Subject: bug#47244: 28.0.50; SIGSEGV in long-runnning Emacs

From: martin rudalics <[email-masked]>
Date: Wed, 7 Apr 2021 11:00:36 +0200

> Just a shot in the dark here.  The only way I can think of this
 > occurring (Vwindow_list being too small), is that something modified
 > Vwindow_list when we don't expect it to be modified.  I looked through
 > all the uses of Vwindow_list and window_list(), and they all look
 > kosher.  The only modification seems to be setting Vwindow_list to nil.
 > But window_list() does call into lisp (nconc2, which calls Fnconc) when
 > constructing Vwindow_list, and it uses Vwindow_list as a temporary in
 > the process.  Is it possible that during that call something happens
 > asynchronously (via gc?) that calls something that sets Vwindow_list
 > to nil again, while Vwindow_list is being constructed?

Improbable but I attached some code that could detect such an
incidence.  Put a breakpoint at the

	Vwindow_list = window_list_2;

line.

 > I don't think
 > so, off hand, but I'm grasping at straws.

I also added a variable called `window-list-lengths' which records the
last 20 or so last changes in the number of windows seen by window_list.
Maybe you can detect some strange 2 ~> 6 or 6 ~> 2 change while running
Gnus.  I'm still not sure whether your daemon setup could be involved
somehow.

martin
diff --git a/src/buffer.c b/src/buffer.c
index 8e33162989..70b37e61a2 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1951,6 +1951,9 @@ DEFUN ("kill-buffer", Fkill_buffer, Skill_buffer, 0, 1, "bKill buffer: ",
      This gets rid of them for certain.  */
   reset_buffer_local_variables (b, 1);
 
+  if (!b->base_buffer)
+    eassert (b->window_count == 0);
+
   bset_name (b, Qnil);
 
   block_input ();
@@ -1991,6 +1994,8 @@ DEFUN ("kill-buffer", Fkill_buffer, Skill_buffer, 0, 1, "bKill buffer: ",
 
   run_buffer_list_update_hook (b);
 
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   return Qt;
 }
 
@@ -2151,6 +2156,9 @@ set_buffer_internal_1 (register struct buffer *b)
 
   BUFFER_CHECK_INDIRECTION (b);
 
+  if (!BUFFER_LIVE_P (b))
+    error ("Selecting deleted buffer");
+
   old_buf = current_buffer;
   current_buffer = b;
   last_known_column_point = -1;   /* Invalidate indentation cache.  */
diff --git a/src/window.c b/src/window.c
index eb16e2a433..16c75a13d5 100644
--- a/src/window.c
+++ b/src/window.c
@@ -307,7 +307,11 @@ wset_buffer (struct window *w, Lisp_Object val)
   if (BUFFERP (val))
     /* Make sure that we do not assign the buffer
        to an internal window.  */
-    eassert (MARKERP (w->start) && MARKERP (w->pointm));
+    {
+      eassert (BUFFER_LIVE_P (XBUFFER (val)));
+      eassert (MARKERP (w->start) && MARKERP (w->pointm));
+    }
+
   w->contents = val;
   adjust_window_count (w, 1);
 }
@@ -315,6 +319,7 @@ wset_buffer (struct window *w, Lisp_Object val)
 static void
 wset_old_buffer (struct window *w, Lisp_Object val)
 {
+  eassert (BUFFER_LIVE_P (XBUFFER (val)));
   w->old_buffer = val;
 }
 
@@ -594,6 +599,8 @@ select_window_1 (Lisp_Object window, bool inhibit_point_swap)
 
   selected_window = window;
 
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   /* Go to the point recorded in the window.
      This is important when the buffer is in more
      than one window.  It also matters when
@@ -2556,6 +2563,7 @@ window_list (void)
   if (!CONSP (Vwindow_list))
     {
       Lisp_Object tail, frame;
+      Lisp_Object window_list_2 = Qnil;
 
       Vwindow_list = Qnil;
       FOR_EACH_FRAME (tail, frame)
@@ -2569,6 +2577,42 @@ window_list (void)
 	  arglist = Fnreverse (arglist);
 	  Vwindow_list = nconc2 (Vwindow_list, arglist);
 	}
+
+      if (NILP (window_list_lengths))
+	window_list_lengths
+	  = Fcons (make_fixnum (list_length (Vwindow_list)), Qnil);
+      else
+	{
+	  int old_length = XFIXNUM (Fcar (window_list_lengths));
+	  int new_length = list_length (Vwindow_list);
+
+	  if (old_length != new_length)
+	    {
+	      Lisp_Object twentieth;
+
+	      window_list_lengths
+		= Fcons (make_fixnum (new_length), window_list_lengths);
+
+	      /* Crop to 20 elements.  */
+	      twentieth = Fnthcdr (make_fixnum (20), window_list_lengths);
+
+	      if (!NILP (twentieth))
+		XSETCDR (twentieth, Qnil);
+	    }
+	}
+
+      FOR_EACH_FRAME (tail, frame)
+	{
+	  Lisp_Object arglist = Qnil;
+
+	  foreach_window (XFRAME (frame), add_window_to_list, &arglist);
+	  arglist = Fnreverse (arglist);
+	  window_list_2 = nconc2 (window_list_2, arglist);
+	}
+
+      if (NILP (Fequal (Vwindow_list, window_list_2)))
+	Vwindow_list = window_list_2;
+
     }
 
   return Vwindow_list;
@@ -2950,6 +2994,8 @@ DEFUN ("window-list-1", Fwindow_list_1, Swindow_list_1, 0, 3, 0,
   CHECK_ALL_WINDOWS                 /* Arg is ignored */
 };
 
+static int replace_buffer_in_windows_safely_count;
+
 static Lisp_Object
 window_loop (enum window_loop type, Lisp_Object obj, bool mini,
 	     Lisp_Object frames)
@@ -3048,6 +3094,8 @@ window_loop (enum window_loop type, Lisp_Object obj, bool mini,
 		if (EQ (window, selected_window)
 		    && XBUFFER (w->contents) == current_buffer)
 		  Fset_buffer (w->contents);
+
+		replace_buffer_in_windows_safely_count += 1;
 	      }
 	    break;
 
@@ -3411,6 +3459,8 @@ DEFUN ("delete-other-windows-internal", Fdelete_other_windows_internal,
 replace_buffer_in_windows (Lisp_Object buffer)
 {
   call1 (Qreplace_buffer_in_windows, buffer);
+
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
 }
 
 /* If BUFFER is shown in a window, safely replace it with some other
@@ -3419,7 +3469,28 @@ replace_buffer_in_windows (Lisp_Object buffer)
 void
 replace_buffer_in_windows_safely (Lisp_Object buffer)
 {
-  if (buffer_window_count (XBUFFER (buffer)))
+  if (!NILP (Fstring_equal (XBUFFER (buffer)->name_,
+			    build_string ("*Server*"))))
+    {
+      int count = buffer_window_count (XBUFFER (buffer));
+
+      replace_buffer_in_windows_safely_count = 0;
+
+
+      if (buffer_window_count (XBUFFER (buffer)))
+	{
+	  Lisp_Object tail, frame;
+
+	  /* A single call to window_loop won't do the job because it only
+	     considers frames on the current keyboard.  So loop manually over
+	     frames, and handle each one.  */
+	  FOR_EACH_FRAME (tail, frame)
+	    window_loop (REPLACE_BUFFER_IN_WINDOWS_SAFELY, buffer, true, frame);
+	}
+
+      eassert (count == replace_buffer_in_windows_safely_count);
+    }
+  else if (buffer_window_count (XBUFFER (buffer)))
     {
       Lisp_Object tail, frame;
 
@@ -3429,6 +3500,7 @@ replace_buffer_in_windows_safely (Lisp_Object buffer)
       FOR_EACH_FRAME (tail, frame)
 	window_loop (REPLACE_BUFFER_IN_WINDOWS_SAFELY, buffer, true, frame);
     }
+
 }
 
 /* The following three routines are needed for running a window's
@@ -3960,6 +4032,8 @@ run_window_change_functions (void)
   /* Record changes for all frames (if asked for), selected window and
      frame.  */
   unbind_to (count, Qnil);
+
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
 }
 
 /* Make WINDOW display BUFFER.  RUN_HOOKS_P means it's allowed
@@ -4054,6 +4128,9 @@ set_window_buffer (Lisp_Object window, Lisp_Object buffer,
   if (!samebuf && !MINI_WINDOW_P (w) && !WINDOW_PSEUDO_P (w))
     FRAME_WINDOW_CHANGE (XFRAME (w->frame)) = true;
 
+  if (WINDOWP (selected_window))
+    eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   unbind_to (count, Qnil);
 }
 
@@ -5174,6 +5251,8 @@ DEFUN ("delete-window-internal", Fdelete_window_internal, Sdelete_window_interna
       error ("Deletion failed");
     }
 
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   return Qnil;
 }
 
@@ -7233,6 +7312,9 @@ DEFUN ("set-window-configuration", Fset_window_configuration,
   minibuf_selected_window = data->minibuf_selected_window;
 
   SAFE_FREE ();
+
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   return FRAME_LIVE_P (f) ? Qt : Qnil;
 }
 
@@ -8479,6 +8561,10 @@ syms_of_window (void)
 displayed after a scrolling operation to be somewhat inaccurate.  */);
   fast_but_imprecise_scrolling = false;
 
+  DEFVAR_LISP ("window-list-lengths", window_list_lengths,
+               doc: /* Last lengths of `window-list'.  */);
+  window_list_lengths = Qnil;
+
   defsubr (&Sselected_window);
   defsubr (&Sold_selected_window);
   defsubr (&Sminibuffer_window);
diff --git a/src/xdisp.c b/src/xdisp.c
index cc0a689ba3..ea64e5037a 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -12980,6 +12980,8 @@ restore_selected_window (Lisp_Object window)
       /* Abort if we cannot find a live frame.  */
       emacs_abort ();
     }
+
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
 }
 
 /* Restore WINDOW, if live, as its frame's selected window.  */
@@ -12997,6 +12999,8 @@ restore_frame_selected_window (Lisp_Object window)
       if (EQ (frame, selected_frame))
 	selected_window = window;
     }
+
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
 }
 
 /* Update the tab-bar item list for frame F.  This has to be done
@@ -13074,6 +13078,8 @@ update_tab_bar (struct frame *f, bool save_match_data)
 	  XSETFRAME (frame, f);
 	  selected_frame = frame;
 	  selected_window = FRAME_SELECTED_WINDOW (f);
+
+	  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
 #endif
 
 	  /* Build desired tab-bar items from keymaps.  */
@@ -14010,6 +14016,8 @@ update_tool_bar (struct frame *f, bool save_match_data)
 	  selected_frame = frame;
 	  selected_window = FRAME_SELECTED_WINDOW (f);
 
+	  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
 	  /* Build desired tool-bar items from keymaps.  */
           new_tool_bar
 	    = tool_bar_items (Fcopy_sequence (f->tool_bar_items),
@@ -17246,6 +17254,8 @@ run_window_scroll_functions (Lisp_Object window, struct text_pos startp)
       set_buffer_internal (XBUFFER (w->contents));
     }
 
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   return startp;
 }
 
@@ -25450,6 +25460,8 @@ display_mode_lines (struct window *w)
 
   unbind_to (count, Qnil);
 
+  eassert (BUFFER_LIVE_P (XBUFFER (XWINDOW (selected_window)->contents)));
+
   if (n > 0)
     w->must_be_updated_p = true;
   return n;