gdatetime: Fix handling of unsupported nl_langinfo() items

If nl_langinfo() doesn’t support a particular item, it returns the empty
string. We should check for that and return NULL from
g_date_time_format() accordingly, otherwise the user could unwittingly
end up with a formatted date/time which is missing some or all of its
components.

This arose with %r in de_DE, which is unsupported by nl_langinfo()
because Germans almost never write time in 12-hour format.

Add a unit test.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790416

glib/gdatetime.c

@@ -2695,6 +2695,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	{
 	case 'a':
 	  name = WEEKDAY_ABBR (datetime);
+          if (g_strcmp0 (name, "") == 0)
+            return FALSE;
 #if !defined (HAVE_LANGINFO_TIME)
 	  if (!locale_is_utf8)
 	    {
@@ -2712,6 +2714,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'A':
 	  name = WEEKDAY_FULL (datetime);
+          if (g_strcmp0 (name, "") == 0)
+            return FALSE;
 #if !defined (HAVE_LANGINFO_TIME)
 	  if (!locale_is_utf8)
 	    {
@@ -2729,6 +2733,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'b':
 	  name = MONTH_ABBR (datetime);
+          if (g_strcmp0 (name, "") == 0)
+            return FALSE;
 #if !defined (HAVE_LANGINFO_TIME)
 	  if (!locale_is_utf8)
 	    {
@@ -2746,6 +2752,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'B':
 	  name = MONTH_FULL (datetime);
+          if (g_strcmp0 (name, "") == 0)
+            return FALSE;
 #if !defined (HAVE_LANGINFO_TIME)
 	  if (!locale_is_utf8)
 	    {
@@ -2763,6 +2771,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'c':
 	  {
+            if (g_strcmp0 (PREFERRED_DATE_TIME_FMT, "") == 0)
+              return FALSE;
 	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_DATE_TIME_FMT,
 						   outstr, locale_is_utf8))
 	      return FALSE;
@@ -2796,6 +2806,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'h':
 	  name = MONTH_ABBR (datetime);
+          if (g_strcmp0 (name, "") == 0)
+            return FALSE;
 #if !defined (HAVE_LANGINFO_TIME)
 	  if (!locale_is_utf8)
 	    {
@@ -2855,6 +2867,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'r':
 	  {
+            if (g_strcmp0 (PREFERRED_12HR_TIME_FMT, "") == 0)
+              return FALSE;
 	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_12HR_TIME_FMT,
 						   outstr, locale_is_utf8))
 	      return FALSE;
@@ -2895,6 +2909,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'x':
 	  {
+            if (g_strcmp0 (PREFERRED_DATE_FMT, "") == 0)
+              return FALSE;
 	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_DATE_FMT,
 						   outstr, locale_is_utf8))
 	      return FALSE;
@@ -2902,6 +2918,8 @@ g_date_time_format_locale (GDateTime   *datetime,
 	  break;
 	case 'X':
 	  {
+            if (g_strcmp0 (PREFERRED_TIME_FMT, "") == 0)
+              return FALSE;
 	    if (!g_date_time_locale_format_locale (datetime, PREFERRED_TIME_FMT,
 						   outstr, locale_is_utf8))
 	      return FALSE;
@@ -3059,7 +3077,8 @@ g_date_time_format_locale (GDateTime   *datetime,
  *   for the specifier.
  *
  * Returns: a newly allocated string formatted to the requested format
- *     or %NULL in the case that there was an error. The string
+ *     or %NULL in the case that there was an error (such as a format specifier
+ *     not being supported in the current locale). The string
  *     should be freed with g_free().
  *
  * Since: 2.26

glib/tests/gdatetime.c

@@ -1818,6 +1818,27 @@ test_strftime (void)
 }
 #pragma GCC diagnostic pop
 
+/* Check that g_date_time_format() correctly returns %NULL for format
+ * placeholders which are not supported in the current locale. */
+static void
+test_GDateTime_strftime_error_handling (void)
+{
+  gchar *oldlocale;
+
+  oldlocale = g_strdup (setlocale (LC_ALL, NULL));
+  setlocale (LC_ALL, "de_DE.utf-8");
+  if (strstr (setlocale (LC_ALL, NULL), "de_DE") != NULL)
+    {
+      /* de_DE doesn’t ever write time in 12-hour notation, so %r is
+       * unsupported for it. */
+      TEST_PRINTF_TIME (23, 0, 0, "%r", NULL);
+    }
+  else
+    g_test_skip ("locale de_DE not available, skipping error handling tests");
+  setlocale (LC_ALL, oldlocale);
+  g_free (oldlocale);
+}
+
 static void
 test_find_interval (void)
 {
@@ -2141,6 +2162,7 @@ main (gint   argc,
   g_test_add_func ("/GDateTime/printf", test_GDateTime_printf);
   g_test_add_func ("/GDateTime/non_utf8_printf", test_non_utf8_printf);
   g_test_add_func ("/GDateTime/strftime", test_strftime);
+  g_test_add_func ("/GDateTime/strftime/error_handling", test_GDateTime_strftime_error_handling);
   g_test_add_func ("/GDateTime/modifiers", test_modifiers);
   g_test_add_func ("/GDateTime/to_local", test_GDateTime_to_local);
   g_test_add_func ("/GDateTime/to_unix", test_GDateTime_to_unix);