fix expert lsp stdio and register capability (#5113)

* pass --stdio flag to expert lsp server

expert requires --stdio to use stdio transport but the linter
definition launched it without the flag, so the server never
entered lsp mode

* handle client/registerCapability requests from lsp servers

some lsp servers like expert send a client/registerCapability
request after initialization to dynamically register capabilities,
if ale does not respond the server blocks and never processes
further messages like textDocument/didOpen

test/linter/test_elixir_expert.vader

@@ -5,12 +5,12 @@ After:
   call ale#assert#TearDownLinterTest()
 
 Execute(should set correct defaults):
-  AssertLinter 'expert', ale#Escape('expert')
+  AssertLinter 'expert', ale#Escape('expert') . ' --stdio'
 
 Execute(The executable should be configurable):
   let b:ale_elixir_expert_executable = 'foobar'
 
-  AssertLinter 'foobar', ale#Escape('foobar')
+  AssertLinter 'foobar', ale#Escape('foobar') . ' --stdio'
 
 Execute(should set correct LSP values):
   call ale#test#SetFilename('../test-files/elixir/umbrella_project/apps/app1/lib/app.ex')

test/lsp/test_engine_lsp_response_handling.vader

@@ -562,6 +562,30 @@ Execute(workspace/configuration requests should be answered with the connection
   unlet! g:sent_responses
   runtime autoload/ale/lsp.vim
 
+Execute(client/registerCapability requests should be acknowledged):
+  let g:sent_responses = []
+
+  function! ale#lsp#SendResponse(conn_id, id, result) abort
+    call add(g:sent_responses, [a:conn_id, a:id, a:result])
+  endfunction
+
+  call ale#lsp_linter#SetLSPLinterMap({'1': {'name': 'expert', 'aliases': [], 'lsp': 'stdio'}})
+  call ale#lsp_linter#HandleLSPResponse(1, {
+  \ 'jsonrpc': '2.0',
+  \ 'id': 12,
+  \ 'method': 'client/registerCapability',
+  \ 'params': {
+  \   'registrations': [{'id': 'abc', 'method': 'textDocument/didSave'}],
+  \ },
+  \})
+
+  AssertEqual
+  \ [[1, 12, v:null]],
+  \ g:sent_responses
+
+  unlet! g:sent_responses
+  runtime autoload/ale/lsp.vim
+
 Execute(LSP errors should be logged in the history):
   call ale#lsp_linter#SetLSPLinterMap({'347': {'name': 'foobar', 'aliases': [], 'lsp': 'stdio'}})
   call ale#lsp_linter#HandleLSPResponse(347, {