diff --git a/docs/reference/glib/Makefile.am b/docs/reference/glib/Makefile.am
index 4d4c8d80c..a3e716867 100644
--- a/docs/reference/glib/Makefile.am
+++ b/docs/reference/glib/Makefile.am
@@ -64,6 +64,7 @@ IGNORE_HFILES = \
 	glib-init.h		\
 	gconstructor.h		\
 	valgrind.h		\
+	gutilsprivate.h		\
 	gvalgrind.h		\
 	$(NULL)
 
diff --git a/docs/reference/glib/meson.build b/docs/reference/glib/meson.build
index eca891a20..2fb82481f 100644
--- a/docs/reference/glib/meson.build
+++ b/docs/reference/glib/meson.build
@@ -36,6 +36,7 @@ if get_option('gtk_doc')
     'glib-init.h',
     'gconstructor.h',
     'valgrind.h',
+    'gutilsprivate.h',
     'gvalgrind.h',
   ]
 
diff --git a/glib.supp b/glib.supp
index c8f3682ef..3ff1fdbc0 100644
--- a/glib.supp
+++ b/glib.supp
@@ -544,6 +544,26 @@
 	fun:g_object_new_valist
 }
 
+# g_set_user_dirs() deliberately leaks the previous cached g_get_user_*() values.
+{
+	g_set_user_dirs_str
+	Memcheck:Leak
+	fun:malloc
+	...
+	fun:set_str_if_different
+	fun:g_set_user_dirs
+}
+
+# g_set_user_dirs() deliberately leaks the previous cached g_get_user_*() values.
+{
+	g_set_user_dirs_strv
+	Memcheck:Leak
+	fun:malloc
+	...
+	fun:set_strv_if_different
+	fun:g_set_user_dirs
+}
+
 # g_get_system_data_dirs() caches a one-time allocation
 {
 	g_get_system_data_dirs
diff --git a/glib/Makefile.am b/glib/Makefile.am
index e072b8829..e433d48f3 100644
--- a/glib/Makefile.am
+++ b/glib/Makefile.am
@@ -191,6 +191,7 @@ libglib_2_0_la_SOURCES = 	\
 	gunicodeprivate.h	\
 	gurifuncs.c 		\
 	gutils.c		\
+	gutilsprivate.h		\
 	guuid.c			\
 	gvalgrind.h		\
 	gvariant.h		\
diff --git a/glib/gutils.c b/glib/gutils.c
index 51ea3426b..1e267602a 100644
--- a/glib/gutils.c
+++ b/glib/gutils.c
@@ -29,6 +29,7 @@
 #include "config.h"
 
 #include "gutils.h"
+#include "gutilsprivate.h"
 
 #include <stdarg.h>
 #include <stdlib.h>
@@ -1177,6 +1178,113 @@ g_set_application_name (const gchar *application_name)
     g_warning ("g_set_application_name() called multiple times");
 }
 
+/* Set @global_str to a copy of @new_value if it’s currently unset or has a
+ * different value. If its current value matches @new_value, do nothing. If
+ * replaced, we have to leak the old value as client code could still have
+ * pointers to it. */
+static void
+set_str_if_different (gchar       **global_str,
+                      const gchar  *type,
+                      const gchar  *new_value)
+{
+  if (*global_str == NULL ||
+      !g_str_equal (new_value, *global_str))
+    {
+      g_debug ("g_set_user_dirs: Setting %s to %s", type, new_value);
+
+      /* We have to leak the old value, as user code could be retaining pointers
+       * to it. */
+      *global_str = g_strdup (new_value);
+    }
+}
+
+static void
+set_strv_if_different (gchar                ***global_strv,
+                       const gchar            *type,
+                       const gchar  * const   *new_value)
+{
+  if (*global_strv == NULL ||
+      !g_strv_equal (new_value, (const gchar * const *) *global_strv))
+    {
+      gchar *new_value_str = g_strjoinv (":", (gchar **) new_value);
+      g_debug ("g_set_user_dirs: Setting %s to %s", type, new_value_str);
+      g_free (new_value_str);
+
+      /* We have to leak the old value, as user code could be retaining pointers
+       * to it. */
+      *global_strv = g_strdupv ((gchar **) new_value);
+    }
+}
+
+/*
+ * g_set_user_dirs:
+ * @first_dir_type: Type of the first directory to set
+ * @...: Value to set the first directory to, followed by additional type/value
+ *    pairs, followed by %NULL
+ *
+ * Set one or more ‘user’ directories to custom values. This is intended to be
+ * used by test code (particularly with the %G_TEST_OPTION_ISOLATE_DIRS option)
+ * to override the values returned by the following functions, so that test
+ * code can be run without touching an installed system and user data:
+ *
+ *  - g_get_home_dir() — use type `HOME`, pass a string
+ *  - g_get_user_cache_dir() — use type `XDG_CACHE_HOME`, pass a string
+ *  - g_get_system_config_dirs() — use type `XDG_CONFIG_DIRS`, pass a
+ *    %NULL-terminated string array
+ *  - g_get_user_config_dir() — use type `XDG_CONFIG_HOME`, pass a string
+ *  - g_get_system_data_dirs() — use type `XDG_DATA_DIRS`, pass a
+ *    %NULL-terminated string array
+ *  - g_get_user_data_dir() — use type `XDG_DATA_HOME`, pass a string
+ *  - g_get_user_runtime_dir() — use type `XDG_RUNTIME_DIR`, pass a string
+ *
+ * The list must be terminated with a %NULL type. All of the values must be
+ * non-%NULL — passing %NULL as a value won’t reset a directory. If a reference
+ * to a directory from the calling environment needs to be kept, copy it before
+ * the first call to g_set_user_dirs(). g_set_user_dirs() can be called multiple
+ * times.
+ *
+ * Since: 2.60
+ */
+/*< private > */
+void
+g_set_user_dirs (const gchar *first_dir_type,
+                 ...)
+{
+  va_list args;
+  const gchar *dir_type;
+
+  G_LOCK (g_utils_global);
+
+  va_start (args, first_dir_type);
+
+  for (dir_type = first_dir_type; dir_type != NULL; dir_type = va_arg (args, const gchar *))
+    {
+      gconstpointer dir_value = va_arg (args, gconstpointer);
+      g_assert (dir_value != NULL);
+
+      if (g_str_equal (dir_type, "HOME"))
+        set_str_if_different (&g_home_dir, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_CACHE_HOME"))
+        set_str_if_different (&g_user_cache_dir, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_CONFIG_DIRS"))
+        set_strv_if_different (&g_system_config_dirs, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_CONFIG_HOME"))
+        set_str_if_different (&g_user_config_dir, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_DATA_DIRS"))
+        set_strv_if_different (&g_system_data_dirs, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_DATA_HOME"))
+        set_str_if_different (&g_user_data_dir, dir_type, dir_value);
+      else if (g_str_equal (dir_type, "XDG_RUNTIME_DIR"))
+        set_str_if_different (&g_user_runtime_dir, dir_type, dir_value);
+      else
+        g_assert_not_reached ();
+    }
+
+  va_end (args);
+
+  G_UNLOCK (g_utils_global);
+}
+
 static gchar *
 g_build_user_data_dir (void)
 {
diff --git a/glib/gutilsprivate.h b/glib/gutilsprivate.h
new file mode 100644
index 000000000..5a0686086
--- /dev/null
+++ b/glib/gutilsprivate.h
@@ -0,0 +1,33 @@
+/*
+ * Copyright © 2018 Endless Mobile, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Philip Withnall <withnall@endlessm.com>
+ */
+
+#ifndef __G_UTILS_PRIVATE_H__
+#define __G_UTILS_PRIVATE_H__
+
+#include "gtypes.h"
+
+G_BEGIN_DECLS
+
+GLIB_AVAILABLE_IN_2_60
+void g_set_user_dirs (const gchar *first_dir_type,
+                      ...) G_GNUC_NULL_TERMINATED;
+
+G_END_DECLS
+
+#endif /* __G_UTILS_PRIVATE_H__ */
diff --git a/glib/meson.build b/glib/meson.build
index 3275ee6e1..1fe4a6e73 100644
--- a/glib/meson.build
+++ b/glib/meson.build
@@ -199,6 +199,7 @@ glib_sources = files(
   'gunidecomp.c',
   'gurifuncs.c',
   'gutils.c',
+  'gutilsprivate.h',
   'guuid.c',
   'gvariant.c',
   'gvariant-core.c',