fix(lsp): :lsp restart restarts on client exit #37125

Problem:
`:lsp restart` detects when a client has exited by using the `LspDetach`
autocommand. This works correctly in common cases, but breaks when
restarting a client which is not attached to any buffer. It also breaks
if a client is detached in between `:lsp restart` and the actual
stopping of the client.

Solution:
Move restart logic into `vim/lsp/client.lua`, so it can hook in to
`_on_exit()`. The public `on_exit` callback cannot be used for this, as
`:lsp restart` needs to ensure the restart only happens once, even if
the command is run multiple times on the same client.

runtime/doc/lsp.txt

@@ -123,7 +123,7 @@ COMMANDS                                                 *:lsp* *lsp-commands*
     Activates LSP for current and future buffers. See |vim.lsp.enable()|.
 
 :lsp disable [config_name]                                *:lsp-disable*
-    Disables (and stops) LSP for current and future buffers. See
+    Disables LSP (and stops if running) for current and future buffers. See
     |vim.lsp.enable()|.
 
 :lsp restart [client_name]                                *:lsp-restart*

runtime/lua/vim/_core/ex_cmd.lua

@@ -122,7 +122,7 @@ end
 --- @param client_names string[]
 --- @return vim.lsp.Client[]
 local function get_clients_from_names(client_names)
-  -- Default to stopping all active clients attached to the current buffer.
+  -- Default to all active clients attached to the current buffer.
   if #client_names == 0 then
     local clients = lsp.get_clients { bufnr = api.nvim_get_current_buf() }
     if #clients == 0 then
@@ -149,29 +149,7 @@ local function ex_lsp_restart(client_names)
   local clients = get_clients_from_names(client_names)
 
   for _, client in ipairs(clients) do
-    --- @type integer[]
-    local attached_buffers = vim.tbl_keys(client.attached_buffers)
-
-    -- Reattach new client once the old one exits
-    api.nvim_create_autocmd('LspDetach', {
-      group = api.nvim_create_augroup('nvim.lsp.ex_restart_' .. client.id, {}),
-      callback = function(info)
-        if info.data.client_id ~= client.id then
-          return
-        end
-
-        local new_client_id = lsp.start(client.config, { attach = false })
-        if new_client_id then
-          for _, buffer in ipairs(attached_buffers) do
-            lsp.buf_attach_client(buffer, new_client_id)
-          end
-        end
-
-        return true -- Delete autocmd
-      end,
-    })
-
-    client:stop(client.exit_timeout)
+    client:_restart(client.exit_timeout)
   end
 end
 

runtime/lua/vim/lsp/client.lua

@@ -908,6 +908,31 @@ function Client:stop(force)
   end)
 end
 
+--- Stops a client, then starts a new client with the same config and attached
+--- buffers.
+---
+--- @param force? integer|boolean See [Client:stop()] for details.
+---                               (default: `self.exit_timeout`)
+function Client:_restart(force)
+  validate('force', force, { 'number', 'boolean' }, true)
+
+  self._handle_restart = function()
+    --- @type integer[]
+    local attached_buffers = vim.tbl_keys(self.attached_buffers)
+
+    vim.schedule(function()
+      local new_client_id = lsp.start(self.config, { attach = false })
+      if new_client_id then
+        for _, buffer in ipairs(attached_buffers) do
+          lsp.buf_attach_client(buffer, new_client_id)
+        end
+      end
+    end)
+  end
+
+  self:stop(force)
+end
+
 --- Get options for a method that is registered dynamically.
 --- @param method vim.lsp.protocol.Method | vim.lsp.protocol.Method.Registration
 function Client:_supports_registration(method)
@@ -1334,6 +1359,11 @@ function Client:_on_exit(code, signal)
     end
   end)
 
+  if self._handle_restart ~= nil then
+    self._handle_restart()
+    self._handle_restart = nil
+  end
+
   -- Schedule the deletion of the client object so that it exists in the execution of LspDetach
   -- autocommands
   vim.schedule(function()

test/functional/ex_cmds/lsp_spec.lua

@@ -62,18 +62,22 @@ describe(':lsp', function()
     end)
 
     it('restart' .. test_message_suffix, function()
-      local ids_differ = exec_lua(function()
+      --- @type boolean, integer?
+      local ids_differ, attached_buffer_count = exec_lua(function()
         vim.lsp.enable('dummy')
         local old_id = vim.lsp.get_clients()[1].id
 
         vim.cmd('lsp restart' .. lsp_command_suffix)
-        vim.wait(1000, function()
-          return old_id ~= vim.lsp.get_clients()[1].id
+        return vim.wait(1000, function()
+          local new_client = vim.lsp.get_clients()[1]
+          if new_client == nil then
+            return false
+          end
+          return old_id ~= new_client.id, #new_client.attached_buffers
         end)
-        local new_id = vim.lsp.get_clients()[1].id
-        return old_id ~= new_id
       end)
       eq(true, ids_differ)
+      eq(1, attached_buffer_count)
     end)
 
     it('stop' .. test_message_suffix, function()