gtimezone: Fix assertion failure when called with a huge offset

This looks like a regression from commit 3356934db5, but prior to that
commit there was always an assertion failure when calling
`g_time_zone_new_offset()` with an offset which is too large (such as 44
hours), ever since the function was added in commit cf24867b93.

It would be ideal if we could return a `NULL` timezone to indicate the
error, but that’s not part of the API for `g_time_zone_new_offset()`, so
we have to go with the dated and not-ideal approach of returning the UTC
timezone and letting the caller figure it out.

Another potential approach would be to reduce the `offset` modulo 24
hours. This makes the error less easily detectable than if returning
UTC, though, and still returns an invalid result: `+44:00` is not the
same timezone as `+20:00` (it’s one day further ahead).

Add a unit test.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2620

glib/gtimezone.c

@@ -2028,7 +2028,13 @@ g_time_zone_new_local (void)
  * This is equivalent to calling g_time_zone_new() with a string in the form
  * `[+|-]hh[:mm[:ss]]`.
  *
- * Returns: (transfer full): a timezone at the given offset from UTC
+ * It is possible for this function to fail if @seconds is too big (greater than
+ * 24 hours), in which case this function will return the UTC timezone for
+ * backwards compatibility. To detect failures like this, use
+ * g_time_zone_new_identifier() directly.
+ *
+ * Returns: (transfer full): a timezone at the given offset from UTC, or UTC on
+ *   failure
  * Since: 2.58
  */
 GTimeZone *
@@ -2048,11 +2054,15 @@ g_time_zone_new_offset (gint32 seconds)
                                 (ABS (seconds) / 60) % 60,
                                 ABS (seconds) % 60);
   tz = g_time_zone_new_identifier (identifier);
+
+  if (tz == NULL)
+    tz = g_time_zone_new_utc ();
+  else
+    g_assert (g_time_zone_get_offset (tz, 0) == seconds);
+
   g_assert (tz != NULL);
   g_free (identifier);
 
-  g_assert (g_time_zone_get_offset (tz, 0) == seconds);
-
   return tz;
 }
 

glib/tests/gdatetime.c

@@ -2940,19 +2940,26 @@ test_identifier (void)
 static void
 test_new_offset (void)
 {
-  const gint32 vectors[] =
+  const struct
     {
-      -10000,
+      gint32 offset;
-      -3600,
+      gboolean expected_success;
-      -61,
+    }
-      -60,
+  vectors[] =
-      -59,
+    {
-      0,
+      { -158400, FALSE },
-      59,
+      { -10000, TRUE },
-      60,
+      { -3600, TRUE },
-      61,
+      { -61, TRUE },
-      3600,
+      { -60, TRUE },
-      10000,
+      { -59, TRUE },
+      { 0, TRUE },
+      { 59, TRUE },
+      { 60, TRUE },
+      { 61, TRUE },
+      { 3600, TRUE },
+      { 10000, TRUE },
+      { 158400, FALSE },
     };
   gsize i;
 
@@ -2960,12 +2967,21 @@ test_new_offset (void)
     {
       GTimeZone *tz = NULL;
 
-      g_test_message ("Vector %" G_GSIZE_FORMAT ": %d", i, vectors[i]);
+      g_test_message ("Vector %" G_GSIZE_FORMAT ": %d", i, vectors[i].offset);
 
-      tz = g_time_zone_new_offset (vectors[i]);
+      tz = g_time_zone_new_offset (vectors[i].offset);
       g_assert_nonnull (tz);
-      g_assert_cmpstr (g_time_zone_get_identifier (tz), !=, "UTC");
+
-      g_assert_cmpint (g_time_zone_get_offset (tz, 0), ==, vectors[i]);
+      if (vectors[i].expected_success)
+        {
+          g_assert_cmpstr (g_time_zone_get_identifier (tz), !=, "UTC");
+          g_assert_cmpint (g_time_zone_get_offset (tz, 0), ==, vectors[i].offset);
+        }
+      else
+        {
+          g_assert_cmpstr (g_time_zone_get_identifier (tz), ==, "UTC");
+        }
+
       g_time_zone_unref (tz);
     }
 }