Before:
  runtime autoload/ale/linter.vim
  runtime autoload/ale/lsp.vim
  runtime autoload/ale/lsp_linter.vim

  let g:address = 'ccls_address'
  let g:callback_result = 0
  let g:conn_id = -1
  let g:executable = 'ccls'
  let g:linter_name = 'ccls'
  let g:magic_number = 42
  let g:message = -1
  let g:message_id = 1
  let g:method = '$ccls/call'
  let g:parameters = {}
  let g:project = '/project/root'
  let g:return_value = -1

  let g:linter_list = [{
  \   'output_stream': 'stdout',
  \   'lint_file': 0,
  \   'language': 'cpp',
  \   'name': g:linter_name,
  \   'project_root': {b -> g:project},
  \   'aliases': [],
  \   'language_callback': {b -> 'cpp'},
  \   'read_buffer': 1,
  \   'command': '%e'
  \ }]

  " Encode dictionary to jsonrpc
  function! Encode(obj) abort
    let l:body = json_encode(a:obj)
    return 'Content-Length: ' . strlen(l:body) . "\r\n\r\n" . l:body
  endfunction

  " Register the server with given executable or address
  function! InitServer(executable_or_address) abort
    let g:conn_id = ale#lsp#Register(a:executable_or_address, g:project, {})
    call ale#lsp#HandleMessage(g:conn_id, Encode({'method': 'initialize'}))
  endfunction

  " Dummy callback
  function! Callback(response) abort
    let g:callback_result = a:response.result.value
  endfunction

  " Replace the GetAll function to mock an LSP linter
  function! ale#linter#GetAll(filetype) abort
    return g:linter_list
  endfunction

  " Replace the Send function to mock an LSP linter
  function! ale#lsp#Send(conn_id, message) abort
    let g:message = a:message
    return g:message_id
  endfunction

  " Code for a test case
  function! TestCase() abort
    " Test sending a custom request
    let g:return_value = ale#lsp_linter#SendRequest(bufnr('%'), g:linter_name, g:method, g:parameters, function('Callback'))

    AssertEqual
    \ 0,
    \ g:return_value

    AssertEqual
    \ [0, g:method, g:parameters],
    \ g:message

    " Mock an incoming response to the request
    call ale#lsp#HandleMessage(g:conn_id, Encode({'id': g:message_id, 'jsonrpc': '2.0', 'result': {'value': g:magic_number}}))

    AssertEqual
    \ g:magic_number,
    \ g:callback_result
  endfunction

After:
  if g:conn_id isnot v:null
    call ale#lsp#RemoveConnectionWithID(g:conn_id)
  endif

  unlet! g:callback_result
  unlet! g:conn_id
  unlet! g:executable
  unlet! g:linter_name
  unlet! g:magic_number
  unlet! g:message
  unlet! g:message_id
  unlet! g:method
  unlet! g:parameters
  unlet! g:project
  unlet! g:return_value

  delfunction Encode
  delfunction InitServer
  delfunction Callback
  delfunction TestCase

  runtime autoload/ale/linter.vim
  runtime autoload/ale/lsp.vim
  runtime autoload/ale/lsp_linter.vim

Given cpp(Empty cpp file):
Execute(Test custom request to server identified by executable):
  call InitServer(g:executable)
  let g:linter_list[0].executable = {b -> g:executable}
  let g:linter_list[0].lsp = 'stdio'

  call TestCase()

Given cpp(Empty cpp file):
Execute(Test custom request to server identified by address):
  call InitServer(g:address)
  let g:linter_list[0].address = {b -> g:address}
  let g:linter_list[0].lsp = 'socket'

  call TestCase()