Add configuration of multipathServiceType in NIOTSConnectionBootstrap

[Aperence]

Motivation:

Allow different devices to leverage the capabilities of Mutipath TCP (MPTCP) to enhance the network reliability. Thanks to MPTCP, a connection can for example automatically migrate to another interface if it deteriorates on the first one. This can be especially interesting on MacOS X and iOS, where devices may frequently benefit from multiple interfaces (ethernet + Wi-Fi for Macs and Wi-fi + cellular for iOS). Allowing developers to enable MPTCP on their connections seems thus like a fine addition to this library.

Modifications:

Added a function "withMultipath" on NIOTSConnectionBootstrap, that allow to configure the type of service used for MPTCP (defaults to disabled). This value will be stored in a field, and then propagated to the underlying channel when the connect method will be called. Also updated the parameters field of NIOTSConnectionChannel to set the multipathServiceType accordingly.

Result:

Users will now be able to easily enable Multipath TCP, allowing them to benefit from seamless handover, interactive mode to automatically use the lowest delay interface or aggregate mode to send data in parallel on both interfaces.

Example:

let group = NIOTSEventLoopGroup()
defer {
     try! group.syncShutdownGracefully()
}
let bootstrap = NIOTSConnectionBootstrap(group: group)
    .withMultipath(.handover)   // set handover mode
    .channelInitializer { channel in
        channel.pipeline.addHandler(MyChannelHandler())
    }
try! bootstrap.connect(host: "example.org", port: 12345).wait()
/* the Channel is now connected */

[Lukasa]

Thanks for filing this! This is a nice addition, as offering a convenience API for multipath service type is useful. However, the multipath service type is already configurable via the NIOTSChannelOptions.multipathServiceType option. I'm all for offering a convenience abstraction for this, but I recommend we have it decay down to just setting the channel option, as that will ensure we only have a single path for getting this to work correctly.

[Aperence]

Thank you for your feedback, I'll try to simplify it to use the channel options !

[Aperence]

Hello,

I've simplified (a lot) the code thanks to your feedback, it now uses the NIOTSChannelOptions, limiting drastically the modifications needed

[Lukasa]

Thanks for this, and sorry for the delay in getting to it! Can you briefly add a unit test that configures both a listener and a connection bootstrap and connects them, and checks the set values using the channel options? I'd just like to make sure we don't accidentally regress this API in future.

[Aperence]

No problem, I've just added this test now !

[Lukasa]

@swift-server-bot test this please

[Lukasa]

Thank you very much @Aperence, and thanks for being very game with the feedback!