Concurrency warning in example server: Conformance of 'NIOSSHHandler' to 'Sendable' is unavailable

[yusefnapora]

I'm experimenting with porting a simple server app I'm working on from the (quite nice) Citadel wrapper library to use NIOSSH directly. I've got it basically working, but I get a concurrency warning that says it will be an error in Swift 6 mode.

It seems that the ChannelPipeline.addHandler method used in the example server app wants its handlers input to be [ChannelHandler & Sendable], but the NIOSSHHandler type is explicitly marked as non-Sendable.

It looks like there were some updates to that type related to strict concurrency in #196 - is it possible that the type is now effectively Sendable if the availability annotation were removed? I don't know enough about the internals here to have a clue, but perhaps a maintainer would.

[glbrntt]

Thanks for filing this! The NIOSSHHandler isn't Sendable, but we have foreknowledge that the childChannelInitializer is run on the channel's event loop. This means we can use a different API to create the SSH handler and add it to the pipeline without moving it across an isolation domain. That part of the example would be:

let bootstrap = ServerBootstrap(group: group)
    .childChannelInitializer { channel in
        channel.eventLoop.makeCompletedFuture {
            let sync = channel.pipeline.syncOperations
            try sync.addHandler(
                NIOSSHHandler(
                    role: .server(
                        .init(
                            hostKeys: [hostKey],
                            userAuthDelegate: HardcodedPasswordDelegate(),
                            globalRequestDelegate: RemotePortForwarderGlobalRequestDelegate()
                        )
                    ),
                    allocator: channel.allocator,
                    inboundChildChannelInitializer: sshChildChannelInitializer(_:_:)
                )
            )
            try sync.addHandler(ErrorHandler())
        }
    }
    .serverChannelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEADDR), value: 1)
    .serverChannelOption(ChannelOptions.socket(SocketOptionLevel(IPPROTO_TCP), TCP_NODELAY), value: 1)

As a side note, it looks like the #if canImport statements are no longer effective and the #else branch is always being hit which is why we never saw the warnings when doing the strict concurrency work.