patch 8.1.0066: nasty autocommand causes using freed memory

Problem:    Nasty autocommand causes using freed memory. (Dominique Pelle)
Solution:   Do not force executing autocommands if the value of 'syntax' or
            'filetype' did not change.

Author: brammool

src/option.c

@@ -6029,7 +6029,7 @@ did_set_string_option(
     /* set when changing an option that only requires a redraw in the GUI */
     int		redraw_gui_only = FALSE;
 #endif
-    int		ft_changed = FALSE;
+    int		value_changed = FALSE;
 #if defined(FEAT_VTP) && defined(FEAT_TERMGUICOLORS)
     int		did_swaptcap = FALSE;
 #endif
@@ -7437,14 +7437,16 @@ did_set_string_option(
 	if (!valid_filetype(*varp))
 	    errmsg = e_invarg;
 	else
-	    ft_changed = STRCMP(oldval, *varp) != 0;
+	    value_changed = STRCMP(oldval, *varp) != 0;
     }
 
 #ifdef FEAT_SYN_HL
     else if (gvarp == &p_syn)
     {
 	if (!valid_filetype(*varp))
 	    errmsg = e_invarg;
+	else
+	    value_changed = STRCMP(oldval, *varp) != 0;
     }
 #endif
 
@@ -7565,20 +7567,24 @@ did_set_string_option(
 	/* When 'syntax' is set, load the syntax of that name */
 	if (varp == &(curbuf->b_p_syn))
 	{
+	    // Only pass TRUE for "force" when the value changed, to avoid
+	    // endless recurrence. */
 	    apply_autocmds(EVENT_SYNTAX, curbuf->b_p_syn,
-					       curbuf->b_fname, TRUE, curbuf);
+				       curbuf->b_fname, value_changed, curbuf);
 	}
 #endif
 	else if (varp == &(curbuf->b_p_ft))
 	{
 	    /* 'filetype' is set, trigger the FileType autocommand.
 	     * Skip this when called from a modeline and the filetype was
-	     * already set to this value. */
-	    if (!(opt_flags & OPT_MODELINE) || ft_changed)
+	     * already set to this value.
+	     * Only pass TRUE for "force" when the value changed, to avoid
+	     * endless recurrence. */
+	    if (!(opt_flags & OPT_MODELINE) || value_changed)
 	    {
 		did_filetype = TRUE;
 		apply_autocmds(EVENT_FILETYPE, curbuf->b_p_ft,
-					       curbuf->b_fname, TRUE, curbuf);
+				       curbuf->b_fname, value_changed, curbuf);
 		/* Just in case the old "curbuf" is now invalid. */
 		if (varp != &(curbuf->b_p_ft))
 		    varp = NULL;

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    66,
 /**/
     65,
 /**/