[gnome-shell/wip/chergert/3-28-perf-fixes: 3/4] environment: reduce calls to g_time_zone_new_local()

[Christian Hergert <chergert src gnome org>]

commit e63fab0cb08e0d3ae5dba433a08b2b655314abbe
Author: Christian Hergert <chergert redhat com>
Date:   Thu Feb 27 19:43:58 2020 -0800

    environment: reduce calls to g_time_zone_new_local()
    
    Creating a new GTimeZone for the local timezone can be quite expensive if
    done repeatedly. It requires an open(), mmap(), and parsing of
    /etc/localtime.
    
    This patch was provided by Florian, and I've tested it as far back as
    3.28.4 to ensure that we are really reducing the number of open() calls
    on the compositor thread.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1051
    
    Signed-off-by: Christian Hergert <chergert redhat com>
    
    # Conflicts:
    #       js/ui/environment.js

 js/ui/environment.js | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)
---
diff --git a/js/ui/environment.js b/js/ui/environment.js
index 719680f615..78a24f888f 100644
--- a/js/ui/environment.js
+++ b/js/ui/environment.js
@@ -17,6 +17,9 @@ const Gtk = imports.gi.Gtk;
 const Lang = imports.lang;
 const Shell = imports.gi.Shell;
 const St = imports.gi.St;
+const System = imports.system;
+
+let _localTimeZone = null;
 
 // We can't import shell JS modules yet, because they may have
 // variable initializations, etc, that depend on init() already having
@@ -116,9 +119,26 @@ function init() {
         }
     };
 
+    // Override to clear our own timezone cache as well
+    const origClearDateCaches = System.clearDateCaches;
+    System.clearDateCaches = function () {
+        _localTimeZone = null;
+        origClearDateCaches();
+    };
+
     // Work around https://bugzilla.mozilla.org/show_bug.cgi?id=508783
     Date.prototype.toLocaleFormat = function(format) {
-        return Shell.util_format_date(format, this.getTime());
+        if (_localTimeZone === null)
+            _localTimeZone = GLib.TimeZone.new_local();
+
+        let dt = GLib.DateTime.new(_localTimeZone,
+            this.getYear(),
+            this.getMonth() + 1,
+            this.getDate(),
+            this.getHours(),
+            this.getMinutes(),
+            this.getSeconds());
+        return dt ? dt.format(format) : '';
     };
 
     let slowdownEnv = GLib.getenv('GNOME_SHELL_SLOWDOWN_FACTOR');