Suppress debug output when completing

Any log output interferes with comletion.
We already refrain from installing our verbose
log handlers in this case, but we are still
vulnerable to GLib default message handling,
e.g. via the G_MESSAGES_DEBUG environment
variable. To avoid that, explicitly install
a message handler that suppresses all output.

Closes: flatpak/flatpak#2860

Closes: #2862
Approved by: alexlarsson

app/flatpak-main.c

@@ -182,6 +182,14 @@ message_handler (const gchar   *log_domain,
   g_printerr ("F: %s\n", message);
 }
 
+static void
+no_message_handler (const char     *log_domain,
+                    GLogLevelFlags  log_level,
+                    const char     *message,
+                    gpointer        user_data)
+{
+}
+
 static GOptionContext *
 flatpak_option_context_new_with_commands (FlatpakCommand *commands)
 {
@@ -320,7 +328,11 @@ flatpak_option_context_parse (GOptionContext     *context,
     return FALSE;
 
   /* We never want verbose output in the complete case, that breaks completion */
-  if (!is_in_complete)
+  if (is_in_complete)
+    {
+      g_log_set_default_handler (no_message_handler, NULL);
+    }
+  else
     {
       if (opt_verbose > 0)
         g_log_set_handler (G_LOG_DOMAIN, G_LOG_LEVEL_DEBUG, message_handler, NULL);