

An improvement I'd like is for a way to notify the underlying service that shutdown is desired, and hyper can turn off keep-alive for all the existing connections, so that after the next response, they will close.

I've been thinking about how to do this, with some possibilities:

• Adjust the Service trait with some sort of try_close method, signalling that it should shutdown.
• Allow TcpServer to take an on_shutdown option, so we can pipe the signalling through with whatever custom crazy code we want.

[carllerche]

I don't love TcpServerInstance as a name, but I also don't have a great alternative and it doesn't seem worth bike shedding at this point.

Maybe once tokio-proto evolves a bit more, the names of all the types can be rethought.

[tikue]

Is there any way this code could be made more modular so that folks not using TcpServer could get at it?

[wspeirs]

Just wondering about the status of this bug/issue... still doesn't seem to be a clean way to shutdown a TcpServer, or am I missing something? Thanks!