patch 9.0.0873: using freed memory when executing mapclear at more prompt

Problem:    Using freed memory when executing mapclear at the more prompt.
Solution:   Do not clear mappings while listing them. (closes #11438)

src/errors.h

@@ -3333,3 +3333,5 @@ EXTERN char e_argument_nr_trying_to_modify_const_str[]
 EXTERN char e_cannot_resize_window_in_another_tab_page[]
 	INIT(= N_("E1308: Cannot resize a window in another tab page"));
 #endif
+EXTERN char e_cannot_change_mappings_while_listing[]
+	INIT(= N_("E1309: Cannot change mappings while listing"));

src/map.c

@@ -24,6 +24,10 @@ static mapblock_T	*first_abbr = NULL; // first entry in abbrlist
 static mapblock_T	*(maphash[256]);
 static int		maphash_valid = FALSE;
 
+// When non-zero then no mappings can be added or removed.  Prevents mappings
+// to change while listing them.
+static int		map_locked = 0;
+
 /*
  * Make a hash value for a mapping.
  * "mode" is the lower 4 bits of the State for the mapping.
@@ -150,11 +154,15 @@ showmap(
     if (message_filtered(mp->m_keys) && message_filtered(mp->m_str))
 	return;
 
+    // Prevent mappings to be cleared while at the more prompt.
+    // Must jump to "theend" instead of returning.
+    ++map_locked;
+
     if (msg_didout || msg_silent != 0)
     {
 	msg_putchar('\n');
 	if (got_int)	    // 'q' typed at MORE prompt
-	    return;
+	    goto theend;
     }
 
     mapchars = map_mode_to_chars(mp->m_mode);
@@ -200,6 +208,9 @@ showmap(
 #endif
     msg_clr_eos();
     out_flush();			// show one line at a time
+
+theend:
+    --map_locked;
 }
 
     static int
@@ -298,6 +309,9 @@ list_mappings(
 	int	mode,
 	int	*did_local)
 {
+    // Prevent mappings to be cleared while at the more prompt.
+    ++map_locked;
+
     if (p_verbose > 0 && keyround == 1 && seenModifyOtherKeys)
 	msg_puts(_("Seen modifyOtherKeys: true"));
 
@@ -337,6 +351,8 @@ list_mappings(
 	    }
 	}
     }
+
+    --map_locked;
 }
 
 /*
@@ -954,6 +970,21 @@ map_clear(
     map_clear_mode(curbuf, mode, local, abbr);
 }
 
+/*
+ * If "map_locked" is set then give an error and return TRUE.
+ * Otherwise return FALSE.
+ */
+    static int
+is_map_locked(void)
+{
+    if (map_locked > 0)
+    {
+	emsg(_(e_cannot_change_mappings_while_listing));
+	return TRUE;
+    }
+    return FALSE;
+}
+
 /*
  * Clear all mappings in "mode".
  */
@@ -968,6 +999,9 @@ map_clear_mode(
     int		hash;
     int		new_hash;
 
+    if (is_map_locked())
+	return;
+
     validate_maphash();
 
     for (hash = 0; hash < 256; ++hash)

src/testdir/test_mapping.vim

@@ -1774,5 +1774,29 @@ func Test_using_past_typeahead()
   nunmap :00
 endfunc
 
+func Test_mapclear_while_listing()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      set nocompatible
+      mapclear
+      for i in range(1, 999)
+        exe 'map ' .. 'foo' .. i .. ' bar'
+      endfor
+      au CmdlineLeave : call timer_start(0, {-> execute('mapclear')})
+  END
+  call writefile(lines, 'Xmapclear', 'D')
+  let buf = RunVimInTerminal('-S Xmapclear', {'rows': 10})
+
+  " this was using freed memory
+  call term_sendkeys(buf, ":map\<CR>")
+  call TermWait(buf, 50)
+  call term_sendkeys(buf, "G")
+  call TermWait(buf, 50)
+  call term_sendkeys(buf, "\<CR>")
+
+  call StopVimInTerminal(buf)
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    873,
 /**/
     872,
 /**/