Provide a way to access Client outside the callbacks

[icsaszar]

Currently tower-lsp works great for servers that use a task based model. But consider the case when
you have a file system watcher, as a process task/process that is idependent from the language server task.

let server = initialize_server(...);
let watcher = initialize_watcher(...);
futures::join!(server, watcher);

When the watcher sees a change, it reparses a file, does some analysis and sends the diagnostics to the client. The problem is how can it acces the Client that is normally provided by tower-lsp in the callbacks?

One way could be to set up the language server with a oneshot channel with the other process waiting on the other side and send the Client from the initialized method, but this seems a bit hacky.

What would be a much more elegant solution would be returning a clone of the client when the service is created, like this:

let stdin = tokio::io::stdin();
let stdout = tokio::io::stdout();

let (client, service, messages) = LspService::new(backend);
  // ^^^^^^ access to client
Server::new(stdin, stdout)
   .interleave(messages)
   .serve(service)
   .await;

Of course with the caveat that the client will panic if it's used before the server is initalized. But this can be solved by providing a method on the client that can be used to check if the server is initialized before sending a message.

As with other issues, this will be affected by #102 and related rewrite issues (#177, #58), so maybe it's worth waiting with this, as it's not a major inconvenience.

[ebkalderon]

Thanks for the suggestion, @icsaszar! I'm not fully convinced we should expose the Client outside the LspService, since I feel it somewhat breaks the encapsulation of server and client, design-wise, and I can't really imagine what access to the Client would be used for outside the server. Would you mind elaborating on a potential use case for this?

[ebkalderon]

Is it not possible for the watcher to be spawned at initialization time as part of the server using tokio::spawn()?

[ebkalderon]

Continuing discussion in #189.