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

4 files changed, 29 insertions, 3 deletions

diff --git a/ale_linters/elixir/expert.vim b/ale_linters/elixir/expert.vim
index aba88ab06..521a59639 100644
--- a/ale_linters/elixir/expert.vim
+++ b/ale_linters/elixir/expert.vim
@@ -7,6 +7,6 @@ call ale#linter#Define('elixir', {
 \   'name': 'expert',
 \   'lsp': 'stdio',
 \   'executable': {b -> ale#Var(b, 'elixir_expert_executable')},
-\   'command': '%e',
+\   'command': '%e --stdio',
 \   'project_root': function('ale#handlers#elixir#FindMixUmbrellaRoot'),
 \})
diff --git a/autoload/ale/lsp_linter.vim b/autoload/ale/lsp_linter.vim
index fae9f7098..fdd492ddc 100644
--- a/autoload/ale/lsp_linter.vim
+++ b/autoload/ale/lsp_linter.vim
@@ -241,6 +241,8 @@ function! ale#lsp_linter#HandleLSPResponse(conn_id, response) abort
         \   : a:response.result.items
 
         call ale#lsp_linter#HandleLSPDiagnostics(a:conn_id, l:uri, l:diagnostics)
+    elseif l:method is# 'client/registerCapability'
+        call ale#lsp#SendResponse(a:conn_id, a:response.id, v:null)
     elseif l:method is# 'workspace/configuration'
         let l:items = get(get(a:response, 'params', {}), 'items', [])
         let l:config = ale#lsp#GetConnectionConfig(a:conn_id)
diff --git a/test/linter/test_elixir_expert.vader b/test/linter/test_elixir_expert.vader
index a36f4e472..2e9c0ed84 100644
--- a/test/linter/test_elixir_expert.vader
+++ b/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')
diff --git a/test/lsp/test_engine_lsp_response_handling.vader b/test/lsp/test_engine_lsp_response_handling.vader
index 153be2b0e..226051c44 100644
--- a/test/lsp/test_engine_lsp_response_handling.vader
+++ b/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, {