fix(buffer): defer w_buffer clearing to prevent dict watcher crash #36748

(cherry picked from commit f9ef1a4cab)

src/nvim/buffer.c

@@ -679,10 +679,14 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
     return false;
   }
 
+  bool clear_w_buf = false;
   if (win != NULL  // Avoid bogus clang warning.
       && win_valid_any_tab(win)
       && win->w_buffer == buf) {
-    win->w_buffer = NULL;  // make sure we don't use the buffer now
+    // Defer clearing w_buffer until after operations that may invoke dict
+    // watchers (e.g., buf_clear_file()), so callers like tabpagebuflist()
+    // never see a window in the winlist with a NULL buffer.
+    clear_w_buf = true;
   }
 
   // Autocommands may have opened or closed windows for this buffer.
@@ -693,6 +697,9 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
 
   // Remove the buffer from the list.
   if (wipe_buf) {
+    if (clear_w_buf) {
+      win->w_buffer = NULL;
+    }
     // Do not wipe out the buffer if it is used in a window.
     if (buf->b_nwindows > 0) {
       return false;
@@ -730,6 +737,9 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
       buf->b_p_initialized = false;
     }
     buf_clear_file(buf);
+    if (clear_w_buf) {
+      win->w_buffer = NULL;
+    }
     if (del_buf) {
       buf->b_p_bl = false;
     }

test/functional/ex_cmds/dict_notifications_spec.lua

@@ -524,3 +524,50 @@ describe('Vimscript dictionary notifications', function()
     eq({ 'W1', 'W2', 'W2', 'W1' }, eval('g:calls'))
   end)
 end)
+describe('tabpagebuflist() with dict watcher during buffer close/wipe', function()
+  before_each(function()
+    clear()
+  end)
+
+  it(
+    'does not segfault when called from dict watcher on b:changedtick (bufhidden=unload)',
+    function()
+      command([[
+    new
+    set bufhidden=unload
+    call dictwatcheradd(b:, 'changedtick', {-> tabpagebuflist()})
+    close
+    ]])
+
+      assert_alive()
+    end
+  )
+
+  it('does not segfault when wiping buffer with dict watcher', function()
+    command([[
+    new
+    call setline(1, 'test')
+    call dictwatcheradd(b:, 'changedtick', {-> tabpagebuflist()})
+    bwipeout!
+    ]])
+
+    assert_alive()
+  end)
+
+  it('does not segfault with multiple windows in the tabpage', function()
+    command([[
+    " create two windows in the current tab
+    edit foo
+    vnew
+    call setline(1, 'bar')
+
+    " attach watcher to the current buffer in the split
+    call dictwatcheradd(b:, 'changedtick', {-> tabpagebuflist()})
+
+    " close the split window (triggers close_buffer on this buffer)
+    close
+    ]])
+
+    assert_alive()
+  end)
+end)