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

From: martin rudalics <[email-masked]>
Date: Thu, 8 Apr 2021 20:52:29 +0200

> I hit the breakpoint in quit.  It looks like Fnconc uses FOR_EACH_TAIL,
 > which uses FOR_EACH_TAIL_INTENAL, which calls maybe_quit.

Not just around the corner but near.  Well done.

 > The question
 > in my mind now is whether block/unblock_input belongs in window_list or
 > in Fnconc.

In window_list and we then should bind inhibit_quit IIUC.  Fnconc must be
able to deal with circular lists.  I attach a patch.

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/nsfns.m b/src/nsfns.m
index 5c4cc915e7..82136b0639 100644
--- a/src/nsfns.m
+++ b/src/nsfns.m
@@ -1447,7 +1447,7 @@ Turn the input menu (an NSMenu) into a lisp list for tracking on lisp side.
 
   /* Make sure windows on this frame appear in calls to next-window
      and similar functions.  */
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 
   return unbind_to (count, frame);
 }
diff --git a/src/w32fns.c b/src/w32fns.c
index 9db367bfaf..f2d5e34c3e 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -6148,7 +6148,7 @@ DEFUN ("x-create-frame", Fx_create_frame, Sx_create_frame,
 
   /* Make sure windows on this frame appear in calls to next-window
      and similar functions.  */
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 
   return unbind_to (count, frame);
 }
diff --git a/src/window.c b/src/window.c
index eb16e2a433..c858ddb4ff 100644
--- a/src/window.c
+++ b/src/window.c
@@ -86,6 +86,7 @@
    Functions creating or deleting windows should invalidate this cache
    by setting it to nil.  */
 Lisp_Object Vwindow_list;
+Lisp_Object Vwindow_list_2;
 
 /* True mean window_change_record has to record all live frames.  */
 static bool window_change_record_frames;
@@ -307,7 +308,11 @@ 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 +320,7 @@ 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 +600,8 @@ 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
@@ -2556,8 +2564,10 @@ window_list (void)
   if (!CONSP (Vwindow_list))
     {
       Lisp_Object tail, frame;
+      ptrdiff_t count = SPECPDL_INDEX ();
 
       Vwindow_list = Qnil;
+      specbind (Qinhibit_quit, Qt);
       FOR_EACH_FRAME (tail, frame)
 	{
 	  Lisp_Object arglist = Qnil;
@@ -2569,7 +2579,12 @@ window_list (void)
 	  arglist = Fnreverse (arglist);
 	  Vwindow_list = nconc2 (Vwindow_list, arglist);
 	}
+
+      Vwindow_list_2 = Fcopy_sequence (Vwindow_list);
+      unbind_to (count, Qnil);
     }
+  else
+    eassert (!NILP (Fequal (Vwindow_list, Vwindow_list_2)));
 
   return Vwindow_list;
 }
@@ -2950,6 +2965,8 @@ 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 +3065,8 @@ 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;
 
@@ -3277,7 +3296,7 @@ DEFUN ("delete-other-windows-internal", Fdelete_other_windows_internal,
   free_window_matrices (r);
 
   fset_redisplay (f);
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 
   if (!WINDOW_LEAF_P (w))
     {
@@ -3411,6 +3430,8 @@ 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 +3440,28 @@ 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;
 
@@ -3960,6 +4002,8 @@ 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 +4098,9 @@ 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);
 }
 
@@ -4296,7 +4343,7 @@ make_window (void)
   w->scroll_bar_height = -1;
   w->column_number_displayed = -1;
   /* Reset window_list.  */
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
   /* Return window.  */
   XSETWINDOW (window, w);
   return window;
@@ -5086,7 +5133,7 @@ DEFUN ("delete-window-internal", Fdelete_window_internal, Sdelete_window_interna
 	}
 
       fset_redisplay (f);
-      Vwindow_list = Qnil;
+      Vwindow_list = Vwindow_list_2 = Qnil;
 
       wset_next (w, Qnil);  /* Don't delete w->next too.  */
       free_window_matrices (w);
@@ -5174,6 +5221,8 @@ 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 +7282,9 @@ 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;
 }
 
@@ -7279,7 +7331,7 @@ delete_all_child_windows (Lisp_Object window)
       wset_buffer (w, Qnil);
     }
 
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 }
 
 static ptrdiff_t
@@ -8128,8 +8180,9 @@ init_window_once (void)
   selected_window = Qnil;
   staticpro (&selected_window);
 
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
   staticpro (&Vwindow_list);
+  staticpro (&Vwindow_list_2);
 
   minibuf_selected_window = Qnil;
   staticpro (&minibuf_selected_window);
@@ -8148,6 +8201,7 @@ init_window_once (void)
   PDUMPER_RESET_LV (minibuf_window, Qnil);
   PDUMPER_RESET_LV (selected_window, Qnil);
   PDUMPER_RESET_LV (Vwindow_list, Qnil);
+  PDUMPER_RESET_LV (Vwindow_list_2, Qnil);
   PDUMPER_RESET_LV (minibuf_selected_window, Qnil);
 
   /* Hack: if mode_line_in_non_selected_windows is true (which it may
@@ -8178,7 +8232,7 @@ init_window_once (void)
 void
 init_window (void)
 {
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 }
 
 void
diff --git a/src/window.h b/src/window.h
index b6f88e8f55..74d3cbb7be 100644
--- a/src/window.h
+++ b/src/window.h
@@ -1171,6 +1171,7 @@ #define CHECK_LIVE_WINDOW(WINDOW)				\
 
 /* These used to be in lisp.h.  */
 extern Lisp_Object Vwindow_list;
+extern Lisp_Object Vwindow_list_2;
 
 extern Lisp_Object window_list (void);
 extern Lisp_Object window_parameter (struct window *, Lisp_Object parameter);
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;
diff --git a/src/xfns.c b/src/xfns.c
index d90644819b..fbe9014555 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -4225,7 +4225,7 @@ DEFUN ("x-create-frame", Fx_create_frame, Sx_create_frame,
 
   /* Make sure windows on this frame appear in calls to next-window
      and similar functions.  */
-  Vwindow_list = Qnil;
+  Vwindow_list = Vwindow_list_2 = Qnil;
 
  return unbind_to (count, frame);
 }