Fix segfault with mode "default" key bindings (#5007)

ignore bindings when not in a valid mode

Co-authored-by: Ivan Zharov <zhiv.email@gmail.com>
Co-authored-by: Michael Stapelberg <stapelberg@users.noreply.github.com>

release-notes/bugfixes/8-mode-default-sigsegv

@@ -0,0 +1 @@
+Fix segfault with explicit mode "default" key bindings.

src/config_directives.c

@@ -180,6 +180,11 @@ static char *current_mode;
 static bool current_mode_pango_markup;
 
 CFGFUN(mode_binding, const char *bindtype, const char *modifiers, const char *key, const char *release, const char *border, const char *whole_window, const char *exclude_titlebar, const char *command) {
+    if (current_mode == NULL) {
+        /* When using an invalid mode name, e.g. “default” */
+        return;
+    }
+
     configure_binding(bindtype, modifiers, key, release, border, whole_window, exclude_titlebar, command, current_mode, current_mode_pango_markup);
 }
 

testcases/t/547-explicit-mode-default.t

@@ -0,0 +1,34 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test that explicitly defined default mode doesn't cause segfault.
+# Ticket: #5006
+# Bug still in: 4.20-96-gce2665ca
+
+use i3test i3_autostart => 0;
+
+my $config = <<EOT;
+# i3 config file (v4)
+mode "default" {
+    bindsym X resize
+}
+EOT
+
+my $pid = launch_with_config($config);
+does_i3_live;
+
+exit_gracefully($pid);
+done_testing;