feat(lsp): send didChangeConfiguration after init (#18847)

Most LSP servers require the notification to correctly load the
settings and for those who don't it doesn't cause any harm.

So far this is done in lspconfig, but with the addition of vim.lsp.start
it should be part of core.

runtime/lua/vim/lsp.lua

@@ -1117,6 +1117,10 @@ function lsp.start_client(config)
         end
       end
 
+      if next(config.settings) then
+        client.notify('workspace/didChangeConfiguration', { settings = config.settings })
+      end
+
       if config.on_init then
         local status, err = pcall(config.on_init, client, result)
         if not status then

test/functional/fixtures/fake-lsp-server.lua

@@ -116,6 +116,19 @@ function tests.basic_init()
   }
 end
 
+function tests.basic_init_did_change_configuration()
+  skeleton({
+    on_init = function(_)
+      return {
+        capabilities = {},
+      }
+    end,
+    body = function()
+      expect_notification('workspace/didChangeConfiguration', { settings = { dummy = 1 } })
+    end,
+  })
+end
+
 function tests.check_workspace_configuration()
   skeleton {
     on_init = function(_params)

test/functional/plugin/lsp_spec.lua

@@ -45,10 +45,10 @@ local function clear_notrace()
 end
 
 
-local function fake_lsp_server_setup(test_name, timeout_ms, options)
+local function fake_lsp_server_setup(test_name, timeout_ms, options, settings)
   exec_lua([=[
     lsp = require('vim.lsp')
-    local test_name, fixture_filename, logfile, timeout, options = ...
+    local test_name, fixture_filename, logfile, timeout, options, settings = ...
     TEST_RPC_CLIENT_ID = lsp.start_client {
       cmd_env = {
         NVIM_LOG_FILE = logfile;
@@ -79,17 +79,18 @@ local function fake_lsp_server_setup(test_name, timeout_ms, options)
         allow_incremental_sync = options.allow_incremental_sync or false;
         debounce_text_changes = options.debounce_text_changes or 0;
       };
+      settings = settings;
       on_exit = function(...)
         vim.rpcnotify(1, "exit", ...)
       end;
     }
-  ]=], test_name, fake_lsp_code, fake_lsp_logfile, timeout_ms or 1e3, options or {})
+  ]=], test_name, fake_lsp_code, fake_lsp_logfile, timeout_ms or 1e3, options or {}, settings or {})
 end
 
 local function test_rpc_server(config)
   if config.test_name then
     clear_notrace()
-    fake_lsp_server_setup(config.test_name, config.timeout_ms or 1e3, config.options)
+    fake_lsp_server_setup(config.test_name, config.timeout_ms or 1e3, config.options, config.settings)
   end
   local client = setmetatable({}, {
     __index = function(_, name)
@@ -298,6 +299,22 @@ describe('LSP', function()
       }
     end)
 
+    it('should send didChangeConfiguration after initialize if there are settings', function()
+      test_rpc_server({
+        test_name = 'basic_init_did_change_configuration',
+        on_init = function(client, _)
+          client.stop()
+        end,
+        on_exit = function(code, signal)
+          eq(0, code, 'exit code', fake_lsp_logfile)
+          eq(0, signal, 'exit signal', fake_lsp_logfile)
+        end,
+        settings = {
+          dummy = 1,
+        },
+      })
+    end)
+
     it('should succeed with manual shutdown', function()
       if isCI() then
         pending('hangs the build on CI #14028, re-enable with freeze timeout #14204')