fix(:restart): inherit stderr fd on Unix (#38755)

This in turn gives TTY access to channel_from_stdio() in the new server,
if the old server has access to a TTY.

(cherry picked from commit 9927d9259d)

src/nvim/ex_docmd.c

@@ -5023,9 +5023,13 @@ static void ex_restart(exarg_T *eap)
   }
 
   CallbackReader on_err = CALLBACK_READER_INIT;
-  // This temporary bootstrap channel is closed intentionally once we obtain
+#ifdef MSWIN
-  // the new server address. Don't forward child stderr to the current UI.
+  // On Windows, don't forward stderr as it won't work after the current server exits.
   on_err.fwd_err = false;
+#else
+  // On Unix, stderr fd is inherited, so it works even after the current server exits.
+  on_err.fwd_err = true;
+#endif
   bool detach = true;
   varnumber_T exit_status;
 
@@ -5110,8 +5114,18 @@ fail_2:
     api_clear_error(&err);
   }
   arena_mem_free(result_mem);
+  result_mem = NULL;
 
-  // Kill the new nvim server.
+#ifndef MSWIN
+  // Before killing the new server, close its stderr to avoid polluting the current UI.
+  MAXSIZE_TEMP_ARRAY(chanclose_expr_args, 1);
+  ADD_C(chanclose_expr_args, CSTR_AS_OBJ("chanclose(v:stderr)"));
+  rpc_send_call(channel->id, "nvim_eval", chanclose_expr_args, &result_mem, &err);
+  api_clear_error(&err);
+  arena_mem_free(result_mem);
+#endif
+
+  // Kill the new Nvim server.
   proc_stop(&channel->stream.proc);
   if (proc_wait(&channel->stream.proc, -1, NULL) < 0) {
     emsg("killing new nvim server failed");

test/functional/terminal/tui_spec.lua

@@ -224,7 +224,7 @@ describe('TUI :restart', function()
       '--cmd',
       'colorscheme vim',
       '--cmd',
-      'set laststatus=2 background=dark noruler',
+      'set laststatus=2 background=dark noruler noshowcmd',
       -- XXX: New server starts before the UI connects to it.
       -- So checking screen state for this pid is not possible.
       -- '--cmd',
@@ -315,8 +315,15 @@ describe('TUI :restart', function()
     assert_termguicolors_and_no_gui_running()
 
     -- Check ":restart +echo" cannot restart server.
+    -- Check the full screen state to ensure this doesn't pollute the current UI.
     tt.feed_data(':restart +echo\013')
-    screen:expect({ any = vim.pesc('+cmd did not quit the server') })
+    screen:expect([[
+      ^                                                  |
+      {1:~}{18:                                                 }|*3
+      {3:[No Name]                                         }|
+      {9:restart failed: +cmd did not quit the server}      |
+      {5:-- TERMINAL --}                                    |
+    ]])
 
     tt.feed_data('ithis will be removed\027')
     screen:expect({ any = vim.pesc('this will be remove^d') })
@@ -514,7 +521,7 @@ describe('TUI :restart', function()
       '-u',
       config_file,
       '--cmd',
-      'set notermguicolors noswapfile laststatus=0 nowrap noruler',
+      'set notermguicolors noswapfile laststatus=0 nowrap noruler noshowcmd',
     }, { env = env_notermguicolors })
     screen:expect([[
       ^                                                  |
@@ -2906,6 +2913,18 @@ describe('TUI', function()
       { 'tty', 'tty' },
       child_exec_lua('return { vim.uv.guess_handle(0), vim.uv.guess_handle(1) }')
     )
+    -- Also works after :restart #38745
+    feed_data(':restart lua ={ vim.uv.guess_handle(0), vim.uv.guess_handle(1) }\r')
+    screen:expect([[
+      ^                                                  |
+      {100:~                                                 }|*3
+      {3:[No Name]                                         }|
+      { "tty", "tty" }                                  |
+      {5:-- TERMINAL --}                                    |
+    ]])
+    -- The server is now detached and needs to be quit explicitly.
+    feed_data(':qall!\r')
+    screen:expect({ any = vim.pesc('[Process exited 0]') })
   end)
 end)