fix(lsp): buffer messages until connected to server (#28507)

`handle:write(msg)` can fail if the socket is not yet connected to the
server.

Should address #28398 (comment)

runtime/lua/vim/lsp/rpc.lua

@@ -645,9 +645,23 @@ function M.connect(host_or_path, port)
  or assert(uv.new_tcp(), 'Could not create new TCP socket')
  )
  local closing = false
+ -- Connect returns a PublicClient synchronously so the caller
+ -- can immediately send messages before the connection is established
+ -- -> Need to buffer them until that happens
+ local connected = false
+ -- size should be enough because the client can't really do anything until initialization is done
+ -- which required a response from the server - implying the connection got established
+ local msgbuf = vim.ringbuf(10)
  local transport = {
  write = function(msg)
- handle:write(msg)
+ if connected then
+ local _, err = handle:write(msg)
+ if err and not closing then
+ log.error('Error on handle:write: %q', err)
+ end
+ else
+ msgbuf:push(msg)
+ end
  end,
  is_closing = function()
  return closing
@@ -679,6 +693,10 @@ function M.connect(host_or_path, port)
  handle:read_start(M.create_read_loop(handle_body, transport.terminate, function(read_err)
  client:on_error(M.client_errors.READ_ERROR, read_err)
  end))
+ connected = true
+ for msg in msgbuf do
+ handle:write(msg)
+ end
  end
  if port == nil then
  handle:connect(host_or_path, on_connect)