util: cache local GTimeZone

This ensures that we do not create a new GTimeZone with
g_time_zone_new_local() repeatedly. Currently, that will cause GTimeZone
to open(), mmap() and parse /etc/localtime while on the main thread.

We already track timezone changes, so we can cache this value and reuse
it if we:

 1) Clear the cache when timezone changes
 2) Use the only GDateTime API available to us here, which means we
    imply the current time. But this is how environment.js uses the
    date and time anyway, so no loss.

We maintain the old form for plugin compatibility.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2279

js/ui/dateMenu.js

@@ -573,6 +573,11 @@ var DateMenuButton = new Lang.Class({
         // https://bugzilla.gnome.org/show_bug.cgi?id=678507
         System.clearDateCaches();
 
+        // To reduce the number of times that Shell opens /etc/localtime on
+        // the main thread to parse timezone info, we cache the result. That
+        // needs to be cleared when we change timezones.
+        Shell.util_clear_timezone_cache();
+
         this._calendar.updateTimeZone();
     },
 

js/ui/environment.js

@@ -118,7 +118,7 @@ function init() {
 
     // Work around https://bugzilla.mozilla.org/show_bug.cgi?id=508783
     Date.prototype.toLocaleFormat = function(format) {
-        return Shell.util_format_date(format, this.getTime());
+        return Shell.util_format_now(format);
     };
 
     let slowdownEnv = GLib.getenv('GNOME_SHELL_SLOWDOWN_FACTOR');

src/shell-util.c

@@ -20,6 +20,8 @@
 #include <langinfo.h>
 #endif
 
+static GTimeZone *local_tz;
+
 static void
 stop_pick (ClutterActor       *actor,
            const ClutterColor *color)
@@ -154,6 +156,30 @@ shell_util_format_date (const char *format,
   return result;
 }
 
+char *
+shell_util_format_now (const char *format)
+{
+  GDateTime *datetime;
+  char *ret;
+
+  if (local_tz == NULL)
+    local_tz = g_time_zone_new_local ();
+
+  datetime = g_date_time_new_now (local_tz);
+  if (!datetime)
+    return g_strdup ("");
+
+  ret = g_date_time_format (datetime, format);
+  g_date_time_unref (datetime);
+  return ret;
+}
+
+void
+shell_util_clear_timezone_cache (void)
+{
+  g_clear_pointer (&local_tz, g_time_zone_unref);
+}
+
 /**
  * shell_util_get_week_start:
  *

src/shell-util.h

@@ -22,6 +22,8 @@ int      shell_util_get_week_start             (void);
 
 char    *shell_util_format_date                (const char       *format,
                                                 gint64            time_ms);
+char    *shell_util_format_now                 (const char       *format);
+void     shell_util_clear_timezone_cache       (void);
 const char *shell_util_translate_time_string   (const char *str);
 
 char    *shell_util_regex_escape               (const char *str);