[glib/wip/antoniof/fallback-timezone-cache-lookup: 5/6] gtimezone: Cache default timezone indefinitely

[António Fernandes <antoniof src gnome org>]

commit af8b909df7391d209dabb3f1088eb06f340a64eb
Author: António Fernandes <antoniof gnome org>
Date:   Thu Oct 1 21:11:44 2020 +0100

    gtimezone: Cache default timezone indefinitely
    
    We cache GTimeZone instances to avoid expensive construction when the
    same id is requested again.
    
    However, if the NULL id is passed to g_time_zone_new(), we always
    construct a new instance for the default/fallback timezone.
    
    With the recent introduction of some heavy calculations[1], repeated
    instance construction in such cases has visible performance impact in
    nautilus list view and other such GtkTreeView consumers.
    
    To avoid this, cache reference to a constructed default timezone and
    use it the next time g_time_zone_new() is called with NULL argument,
    as long as the default identifier doesn't change. We already did the
    same for the local timezone[2].
    
    Fixes: https://gitlab.gnome.org/GNOME/glib/-/issues/2204
    
    Based on idea proposed by Sebastian Keller <skeller gnome org>.
    
    [1] 25d950b61f92f25cc9ab20d683aa4d6969f93098
    [2] 551e83662de9815d161a82c760cfa77995905740

 glib/gtimezone.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
---
diff --git a/glib/gtimezone.c b/glib/gtimezone.c
index da9c3fe65..daf5c1027 100644
--- a/glib/gtimezone.c
+++ b/glib/gtimezone.c
@@ -195,6 +195,8 @@ struct _GTimeZone
 
 G_LOCK_DEFINE_STATIC (time_zones);
 static GHashTable/*<string?, GTimeZone>*/ *time_zones;
+G_LOCK_DEFINE_STATIC (tz_default);
+static GTimeZone *tz_default = NULL;
 G_LOCK_DEFINE_STATIC (tz_local);
 static gchar *tzenv_cached = NULL;
 static GTimeZone *tz_local = NULL;
@@ -1676,6 +1678,7 @@ g_time_zone_new (const gchar *identifier)
   gchar *resolved_identifier = NULL;
 
   G_LOCK (time_zones);
+  G_LOCK (tz_default);
   if (time_zones == NULL)
     time_zones = g_hash_table_new (g_str_hash, g_str_equal);
 
@@ -1686,6 +1689,7 @@ g_time_zone_new (const gchar *identifier)
         {
           g_atomic_int_inc (&tz->ref_count);
           G_UNLOCK (time_zones);
+          G_UNLOCK (tz_default);
           return tz;
         }
       else
@@ -1698,6 +1702,19 @@ g_time_zone_new (const gchar *identifier)
 #elif defined (G_OS_WIN32)
       resolved_identifier = windows_default_tzname ();
 #endif
+      if (tz_default)
+        /* Flush default if changed */
+        if (g_strcmp0 (tz_default->name, resolved_identifier))
+          g_clear_pointer (&tz_default, g_time_zone_unref);
+        else
+          {
+            tz = g_time_zone_ref (tz_default);
+            G_UNLOCK (time_zones);
+            G_UNLOCK (tz_default);
+
+            g_free (resolved_identifier);
+            return tz;
+          }
     }
 
   tz = g_slice_new0 (GTimeZone);
@@ -1773,9 +1790,16 @@ g_time_zone_new (const gchar *identifier)
     {
       if (identifier)
         g_hash_table_insert (time_zones, tz->name, tz);
+      else if (tz->name)
+        {
+          /* Cache a reference */
+          g_atomic_int_inc (&tz->ref_count);
+          tz_default = tz;
+        }
     }
   g_atomic_int_inc (&tz->ref_count);
   G_UNLOCK (time_zones);
+  G_UNLOCK (tz_default);
 
   return tz;
 }