floatwin: show error if window is closed immediately #11476

Autocmds may close window while it is being entered, then
win_set_minimal_style(wp) operates on an invalid pointer.

We could silently ignore this instead, but it is unlikely to be
intentional, so it is more useful to show an error.

fix #11383

src/nvim/api/vim.c

@@ -1096,6 +1096,10 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dictionary config,
   if (enter) {
     win_enter(wp, false);
   }
+  if (!win_valid(wp)) {
+    api_set_error(err, kErrorTypeException, "Window was closed immediately");
+    return 0;
+  }
   if (buffer > 0) {
     nvim_win_set_buf(wp->handle, buffer, err);
   }

src/nvim/window.c

@@ -4372,9 +4372,10 @@ static void win_goto_hor(bool left, long count)
   }
 }
 
-/*
- * Make window "wp" the current window.
- */
+/// Make window `wp` the current window.
+///
+/// @warning Autocmds may close the window immediately, so caller must check
+///          win_valid(wp).
 void win_enter(win_T *wp, bool undo_sync)
 {
   win_enter_ext(wp, undo_sync, false, false, true, true);

test/functional/ui/float_spec.lua

@@ -2,9 +2,11 @@ local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
 local os = require('os')
 local clear, feed = helpers.clear, helpers.feed
+local assert_alive = helpers.assert_alive
 local command, feed_command = helpers.command, helpers.feed_command
 local eval = helpers.eval
 local eq = helpers.eq
+local exec_lua = helpers.exec_lua
 local insert = helpers.insert
 local meths = helpers.meths
 local curbufmeths = helpers.curbufmeths
@@ -12,7 +14,7 @@ local funcs = helpers.funcs
 local run = helpers.run
 local pcall_err = helpers.pcall_err
 
-describe('floating windows', function()
+describe('floatwin', function()
   before_each(function()
     clear()
   end)
@@ -56,6 +58,31 @@ describe('floating windows', function()
     eq(1000, funcs.win_getid())
   end)
 
+  it('closed immediately by autocmd #11383', function()
+    eq('Error executing lua: [string "<nvim>"]:4: Window was closed immediately',
+      pcall_err(exec_lua, [[
+        local a = vim.api
+        local function crashes(contents)
+          local buf = a.nvim_create_buf(false, true)
+          local floatwin = a.nvim_open_win(buf, true, {
+            relative = 'cursor';
+            style = 'minimal';
+            row = 0; col = 0;
+            height = #contents;
+            width = 10;
+          })
+          a.nvim_buf_set_lines(buf, 0, -1, true, contents)
+          local winnr = vim.fn.win_id2win(floatwin)
+          a.nvim_command('wincmd p')
+          a.nvim_command('autocmd CursorMoved * ++once '..winnr..'wincmd c')
+          return buf, floatwin
+        end
+        crashes{'foo'}
+        crashes{'bar'}
+    ]]))
+    assert_alive()
+  end)
+
   local function with_ext_multigrid(multigrid)
     local screen
     before_each(function()

test/functional/ui/options_spec.lua

@@ -52,10 +52,10 @@ describe('UI receives option updates', function()
     local evs = {}
     screen = Screen.new(20,5)
     -- Override mouse_on/mouse_off handlers.
-    function screen._handle_mouse_on()
+    function screen:_handle_mouse_on()
       table.insert(evs, 'mouse_on')
     end
-    function screen._handle_mouse_off()
+    function screen:_handle_mouse_off()
       table.insert(evs, 'mouse_off')
     end
     screen:attach()