vim-patch:9.1.2085: Use-after-free in winframe_remove()

Problem:  Use-after-free in winframe_remove() (henices)
Solution: Set window_layout_locked() inside winframe_remove()
          and check that writing diff files is disallowed when the
          window layout is locked.

It can happen with a custom diff expression when removing a window:

 1. Buffer was removed, so win_frame_remove() is called to remove the
    window.
 2. win_frame_remove() → frame_new_height() → scroll_to_fraction()
    → diff_check_fill() (checks for filler lines)
 3. diff_check_fill() ends up causing a diff_try_update, and because we
    are not using internal diff, it has to first write the file to a
    buffer using buf_write()
 4. buf_write() is called for a buffer that is not contained within a
    window, so it first calls aucmd_prepbuf() to create a new temporary
    window before writing the buffer and then later calls
    aucmd_restbuf(), which restores the previous window layout, calling
    winframe_remove() again, which will free the window/frame structure,
    eventually freeing stuff that will still be accessed at step 2.

closes: vim/vim#19064

ead1dda74a

Nvim doesn't have this bug as Nvim uses a floating window as autocommand
window, and removing it doesn't need winframe_remove().

Co-authored-by: Christian Brabandt <cb@256bit.org>

test/functional/ui/diff_spec.lua

@@ -3350,3 +3350,32 @@ describe("'diffanchors'", function()
     ]])
   end)
 end)
+
+-- oldtest: Test_diffexpr_wipe_buffers()
+it(':%bwipe does not crash when using diffexpr', function()
+  local screen = Screen.new(70, 20)
+  exec([[
+    func DiffFuncExpr()
+      let in = readblob(v:fname_in)
+      let new = readblob(v:fname_new)
+      let out = v:lua.vim.text.diff(in, new)
+      call writefile(split(out, "\n"), v:fname_out)
+    endfunc
+
+    new
+    vnew
+    set diffexpr=DiffFuncExpr()
+    wincmd l
+    new
+    call setline(1,range(20))
+    windo diffthis
+    wincmd w
+    hide
+    %bw!
+  ]])
+  screen:expect([[
+    ^                                                                      |
+    {1:~                                                                     }|*18
+    4 buffers wiped out                                                   |
+  ]])
+end)