background: Call malloc_trim() after loading background image

Some of the new JXL backgrounds from gnome-backgrounds can allocate
several hundred MB during loading, so let's call malloc_trim() after we
are done with loading the image into a pixbuf.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3327
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3653>

config.h.meson

@@ -132,3 +132,6 @@
 
 /* Supports timerfd_create/timerfd_settime */
 #mesondefine HAVE_TIMERFD
+
+/* Supports malloc_trim */
+#mesondefine HAVE_MALLOC_TRIM

meson.build

@@ -345,6 +345,8 @@ int main (int argc, char ** argv) {
 }
 ''', name : 'timerfd_create(2) system call')
 
+have_malloc_trim = meson.get_compiler('c').has_function('malloc_trim')
+
 have_documentation = get_option('docs')
 have_tests = get_option('tests')
 have_core_tests = false
@@ -570,6 +572,7 @@ cdata.set('HAVE_PROFILER', have_profiler)
 cdata.set('HAVE_LIBDISPLAY_INFO', have_libdisplay_info)
 cdata.set('HAVE_PANGO_FT2', have_pango_ft2)
 cdata.set('HAVE_TIMERFD', have_timerfd)
+cdata.set('HAVE_MALLOC_TRIM', have_malloc_trim)
 
 if have_x11_client
   xkb_base = xkeyboard_config_dep.get_variable('xkb_base')

src/compositor/meta-background-image.c

@@ -23,6 +23,10 @@
 #include <gdk-pixbuf/gdk-pixbuf.h>
 #include <gio/gio.h>
 
+#ifdef HAVE_MALLOC_TRIM
+#include <malloc.h>
+#endif
+
 #include "clutter/clutter.h"
 #include "compositor/cogl-utils.h"
 
@@ -135,6 +139,10 @@ load_file (GTask               *task,
   pixbuf = gdk_pixbuf_new_from_stream (G_INPUT_STREAM (stream), NULL, &error);
   g_object_unref (stream);
 
+#ifdef HAVE_MALLOC_TRIM
+  malloc_trim (0);
+#endif
+
   if (pixbuf == NULL)
     {
       g_task_return_error (task, error);