LSP client that doesn't support any workspace capabilities will cause an error if the server is using workspace/workspaceFolders

[rcjsuen]

If you create your server with let connection = createConnection(ProposedFeatures.all); and use a client that connects to it with only very rudimentary capabilities, an error will be thrown.

function initialize(id: number) {
  send(id, "initialize", {
    rootPath: process.cwd(),
    processId: process.pid,
    capabilities: {
      textDocument: {
        completion: {
          completionItem: {
            snippetSupport: true
          }
        }
      }
    }
  });
}

{
  jsonrpc: '2.0',
  id: 1,
  error: {
    code: -32603,
    message: 'Request initialize failed with message: Cannot read property \'workspaceFolders\' of undefined'
  }
}

This is because workspaceFolders.proposed.ts incorrectly assumes that clientCapabilities.workspace is set.
https://github.com/Microsoft/vscode-languageserver-node/blob/9059f83cd8fb25b4a0fe5429540c562aa899dc6e/server/src/workspaceFolders.proposed.ts#L23-L24

However, according to the protocol, that capability is optional as it is flagged with a ?.

interface ClientCapabilities {
	/**
	 * Workspace specific client capabilities.
	 */
	workspace?: WorkspaceClientCapabilities;

	/**
	 * Text document specific client capabilities.
	 */
	textDocument?: TextDocumentClientCapabilities;

	/**
	 * Experimental client capabilities.
	 */
	experimental?: any;
}

[rcjsuen]

You can reproduce the problem with the following two files.

$ node bug.js
{ jsonrpc: '2.0',
  id: 1,
  error:
   { code: -32603,
     message: 'Request initialize failed with message: Cannot read property \'workspaceFolders\' of undefined' } }

server.js

var langserv = require(".");
let connection = langserv.createConnection(langserv.ProposedFeatures.all);
connection.listen();

bug.js

var langserv = require(".");
var cp = require("child_process");

let lspProcess = cp.fork("server.js", ["--node-ipc"]);
function send(method, params) {
    let message = {
        jsonrpc: "2.0",
        id: 1,
        method: method,
        params: params
    };
    lspProcess.send(message);
}
function initialize() {
    send("initialize", {
        rootPath: process.cwd(),
        processId: process.pid,
        capabilities: {}
    });
}
lspProcess.on('message', function (json) {
    console.log(json);
});
initialize();