fix(api): patch some cmdwin/textlock holes

Problem: there are new ways to escape textlock or break the cmdwin in
nvim_win_set_config and nvim_tabpage_set_win.

Solution: fix them. Use win_goto to check it in nvim_tabpage_set_win and use the
try_start/end pattern like with similar functions such as nvim_set_current_win
(which uses the existing msg_list, if set).

Careful not to use `wp->handle` when printing the window ID in the error message
for nvim_tabpage_set_win, as win_goto autocommands may have freed the window.

On a related note, I have a feeling some API functions ought to be checking
curbuf_locked...

src/nvim/api/tabpage.c

@@ -146,7 +146,11 @@ void nvim_tabpage_set_win(Tabpage tabpage, Window win, Error *err)
   }
 
   if (tp == curtab) {
-    win_enter(wp, true);
+    try_start();
+    win_goto(wp);
+    if (!try_end(err) && curwin != wp) {
+      api_set_error(err, kErrorTypeException, "Failed to switch to window %d", win);
+    }
   } else if (tp->tp_curwin != wp) {
     tp->tp_prevwin = tp->tp_curwin;
     tp->tp_curwin = wp;

src/nvim/api/win_config.c

@@ -451,6 +451,12 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     if (!check_split_disallowed_err(win, err)) {
       return;  // error already set
     }
+    // Can't move the cmdwin or its old curwin to a different tabpage.
+    if ((win == cmdwin_win || win == cmdwin_old_curwin) && parent != NULL
+        && win_find_tabpage(parent) != win_tp) {
+      api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+      return;
+    }
 
     bool to_split_ok = false;
     // If we are moving curwin to another tabpage, switch windows *before* we remove it from the

src/nvim/winfloat.c

@@ -55,6 +55,19 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
       api_set_error(err, kErrorTypeException,
                     "Cannot change window from different tabpage into float");
       return NULL;
+    } else if (cmdwin_win != NULL && !cmdwin_win->w_floating) {
+      // cmdwin can't become the only non-float. Check for others.
+      bool other_nonfloat = false;
+      for (win_T *wp2 = firstwin; wp2 != NULL && !wp2->w_floating; wp2 = wp2->w_next) {
+        if (wp2 != wp && wp2 != cmdwin_win) {
+          other_nonfloat = true;
+          break;
+        }
+      }
+      if (!other_nonfloat) {
+        api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+        return NULL;
+      }
     }
     int dir;
     winframe_remove(wp, &dir, NULL, NULL);

test/functional/api/tabpage_spec.lua

@@ -1,5 +1,7 @@
 local helpers = require('test.functional.helpers')(after_each)
 local clear, eq, ok = helpers.clear, helpers.eq, helpers.ok
+local exec = helpers.exec
+local feed = helpers.feed
 local api = helpers.api
 local fn = helpers.fn
 local request = helpers.request
@@ -86,6 +88,30 @@ describe('api/tabpage', function()
         pcall_err(api.nvim_tabpage_set_win, tab1, win3)
       )
     end)
+
+    it('does not switch window when textlocked or in the cmdwin', function()
+      local target_win = api.nvim_get_current_win()
+      feed('q:')
+      local cur_win = api.nvim_get_current_win()
+      eq(
+        'Vim:E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+        pcall_err(api.nvim_tabpage_set_win, 0, target_win)
+      )
+      eq(cur_win, api.nvim_get_current_win())
+      command('quit!')
+
+      exec(([[
+        new
+        call setline(1, 'foo')
+        setlocal debug=throw indentexpr=nvim_tabpage_set_win(0,%d)
+      ]]):format(target_win))
+      cur_win = api.nvim_get_current_win()
+      eq(
+        'Vim(normal):E5555: API call: Vim:E565: Not allowed to change text or change window',
+        pcall_err(command, 'normal! ==')
+      )
+      eq(cur_win, api.nvim_get_current_win())
+    end)
   end)
 
   describe('{get,set,del}_var', function()

test/functional/api/window_spec.lua

@@ -2297,29 +2297,82 @@ describe('API/win', function()
       try_move_t2_wins_to_t1()
     end)
 
-    it('does not switch window when textlocked or in the cmdwin', function()
+    it('handles cmdwin and textlock restrictions', function()
       command('tabnew')
+      local t2 = api.nvim_get_current_tabpage()
       local t2_win = api.nvim_get_current_win()
       command('tabfirst')
+      local t1_move_win = api.nvim_get_current_win()
+      command('split')
+
+      -- Can't move the cmdwin, or its old curwin to a different tabpage.
+      local old_curwin = api.nvim_get_current_win()
       feed('q:')
-      local cur_win = api.nvim_get_current_win()
       eq(
-        'Failed to switch away from window ' .. cur_win,
+        'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
         pcall_err(api.nvim_win_set_config, 0, { split = 'left', win = t2_win })
       )
       eq(
         'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
-        api.nvim_get_vvar('errmsg')
+        pcall_err(api.nvim_win_set_config, old_curwin, { split = 'left', win = t2_win })
       )
-      eq(cur_win, api.nvim_get_current_win())
+      -- But we can move other windows.
+      api.nvim_win_set_config(t1_move_win, { split = 'left', win = t2_win })
+      eq(t2, api.nvim_win_get_tabpage(t1_move_win))
       command('quit!')
 
+      -- Can't configure windows such that the cmdwin would become the only non-float.
+      command('only!')
+      feed('q:')
+      eq(
+        'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+        pcall_err(
+          api.nvim_win_set_config,
+          old_curwin,
+          { relative = 'editor', row = 0, col = 0, width = 5, height = 5 }
+        )
+      )
+      -- old_curwin is now no longer the only other non-float, so we can make it floating now.
+      local t1_new_win = api.nvim_open_win(
+        api.nvim_create_buf(true, true),
+        false,
+        { split = 'left', win = old_curwin }
+      )
+      api.nvim_win_set_config(
+        old_curwin,
+        { relative = 'editor', row = 0, col = 0, width = 5, height = 5 }
+      )
+      eq('editor', api.nvim_win_get_config(old_curwin).relative)
+      -- ...which means we shouldn't be able to also make the new window floating too!
+      eq(
+        'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+        pcall_err(
+          api.nvim_win_set_config,
+          t1_new_win,
+          { relative = 'editor', row = 0, col = 0, width = 5, height = 5 }
+        )
+      )
+      -- Nothing ought to stop us from making the cmdwin itself floating, though...
+      api.nvim_win_set_config(0, { relative = 'editor', row = 0, col = 0, width = 5, height = 5 })
+      eq('editor', api.nvim_win_get_config(0).relative)
+      -- We can't make our new window from before floating too, as it's now the only non-float.
+      eq(
+        'Cannot change last window into float',
+        pcall_err(
+          api.nvim_win_set_config,
+          t1_new_win,
+          { relative = 'editor', row = 0, col = 0, width = 5, height = 5 }
+        )
+      )
+      command('quit!')
+
+      -- Can't switch away from window before moving it to a different tabpage during textlock.
       exec(([[
         new
         call setline(1, 'foo')
         setlocal debug=throw indentexpr=nvim_win_set_config(0,#{split:'left',win:%d})
       ]]):format(t2_win))
-      cur_win = api.nvim_get_current_win()
+      local cur_win = api.nvim_get_current_win()
       matches(
         'E565: Not allowed to change text or change window$',
         pcall_err(command, 'normal! ==')