patch 9.0.0389: crash when 'tagfunc' closes the window

Problem:    Crash when 'tagfunc' closes the window.
Solution:   Bail out when the window was closed.

src/errors.h

@@ -3324,3 +3324,5 @@ EXTERN char e_non_null_dict_required_for_argument_nr[]
 EXTERN char e_non_null_list_required_for_argument_nr[]
 	INIT(= N_("E1298: Non-NULL List required for argument %d"));
 #endif
+EXTERN char e_window_unexpectedly_close_while_searching_for_tags[]
+	INIT(= N_("E1299: Window unexpectedly closed while searching for tags"));

src/tag.c

@@ -690,6 +690,16 @@ do_tag(
 		max_num_matches = MAXCOL; // If less than max_num_matches
 					  // found: all matches found.
 
+	    // A tag function may do anything, which may cause various
+	    // information to become invalid.  At least check for the tagstack
+	    // to still be the same.
+	    if (tagstack != curwin->w_tagstack)
+	    {
+		emsg(_(e_window_unexpectedly_close_while_searching_for_tags));
+		FreeWild(new_num_matches, new_matches);
+		break;
+	    }
+
 	    // If there already were some matches for the same name, move them
 	    // to the start.  Avoids that the order changes when using
 	    // ":tnext" and jumping to another file.

src/testdir/test_tagfunc.vim

@@ -401,4 +401,17 @@ func Test_tagfunc_wipes_buffer()
   set tagfunc=
 endfunc
 
+func Test_tagfunc_closes_window()
+  split any
+  func MytagfuncClose(pat, flags, info)
+    close
+    return [{'name' : 'mytag', 'filename' : 'Xtest', 'cmd' : '1'}]
+  endfunc
+  set tagfunc=MytagfuncClose
+  call assert_fails('tag xyz', 'E1299:')
+
+  set tagfunc=
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -703,6 +703,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    389,
 /**/
     388,
 /**/